metformin , a biguanide used as first - line treatment for type 2 diabetes mellitus , lowers blood glucose by activating amp - activated protein kinase ( ampk ) . ampk activation , through the reduction of mammalian target of rapamycin ( mtor ) signaling , has also been shown to inhibit cancer cell growth and proliferation ( 1 ) . thus , it is conceivable that ampk activators such as metformin may influence human cancer risk . activation of the mtor pathway has been detected in bladder cancer , while inhibition of this pathway blocks bladder tumorigenesis ( 2,3 ) in animal models and is associated with durable response in some patients with advanced bladder cancer ( 4 ) . these observations provide the rationale for mtor - directed therapy with metformin as a potential strategy for bladder cancer chemoprevention . several observational studies have demonstrated that the use of metformin is associated with a decreased incidence of cancers of the breast ( 5,6 ) , colon ( 79 ) , liver ( 7,10,11 ) , lung ( 9,12 ) , pancreas ( 13 ) , and prostate ( 14 ) . two recent meta - analyses , which included some of these observational studies , reported a 30% reduction in all - cancer incidence in metformin users compared with users of other diabetes therapies ( decensi et al . : relative risk 0.69 [ 95% ci 0.610.79 ] ; noto et al . : 0.68 [ 0.530.88 ] ) . there are limited data , however , examining the effect of metformin on the incidence of bladder cancer ( 17,18 ) . given that bladder cancer occurs more commonly in patients with type 2 diabetes ( 19 ) , it is critically important to determine whether exposure to metformin is associated with a decreased risk of bladder cancer . suissa and azoulay ( 20 ) recently reported that many studies of the impact of metformin on subsequent cancer risk suffered from time - related biases , including immortal - time bias from misclassification of metformin exposure time ( 7,8,12 ) , time - window bias from differential lengths of metformin exposure time ( 5,6,10,11,14 ) , and time - lag bias inherent to comparisons of first - line metformin treatment with second- or third - line treatments ( 8,9 ) . collectively , these biases may have led to the exaggerated reductions in cancer risk observed in these studies . in this study , we aimed to examine the risk of bladder cancer between new users of metformin and sulfonylureas ( sus ) among a cohort of type 2 diabetes patients without previous use of these diabetes therapies , thereby avoiding the time - related biases that may have impacted prior studies of metformin and cancer . the health improvement network ( thin ) is an electronic medical records database that is representative of the broader u.k . data available in thin include demographic information , medical diagnoses , lifestyle characteristics , and other measurements , including glycosylated hemoglobin ( hba1c ) and bmi . the software automatically codes medical diagnoses at entry using the read coding system ( 21 ) . thin also records new and repeat prescriptions written by the general practitioner as the computerized record is used to generate these prescriptions . the accuracy and completeness of thin data are well - documented , and the database has been used for epidemiological studies of several chronic diseases , including diabetes and cancer ( 2224 ) . the database currently contains the electronic medical records of > 10 million patients , allowing for precise estimates of incidence rates of even rare outcomes , such as those for bladder cancer . using thin , we conducted a retrospective cohort study of patients with type 2 diabetes who initiated therapy with either metformin or an su between 1 july 2000 and 31 august 2010 ( fig . 1 ) . we selected this time frame to avoid missing data on prior treatment with thiazolidinediones ( tzds ) , which were licensed in the european union in july 2000 and have been associated with an increased risk of bladder cancer ( 25,26 ) . from this population , we assembled a study and comparator cohort of new users of metformin or an su , respectively . a new user was defined as having had a 6-month baseline period in thin without previous prescriptions for either drug . those initiating therapy with a combination of metformin and an su were excluded from the study . subjects who had received a diagnosis of bladder cancer before cohort entry were excluded to avoid misclassification of prevalent cancers as incident cancers . subjects < 40 years of age at cohort entry were also excluded because bladder cancer is extremely rare prior to age 40 years , and when it occurs , it likely results from a different mechanism . the study protocol was approved by the university of pennsylvania institutional review board and the u.k . scientific review committee . a retrospective cohort study was conducted among type 2 diabetes patients in the thin database . we compared new users of metformin ( met ) with new users of sus and excluded patients with use of these drugs before 1 july 2000 or within 6 months of enrollment in thin . exposure to metformin was defined as the receipt of two prescriptions for metformin or metformin - containing products within 6 months . requiring the second prescription within a reasonably brief period helps to exclude the small fraction of patients who may fill a prescription but never return to their general practitioner or who never consumed the medication in the first prescription . duration of therapy was calculated by dividing prescription quantity by the prescribed number of units per day , while cumulative duration of therapy was determined by summing the supply of a total day for all prescriptions subsequent to the index prescription . identical definitions were used to define exposure to the sus , including first - generation drugs ( acetohexamide , chlorpropamide , tolbutamide , or tolazamide ) and second - generation drugs ( glipizide , gliquidone , glimepiride , glibenclamide , or gliclazide ) . sus were selected as the comparator because , like metformin , the sus may also be used as first - line treatment for type 2 diabetes ( 27 ) . furthermore , in prior studies , no association was observed between the use of sus and bladder cancer or other cancer incidence ( 25,26 ) . the primary outcome was an incident diagnosis with bladder cancer , after the index date . follow - up started on the index date and ended with the first of the following censoring events : incident diagnosis of bladder cancer ; transfer out of thin practice ; death ; or end of available data . the latter date was taken as 31 august 2010 to minimize the chance of surveillance bias given the u.s . food and drug administration warnings of a possible association between tzds and bladder cancer in september 2010 ( 28 ) . incidence rates of bladder cancer ( per 100,000 person - years ) with 95% cis were calculated for the study ( metformin ) and comparator ( su ) cohorts . once a patient met the definition of exposure , the patient was considered exposed from that point forward , even if therapy was discontinued . patients who switched or combined therapy , however , were censored on the date of treatment crossover . cox regression models were used to generate relative hazards of bladder cancer in metformin users compared with su users , adjusted for potential confounders ( 29 ) . as potential confounders , we considered age , sex , smoking , recurrent urinary tract infections , obesity , congestive heart failure , myocardial infarction , renal impairment , diabetes duration , hba1c level , other common diabetes medications ( insulin and tzds ) , and other commonly prescribed medications ( ace inhibitors , statins , aspirin , and nonsteroidal anti - inflammatory drugs ) . all covariates were measured at baseline during the 6 months immediately prior to the initiation of metformin or su therapy , with the exception of smoking . smoking status was measured using data recorded at any time before or during follow - up . covariates changing the hazard ratio ( hr ) between metformin exposure and outcome by > 10% were included in the full model . full models were then reduced by sequentially deleting variables with model p values 0.10 to arrive at the final model . the final model included age , sex , smoking ( ever vs. never ) , obesity , ( bmi 30 ) , and hba1c level . patients with missing baseline bmi or hba1c values were omitted from fully adjusted analyses ( case - wise deletion ) . cox regression was also used to determine whether the risk of bladder cancer decreased with increasing duration of metformin therapy . in these analyses , metformin and su use was entered into the model as time - varying exposures . during the follow - up period , each user was categorized according to their cumulative duration of therapy ( < 1 , 12 , 23 , 34 , 45 , and 5 years ) . to test for trend , duration of exposure the duration analyses were planned a priori given previous studies suggesting that the antitumor effect of metformin increased with each year of metformin use ( 15 ) . stata version 12.0 was used for all statistical analyses ( statacorp , college station , tx ) . a subgroup analysis limited the cohort to patients with no documented use of any diabetes medication prior to the start of follow - up , allowing for comparison between new users of metformin and sus as the initial therapy . we also performed a series of sensitivity analyses to test the robustness of our results . to assess the potential impact of misclassification bias , we extended the baseline period used to define new user from 6 months to 1 year and redefined exposure as the receipt of two prescriptions in 3 months rather than 6 months . to address the possibility of detection bias , we conducted a 1-year lagged analysis in which patients with bladder cancers occurring in the first year of follow - up were excluded . to assess for residual confounding by use of other diabetes therapies during follow - up , we repeated the primary analysis after adjustment for the use of tzds and insulin as time - updating variables . first , we entered mean bmi and mean hba1c level in our final model using data recorded throughout the entire follow - up period . second , linear regression was used to impute missing data on baseline bmi and hba1c levels using all descriptive variables included in the primary analysis . to account for the variability among imputations the health improvement network ( thin ) is an electronic medical records database that is representative of the broader u.k . data available in thin include demographic information , medical diagnoses , lifestyle characteristics , and other measurements , including glycosylated hemoglobin ( hba1c ) and bmi . the software automatically codes medical diagnoses at entry using the read coding system ( 21 ) . thin also records new and repeat prescriptions written by the general practitioner as the computerized record is used to generate these prescriptions . the accuracy and completeness of thin data are well - documented , and the database has been used for epidemiological studies of several chronic diseases , including diabetes and cancer ( 2224 ) . the database currently contains the electronic medical records of > 10 million patients , allowing for precise estimates of incidence rates of even rare outcomes , such as those for bladder cancer . using thin , we conducted a retrospective cohort study of patients with type 2 diabetes who initiated therapy with either metformin or an su between 1 july 2000 and 31 august 2010 ( fig . 1 ) . we selected this time frame to avoid missing data on prior treatment with thiazolidinediones ( tzds ) , which were licensed in the european union in july 2000 and have been associated with an increased risk of bladder cancer ( 25,26 ) . from this population , we assembled a study and comparator cohort of new users of metformin or an su , respectively . a new user was defined as having had a 6-month baseline period in thin without previous prescriptions for either drug . those initiating therapy with a combination of metformin and an su were excluded from the study . subjects who had received a diagnosis of bladder cancer before cohort entry were excluded to avoid misclassification of prevalent cancers as incident cancers . subjects < 40 years of age at cohort entry were also excluded because bladder cancer is extremely rare prior to age 40 years , and when it occurs , it likely results from a different mechanism . the study protocol was approved by the university of pennsylvania institutional review board and the u.k . scientific review committee . a retrospective cohort study was conducted among type 2 diabetes patients in the thin database . we compared new users of metformin ( met ) with new users of sus and excluded patients with use of these drugs before 1 july 2000 or within 6 months of enrollment in thin . exposure to metformin was defined as the receipt of two prescriptions for metformin or metformin - containing products within 6 months . requiring the second prescription within a reasonably brief period helps to exclude the small fraction of patients who may fill a prescription but never return to their general practitioner or who never consumed the medication in the first prescription . duration of therapy was calculated by dividing prescription quantity by the prescribed number of units per day , while cumulative duration of therapy was determined by summing the supply of a total day for all prescriptions subsequent to the index prescription . identical definitions were used to define exposure to the sus , including first - generation drugs ( acetohexamide , chlorpropamide , tolbutamide , or tolazamide ) and second - generation drugs ( glipizide , gliquidone , glimepiride , glibenclamide , or gliclazide ) . sus were selected as the comparator because , like metformin , the sus may also be used as first - line treatment for type 2 diabetes ( 27 ) . furthermore , in prior studies , no association was observed between the use of sus and bladder cancer or other cancer incidence ( 25,26 ) . the primary outcome was an incident diagnosis with bladder cancer , after the index date . follow - up started on the index date and ended with the first of the following censoring events : incident diagnosis of bladder cancer ; transfer out of thin practice ; death ; or end of available data . the latter date was taken as 31 august 2010 to minimize the chance of surveillance bias given the u.s . food and drug administration warnings of a possible association between tzds and bladder cancer in september 2010 ( 28 ) . incidence rates of bladder cancer ( per 100,000 person - years ) with 95% cis were calculated for the study ( metformin ) and comparator ( su ) cohorts . once a patient met the definition of exposure , the patient was considered exposed from that point forward , even if therapy was discontinued . patients who switched or combined therapy , however , were censored on the date of treatment crossover . cox regression models were used to generate relative hazards of bladder cancer in metformin users compared with su users , adjusted for potential confounders ( 29 ) . as potential confounders , we considered age , sex , smoking , recurrent urinary tract infections , obesity , congestive heart failure , myocardial infarction , renal impairment , diabetes duration , hba1c level , other common diabetes medications ( insulin and tzds ) , and other commonly prescribed medications ( ace inhibitors , statins , aspirin , and nonsteroidal anti - inflammatory drugs ) . all covariates were measured at baseline during the 6 months immediately prior to the initiation of metformin or su therapy , with the exception of smoking . smoking status was measured using data recorded at any time before or during follow - up . covariates changing the hazard ratio ( hr ) between metformin exposure and outcome by > 10% were included in the full model . full models were then reduced by sequentially deleting variables with model p values 0.10 to arrive at the final model . the final model included age , sex , smoking ( ever vs. never ) , obesity , ( bmi 30 ) , and hba1c level . patients with missing baseline bmi or hba1c values were omitted from fully adjusted analyses ( case - wise deletion ) . cox regression was also used to determine whether the risk of bladder cancer decreased with increasing duration of metformin therapy . in these analyses , metformin and su use was entered into the model as time - varying exposures . during the follow - up period , each user was categorized according to their cumulative duration of therapy ( < 1 , 12 , 23 , 34 , 45 , and 5 years ) . to test for trend , duration of exposure the duration analyses were planned a priori given previous studies suggesting that the antitumor effect of metformin increased with each year of metformin use ( 15 ) . stata version 12.0 was used for all statistical analyses ( statacorp , college station , tx ) . a subgroup analysis limited the cohort to patients with no documented use of any diabetes medication prior to the start of follow - up , allowing for comparison between new users of metformin and sus as the initial therapy . we also performed a series of sensitivity analyses to test the robustness of our results . to assess the potential impact of misclassification bias , we extended the baseline period used to define new user from 6 months to 1 year and redefined exposure as the receipt of two prescriptions in 3 months rather than 6 months . to address the possibility of detection bias , we conducted a 1-year lagged analysis in which patients with bladder cancers occurring in the first year of follow - up were excluded . to assess for residual confounding by use of other diabetes therapies during follow - up , we repeated the primary analysis after adjustment for the use of tzds and insulin as time - updating variables . first , we entered mean bmi and mean hba1c level in our final model using data recorded throughout the entire follow - up period . second , linear regression was used to impute missing data on baseline bmi and hba1c levels using all descriptive variables included in the primary analysis . to account for the variability among imputations , the final cohort included 87,600 patients with type 2 diabetes who were initiators of therapy with metformin ( n = 71,472 ) or an su ( n = 16,128 ) . approximately 8% of initiators ( n = 7,352 ) left a thin practice during follow - up . the median follow - up time was 2 years for both cohorts ( metformin cohort : median 2.1 years [ interquartile range { iqr } 0.84.0 years ] vs. su cohort : median 2.0 years [ iqr 0.73.9 years ] ) , and > 4 years in 25% of subjects ( table 1 ) . metformin initiators were younger ( median [ iqr ] age 62 years [ 5471 years ] vs. 69 years [ 5978 years ] ) , more obese , and less likely to have renal impairment than su initiators ( table 1 ) . there were small differences in other baseline characteristics including sex , smoking status , hba1c level , and diabetes duration between the metformin and su cohorts . levels were missing in 14% and 20% of metformin and su members , respectively , and bmi was missing in 4% and 9% ( table 1 ) . during the follow - up period , the proportion of patients exposed to a tzd was nearly identical in both cohorts ( 15.4% among metformin initiators vs. 15.2% among su initiators ) . demographics of the study and comparator cohorts values are given as n ( % ) , unless otherwise stated . arb , angiotensin - receptor blocker ; nsaid , nonsteroidal anti - inflammatory drug . * all comparisons have p values < 0.01 except sex ( p = 0.08 ) , use of aspirin ( p = 0.80 ) , and use of a tzd after the index date ( p = 0.43 ) . we identified 262 incident bladder cancers during 221,406 person - years : 196 cancers in the metformin cohort and 66 cancers in the su cohort ( table 2 ) . the unadjusted bladder cancer incidence rates among metformin and su initiators were 107.8 ( 95% ci 93.2123.9 ) and 166.7 ( 95% ci 129.0212.1 ) per 100,000 person - years . in the fully adjusted model ( adjusted for age , sex , smoking , obesity , and hba1c level ) , the use of metformin was not associated with bladder cancer risk ( hr 0.81 [ 95% ci 0.601.09 ] ) . this association did not differ by sex ( p for interaction = 0.20 ) . of note , the association did not change appreciably after adjustment for other therapies that may influence bladder cancer risk , including insulin , tzds , angiotensin receptor blockers , statins , aspirin , and nonsteroidal anti - inflammatory drugs ( 3134 ) . incidence rate and relative risk of bladder cancer in the metformin and su cohorts pys , person - years ; ir , incidence rate . adjusted for age ( < 60 , 6069 , and 70 years ) , sex , smoking ( ever vs. never ) , hba1c level ( < 7% , 77.9% , 88.9% , and 9% ) , and obesity ( bmi 30 kg / m ) . the test of trend was calculated by entering the duration categories in a cox regression model as a continuous variable , whereas for analysis of discrete duration intervals , the variable was included as a categorical variable . in analyses that accounted for the duration of therapy ( table 2 ) , we observed no association between increasing duration of metformin therapy and the risk of bladder cancer . for example , there was no decreased risk of bladder cancer among metformin users with 5 years of use compared with < 1 year of use ( 5 years of use : hr 1.02 [ 95% ci 0.591.75 ] ; ptrend = 0.99 ) . similarly , among su users , there were no clear patterns between increasing duration of su therapy and bladder cancer risk ( ptrend = 0.21 ) . when we compared bladder cancer risk in the metformin users relative to su users , there was likewise no pattern of decreasing relative risk of bladder cancer associated with metformin use with increasing duration of metformin therapy ( metformin vs. su therapy : 3 to < 4 years of use : hr 0.57 [ 95% ci 0.251.34 ] ; 4 to < 5 years of use : 0.93 [ 0.302.85 ] ; 5 years of use : 1.18 [ 0.443.19 ] ; ptrend = 0.26 ) . in a subgroup analysis , we restricted the new user cohort to include only those having had a 6-month baseline period in thin without previous prescriptions for metformin , su , or any other diabetes therapy ( n = 82,570 ) . in this analysis , we obtained similar results to our primary analysis for the overall ( hr 0.81 [ 95% ci 0.601.09 ] ) and duration of therapy analyses ( metformin vs. su : 3 to < 4 years of use , 0.53 [ 0.221.26 ] ; 4 to < 5 years of use , 0.91 [ 0.292.83 ] ; 5 years of use , 1.14 [ 0.413.13 ] ; ptrend = 0.36 ) . we also observed similar results to our primary analysis for all sensitivity analyses described in research design and methods ( table 3 ) . subgroup and sensitivity analyses of the relative risk of bladder cancer in the metformin relative to su cohorts * adjusted for age ( < 60 , 6069 , and 70 years ) , sex , smoking ( ever vs. never ) , hba1c level ( < 7% , 77.9% , 88.9% , and 9% ) , and obesity ( bmi 30 kg / m ) . exclusion of subjects with use of diabetes therapies ( n = 5,030 ; 5.7% ) prior to start of follow - up . linear regression was used to impute missing data on hba1c levels and bmi . to account for the variability between imputations follow - up time continued for metformin or su users who switched therapy or started combination therapy . in this cohort study , we found no difference in the incidence of bladder cancer among new users of metformin compared with new users of an su , the common alternative first - line therapy for type 2 diabetes . we also observed no association with risk of bladder cancer by the duration of metformin relative to su use , or metformin or su use alone . although these results do not support prescribing metformin in preference to sus for the purpose of reducing bladder cancer incidence , the results are important for interpreting prior studies of bladder cancer risk with tzds . specifically , many of these studies included patients treated with metformin and sus as comparators . furthermore , our results help to clarify existing controversy over the potential cancer prevention effects of metformin , focusing on a cancer that is more common in patients with type 2 diabetes ( 17,19 ) , has been associated with exposure to another class of diabetes medications ( 25,26 ) , and is potentially sensitive to the mechanism of action of metformin . our findings differ from previous observational studies that have reported dramatic reductions in cancer risk associated with the use of metformin , ranging from 19 to 94% ( 618 ) . these contrasting results could be explained if metformin is only biologically active against tumors other than bladder cancer , or perhaps more likely if the methods used in the previous epidemiological studies systemically biased results toward a protective effect of metformin on cancer risk . suissa and azoulay ( 20 ) have recently described several time - related biases ( immortal - time , time - window , and time - lag biases ) believed to affect the validity of the previous studies . misclassification of immortal time , time during which the outcome under study ( cancer ) could not have occurred , as metformin - exposed time , leads to immortal - time bias ( 35 ) . time - lag bias arises in cohort studies comparing second- or third - line treatments ( sus or insulin ) with first - line metformin treatment ( 20 ) . inclusion of treatment cohorts with different durations of previous therapy or durations of different diabetes may have differential effects on cancer risk . for example , longer duration of diabetes or use of medications associated with longer duration of diabetes may increase the risk of bladder cancer , as has been demonstrated in some studies ( 31 ) . to avoid the time - related biases that may have impacted the results of previous studies , there are several unique features of our study . we used an incident user design comparing metformin to the common alternative first - line therapy , sus , for patients with type 2 diabetes . the new user design reduces the risk of time - lag and time - window bias by excluding treatment groups with different durations of previous therapy , allowing for direct comparisons between predominantly first - line treatment cohorts . as such , we observed similar durations of diabetes between the metformin and su treatment cohorts ( median diabetes duration : metformin users , 8.1 months [ iqr 1.639.1 ] vs. su users , 7.0 months [ iqr 1.539.3 ] ) . the new user design also assigns a chronological reference point , time zero , for each member of the study and comparator cohorts from which follow - up time begins , thereby minimizing bias from immortal time . we further reduced this bias by adopting a cox model with time - dependent drug exposures , and by censoring patients if they had switched or combined therapy between metformin and an su . our findings remained consistent in several sensitivity analyses designed to assess the impact of other common biases in pharmacoepidemiologic studies of diabetes therapies . for example , we assessed the potential misclassification of use by redefining exposure as receiving two prescriptions for therapy within a 3-month period , and redefining new user as requiring a therapy - free baseline period of 1 year . results from these sensitivity analyses were similar to the primary analysis , indicating minimal misclassification of unexposed patients as exposed and prevalent users as new users . we also considered the impact of detection bias among initiators of metformin or sus by excluding cancers occurring in the first year after the initiation of therapy . compared with the results of the primary analysis , this 1-year lagged analysis did not alter our results appreciably , suggesting that detection bias , even if present , was likely minimal . many studies of diabetes therapies and bladder cancer have relied on administrative data sources that lack information on tobacco use . recorded information on cancer has been validated in a related database ( general practice research database ) , which shares some practices with thin and uses the same electronic software for data collection ( 37 ) . observed cancer incidence within thin is comparable to that reported in u.k cancer registry data , particularly since 2001 ( 24 ) . the incidence rate of bladder cancer in the u.k . population aged 65 years was 80 per 100,000 person - years in 2008 ( 38 ) . the incidence of bladder cancer in our cohort was relatively high ( 113 per 100,000 person - years ) , which is consistent with the estimated 40% increase in the risk of bladder cancer observed among patients with type 2 diabetes ( 17,19 ) . diabetes is a chronic disease requiring treatment with several medication classes over time , such as the tzds . several studies have recently shown that treatment with pioglitazone , a tzd , may increase the risk of bladder cancer . because the proportion of patients exposed to a tzd during follow - up was nearly identical in metformin compared with su users ( 15.4% vs. 15.2% ) , the possible effects of tzds on the risk of bladder cancer are unlikely to have impacted our results . this is evidenced by a sensitivity analysis producing results similar to our primary analysis after adjustment for the use of tzd and insulin as time - updating variables ( primary analysis : hr 0.81 [ 95% ci 0.601.09 ] vs. sensitivity analysis : hr 0.86 [ 95% ci 0.641.15 ] ) . furthermore , given that our study ended prior to publicity of the tzd - bladder cancer warnings , surveillance for bladder cancer in response to these warnings would be nondifferential between the metformin and su cohorts . our study could be subject to confounding by indication due to group differences in diabetes severity or duration . to minimize confounding by indication , we selected a reference group of new users of the alternative first - line antidiabetes drug , the sus , and adjusted for baseline hba1c level . importantly , diabetes duration was comparable among the new user cohorts and was not observed to confound the association between metformin use and risk of bladder cancer . to avoid bias from treatment crossover ending follow - up time at this point , regardless of diabetes duration , may exclude patients with more advanced stage of diabetes . in a sensitivity analysis allowing follow - up time to continue in users who switched therapy or started combination therapy ( i.e. , initial treatment carried forward ) , we observed results that were not substantially different from the censor at switch approach . importantly , while research has shown positive associations between type 2 diabetes and the incidence of bladder cancer , data on the association between diabetes duration or severity ( i.e. , hba1c ) and bladder cancer risk remain controversial ( 39,40 ) . although our cohort consisted predominantly of patients receiving initial therapy , a small subset ( 6% ) had been previously treated with other diabetes medications . exclusion of subjects with previous use of diabetes therapy in a subgroup analysis produced nearly identical results to the primary analysis . levels and bmi were missing in 14% and 20% , and 4% and 9% , respectively , of metformin and su cohort members . when we conducted a sensitivity analysis using multiple imputation for the missing hba1c and bmi values , our results were similar to the primary analysis ( table 3 ) . likewise , we also observed similar results after adjustment for mean hba1c level and mean bmi using data recorded during the entire follow - up period , strongly suggesting that our results are unlikely to be biased from missing data on these confounders . . population , and occupational exposures are not strongly associated with bladder cancer ( 41 ) . thus , it is unlikely for variables such as race and occupation to account for residual confounding in this study . a post hoc analysis demonstrates that we were able to detect a relative hr of bladder cancer in metformin relative to su users of 0.65 with 80% power , consistent with the 95% cis ( 0.611.09 ) observed in our primary analysis . although we were underpowered to detect smaller differences in effect size ( i.e. , hr > 0.65 ) , we were adequately powered to detect large differences ( i.e. , hr < 0.65 ) , such as those reported in the previous studies examining the impact of metformin on subsequent cancer risk ( 6,815 ) . similarly , these data can not exclude a protective effect of even longer - term therapy with metformin . despite not observing a cancer prevention effect , our results should not detract others from investigating the therapeutic potential of metformin on bladder cancer progression among patients with known bladder cancer . metformin inhibits cancer cell growth via ampk activation and subsequent downstream inhibition of the mtor . dysregulation of mtor has been implicated in the progression of several cancers , including bladder cancer ( 2 ) . recently , inhibition of mtor has demonstrated antitumor activity in a subset of patients with advanced bladder cancer ( 4 ) , suggesting that this pathway is active in bladder cancer and could serve as a therapeutic target in this disease . thin patient population , we found no evidence for a decreased risk of bladder cancer in type 2 diabetes patients using metformin . there was also no association with bladder cancer risk by duration of metformin use . to avoid time - related biases , future studies investigating the effect of metformin on cancer incidence likewise , it will be important to use the same methods to study other cancers that have previously been identified as potentially preventable with metformin therapy .
objectiveprevious studies evaluating the effect of metformin on cancer risk have been impacted by time - related biases . to avoid these biases , we examined the incidence of bladder cancer in new users of metformin and sulfonylureas ( sus).research design and methodsthis cohort study included 87,600 patients with type 2 diabetes in the health improvement network database . 
 use of metformin or an su was treated as a time - dependent variable . 
 cox regression  generated hazard ratios ( hrs ) compared metformin use with su use , adjusted for age , sex , smoking , obesity , and hba1c level.resultswe identified 196 incident bladder cancers in the metformin cohort and 66 cancers in the su cohort . 
 use of metformin was not associated with decreased bladder cancer risk ( hr 0.81 [ 95% ci 0.601.09 ] ) . this association did not differ by sex ( p for interaction = 0.20 ) . 
 we observed no association with duration of metformin relative to su use ( 3 to < 4 years of use : 0.57 [ 0.251.34 ] ; 4 to < 5 years of use : 0.93 [ 0.302.85 ; 5 years of use : 1.18 [ 0.443.19 ] ; p for trend = 0.26).conclusionsuse of metformin is not associated with a decreased incidence of bladder cancer . 
 similar methods should be used to study other cancers that have previously been identified as potentially preventable with metformin .