Population normalised daily prescription (PNDPs mg/day/1000inh) were calculated from the total monthly mass of the drug prescribed; these were compared against the population normalised daily intake (PNDIs). As previously mentioned these were calculated in a prior comprehensive literature review investigating the pharmacokinetic data of selected AAs to generate CFs . The calculated CF values were used to back calculate the mass of drug consumed at the community level, using the amount of AA or respective metabolite quantified in wastewater and the proportion that target analyte is excreted via urine or faeces (figure ). The macrolide CLR gave good agreement between the PNDI and PNDP, with CLR in Bath (PNDI: 577 ± 189 mg/day/1000inh, PNDP: 524 ± 88 mg/day/1000inh), and in Keynsham (PNDI: 427 ± 183, PNDP:480 ± 125 mg/day/1000inh). The metabolite dmCLR did not give this same closeness being much higher than its parent compound after correction (PNDI for Bath: 1347 ± 402 mg/day/1000inh, PNDI for Keynsham: 922 ± 323 mg/day/1000inh). . Notably, PNDIs for several AAs were in good agreement between the two sites but the PNDP was different. Agreement between PNDIs between different locations could indicate that WBE provides a better estimate of community AA usage than prescriptions alone. This is because WBE can account for the potential problems with prescriptions. As mentioned earlier, drugs prescribed may not have been used, delayed prescription, stored or disposed of, and some treatments may last many months (e.g. AAs prescribed for acne treatment). Patients of GP surgeries may also live outside the studied wastewater catchment area, so use/disposal of those pharmaceuticals not captured in the studied wastewater data. Vice versa some individuals may be consuming pharmaceuticals that were prescribed elsewhere within the studied catchment area so would show in wastewater data and not the catchment prescription data. Also, whilst some laboratory stability studies in wastewater have been investigated , it is recognised that wastewater from different sites could provide very different matrices and the environment within the sewage system could be very different.