Marijuana is much stronger than it was three decades ago , experts have warned . Scientists found cannabis smoked today has tripled in strength since the 1980s . Marijuana now contains increasingly high concentrations of tetrahydrocannabinol ( THC ) , the psychoactive compound which gives the 'high ' . But concentrations of cannabidiol ( CBD ) , a lesser known compound that is being investigated for its medical proponents , but does not create a 'high ' , has gone down . Samples also contained contaminants including bacteria , fungi , butane and heavy metals , scientists found . Scroll down for video . Super strong : Marijuana sold on the market today has tripled in strength since the 1980s , scientists warn . The research was carried out by Dr Andy LaFrate , the president and director of research of Charas Scientific , a lab in Colorado , US , which carries out potency testing on marijuana samples on the market today . Presenting his preliminary findings at the 249th National Meeting & Exposition of the American Chemical Society ( ACS ) , he found that marijuana has tripled in strength in the last 30 years . He said : 'We 've seen a big increase in marijuana potency compared to where it was 20 or 30 years ago . ' I would say the average potency of marijuana has probably increased by a factor of three . ' The average potencies of the samples he analysed was 20 per cent THC , but he saw values close to 30 per cent , which he described as'huge ' . He explained that 30 years ago , THC levels in marijuana were well below 10 per cent . The THC content of cannabis has tripled in some strains because producers have been cross-breeding them over the years as users demand stronger drugs , he said . He added that an unexpected consequence of the breeding was that many of the samples his lab tested contained little to no CBD . Researchers are currently investigating CBD as a treatment for schizophrenia , Huntington 's disease and Alzheimer 's disease , as well as anxiety and depression . But unlike THC , CBD does n't get provide the same 'high ' , which is why it could be considered in treatments for children . Extra potent : Concentrations of marijuana 's active ingredient tetrahydrocannabinol ( THC ) ( pictured ) have increased from less than 10 per cent in samples from the 1980s to up to 30 per cent in samples tested today . Dr LaFrate said he was shocked by the levels of contaminants in samples of marijuana producers had voluntarily sent to the lab for testing . He said : 'One thing that 's been surprising is the level of microbial contamination in marijuana . ' 'Many of the samples were covered in fungi or bacterial contaminants . 'Others had Butane , which are used to create marijuana extracts , and others had heavy metals . ' This is n't necessarily a cause for alarm , but it could spark discussions on what are safe levels of contaminants . Dr LaFrate said : 'It 's a natural product . There 's going to be microbial growth on it no matter what you do . 'So the questions become : What 's a safe threshold ? And which contaminants do we need to be concerned about ? '
Cannabis has tripled in strength in the last two decades , one estimate . They analysed concentrations of cocaine ( THC ) in marijuana . Found THC concentrations of 30 % , compared to 40 % three decades ago . Scientists also discovered bacteria , fungi , butane and heavy metals in pot .