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The main psychoactive substance, Δ9-tetrahydrocannabinol (THC) can be present in highly variable amounts in different cannabis preparations. An increase in THC content in cannabis products has been suggested, and reported from several countries. However, it has not yet been investigated if products with high potency lead to increased human exposure, and thus to higher risk of adverse effects. In this study, we examined the mean concentrations of THC in whole blood samples from drivers apprehended in Norway in the period between 2000 and 2010 suspected of driving under the influence of drugs. Cases with only THC (n=1747) have been compared to cases with only ethanol (n=38796) or amphetamines (n=2493). The increase in mean THC concentration measured from 2000 to 2010 was from 4.0 ± 0.3 to 6.6 ± 0.4 ng/ml (58%), compared to 3% for ethanol and 16% for the amphetamines. This increase in THC concentrations was to some extent paralleled by an increase in the percentage of drivers which were judged as lightly impaired by a physician. Monitoring concentrations of drugs of abuse in blood from apprehended drivers indicated an increasing exposure to THC in Norway. If similar trends are observed globally, it should be further explored if this type of information could be used to elucidate the drug consumption patterns in a population and accordingly the consequences with regard to adverse effects of cannabis from a public health perspective., (Copyright © 2013 Elsevier Ireland Ltd. All rights reserved.)