Genetic variation has been though to be responsible for the differences between people to metabolize certain drugs. The results of a recent study from the Karolinska Institutet in Sweden suggest that it may be even more complicated. Swedish reserchers have found that the body’s ability to break down medicines may be closely related to sunlight exposure and vitamin D, and thus may vary with the seasons. The study, published in the journal Drug Metabolism & Disposition, offers a completely new model to explain individual differences in the effects of drugs [1].