In observational and retrospective studies, people who use cannabis are more likely than people who do not use cannabis to also use other drugs. People who take medical cannabis are also more likely to report medical and non-medical use of opioid analgesics, stimulants, and tranquilizers. Given that people who take medical cannabis and those who do not are likely to have different underlying morbidity, it is possible that medical cannabis use reduces prescription drug use yet prescription drug use remains relatively high. Studies comparing people who take medical cannabis with people who do not take it cannot draw conclusions about the effect of medical cannabis on drug use. To fully understand the effect of medical cannabis on the use of other drugs, prospective longitudinal studies randomizing individuals to cannabis versus other treatments are urgently needed.