Cannabinoids have a relatively unique safety record, particularly when compared to other therapeutically active substances. Most significantly, the consumption of cannabinoids -- regardless of quantity or potency -- cannot induce a fatal overdose because, unlike alcohol or opiates, they do not act as central nervous system depressants. According to a 1995 review prepared for the World Health Organization, "There are no recorded cases of overdose fatalities attributed to cannabis, and the estimated lethal dose for humans extrapolated from animal studies is so high that it cannot be achieved by users."
Nonetheless, cannabis should not necessarily be viewed as a 'harmless' substance. Consuming cannabis will alter mood, influence emotions, and temporarily alter perception, so consumers are best advised to pay particular attention to their set (emotional state) and setting (environment) prior to using it. It should not be consumed immediately prior to driving. Further, there may be some populations that are susceptible to increased risks from the use of cannabis, such as adolescents, pregnant or nursing mothers, and patients with or who have a family history of mental illness. Patients with hepatitis C, decreased lung function (such as chronic obstructive pulmonary disease), or who have a history of heart disease or stroke may also be at a greater risk of experiencing certain adverse side effects from cannabis. As with any therapy, patients concerned about such risks should consult thoroughly with their physician before deciding whether the medical use of cannabis is safe and appropriate for them.