Background Marijuana is one of most widely used illicit substances globally. The risks of injury experienced by marijuana users relative to non-users have been poorly quantified in prospective studies. We investigated the associations between marijuana use and fatal or hospitalised injury in the New Zealand Blood Donors’ Health Study.

Methods At recruitment to this prospective cohort study in 1998–1999, the 22,389 participants completed a self-report survey on demographic, personal and lifestyle questions including the frequency of marijuana use. Using unique identifiers, these data were record-linked to national mortality and hospital discharge databases to ascertain participants’ injury-related admissions or deaths from recruitment to 31 December 2014. The associations between marijuana use and injury were investigated using Cox proportional hazards analyses, adjusting for socio-demographic and confounding factors.

Results At baseline, 15% of participants of the cohort reported marijuana use in the preceding 12 months. During a median follow-up period of 16.8 years, 3,693 incident injury cases (3,651 non-fatal and 63 fatal) occurred, the majority of which were falls (1204 incident cases) and motor vehicle crashes (779 incident cases). Compared to non-users, marijuana users were more likely to experience injuries related to self-harm (adjusted HR 2.00, 95% CI: 1.39–2.89), assault (adjusted HR 1.83, 95% CI: 1.27–2.66), motor vehicle crashes (adjusted HR 1.35, 95% CI: 1.09–1.69), or unintended cutting or piercing trauma (adjusted HR 1.76, 95% CI: 1.31–2.35). Risk estimates were greater with more frequent use of marijuana.

Conclusions Relative to non-users, participants who used marijuana were at increased risk of most major types of injuries, with an apparent dose-response effect. The pathways and correlates of harm underlying the increased risks of injury (particularly self-harm and assault) associated with marijuana use, require public health attention.