Concomitant use of medical cannabis and drugs associated with risks of interaction in older patients: a longitudinal cohort study.

BACKGROUND: Cannabinoids interact with multiple drugs, with some interactions having significant clinical effects. OBJECTIVES: This study aimed to evaluate amongst older people (i) the trends in the concomitant medical use of cannabis and drugs associated with a risk of significant clinical interaction with cannabinoids (DARSCIC), including those with a narrow therapeutic index (drugs with a narrow therapeutic index; DNTI) such as warfarin, and (ii) the risk of bleeding, drug-related intoxication, thyrotoxicosis and major cardiovascular events. METHODS: For objective 1, we conducted an interrupted time series study amongst 12 599 seniors who received an authorised cannabis prescription in Ontario from 2014-2019. Using clinical and medico-administrative data, dispensations of DARSCIC were assessed pre-post cannabis prescription. For objective 2, longitudinal cohort studies were conducted in patients concomitantly exposed to cannabis and warfarin (bleeding) or to DNTI (intoxication) versus controls. RESULTS: For objective 1, the trends of DARSCIC/DNTI dispensations were similar in the year before and after cannabis prescription. For objective 2, amongst 378 patients exposed to cannabis and warfarin, the risk of bleeding was 1.19, 95%CI (0.71-1.98), compared to 1646 controls. The risk of drug-related intoxication was 2.61, 95%CI (1.42-4.79) amongst 3926 patients exposed to cannabis and DNTI compared to 12 223 controls. Patients exposed to cannabis and levothyroxine (n = 2499) had a significantly higher risk of heart failure but not thyrotoxicosis, acute coronary syndrome or stroke. CONCLUSION: These findings indicate that prescribing practices for cannabis may not adequately consider the implications of DARSCIC/DNTI. Enhancing prescriber awareness of these interactions could mitigate the risk of adverse effects such as drug-related intoxication.