How CBD and THC Interfere with Common Medications: A Controlled Human Study

Randomized, three-way crossover study in 18 healthy adults. Participants received, in random order with ≥1 week washout: (1) placebo brownie, (2) CBD-dominant brownie (640 mg CBD + 20 mg THC), or (3) THC-dominant brownie (20 mg THC). Thirty minutes post-brownie, participants consumed a validated CYP probe drug cocktail (caffeine, losartan, omeprazole, dextromethorphan, midazolam). Plasma and urine were collected over 24 hours to measure enzyme activity through probe drug metabolism.

Millions of people use CBD products — often alongside prescription medications — with little awareness of potential drug interactions. This is the first well-controlled human study using actual cannabis extracts (not isolated pharmaceutical CBD) to quantify these interactions. The finding that CBD inhibits four major metabolic pathways has immediate clinical relevance for anyone combining CBD with prescription drugs, particularly blood thinners, antidepressants, anti-seizure medications, and immunosuppressants.

This study provides the controlled human data that explains mechanisms observed in RTHC-00078 (cannabis users on buprenorphine had 2.7x higher drug levels). It also connects to the drug interaction monitoring in RTHC-00085 (THC/CBD for dementia patients on multiple medications). For the growing population using CBD alongside prescriptions — especially elderly patients on polypharmacy — these enzyme inhibition findings have immediate practical implications.

Single-dose study in healthy young adults — chronic CBD users or elderly/sick patients may show different interaction profiles. The CBD dose (640 mg) is high compared to typical consumer CBD products (25–100 mg), though some medical uses approach this level. The probe drug cocktail measures enzyme activity, not clinical outcomes — inhibiting an enzyme doesn't automatically mean a dangerous interaction with every drug that enzyme handles. Real-world cannabis products have variable CBD content and additional compounds that could modify these effects.