THC Interacts With Common Medications in Ways Most Users and Doctors Don't Know About

This review mapped out the pharmacological interactions between THC and other medications, revealing a more complex picture than most cannabis users — or their doctors — appreciate. THC is primarily broken down by two liver enzymes: CYP3A4 and CYP2C9. Dozens of common medications either inhibit or induce these same enzymes, meaning they can increase or decrease THC's effects in unpredictable ways.

The genetics add another layer. CYP2C9 polymorphisms are highly prevalent — up to 35% of Caucasians carry variants that significantly increase THC bioavailability. These individuals effectively get more THC from the same dose, which matters both for effects and side effects.

Beyond metabolism, THC has pharmacodynamic interactions: it can enhance sedation from CNS depressants, affect heart rate and blood pressure when combined with cardiovascular drugs, and potentially impact immune function. The review also noted that THC itself can inhibit or induce various drug-metabolizing enzymes, meaning cannabis use could alter the effectiveness of other medications a patient is taking.

Medical cannabis is increasingly prescribed to people who are already taking multiple medications — exactly the population where drug interactions matter most. The safety profile of cannabis has historically been extrapolated from recreational use by young, healthy people. This review showed that assumption breaks down for medically complex patients.

The CYP2C9 genetic finding is particularly significant. Over a third of Caucasian patients may metabolize THC differently than expected, leading to stronger or longer effects from the same dose. Pharmacogenomic testing before prescribing cannabis is not standard practice, but this paper suggests it probably should be.