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

The takeaway

THC is metabolized by CYP3A4 and CYP2C9 enzymes that many common medications also use, creating potential for drug interactions — especially in medical cannabis patients with multiple prescriptions.

Medical cannabis patients taking other medications, or anyone whose doctor prescribes cannabis without discussing drug interactions.

35%
Of Caucasians carry CYP2C9 variants that increase THC bioavailability

What the researchers found

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.

Why it matters

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.

The numbers in context

• Primary THC metabolism: CYP3A4 and CYP2C9 enzymes

• CYP2C9 polymorphism prevalence: up to 35% in Caucasians

• Carriers get increased THC bioavailability from the same dose

• Interaction categories: metabolic, CNS depression, cardiovascular, immunological

How the study worked

Narrative review of published literature on THC pharmacokinetics, drug-metabolizing enzyme interactions, genetic polymorphisms affecting THC metabolism, and pharmacodynamic interactions with other medication classes.

Who was studied

Review of THC interactions with other medications, no specific population size reported.

What this study cannot tell us

Many potential interactions are theoretical, based on known enzyme pathways rather than documented clinical cases. The clinical significance of most interactions hasn't been studied in controlled settings. THC metabolism research is largely based on oral or injected THC, which may not reflect smoked or vaporized cannabis pharmacokinetics. CBD was not the focus but also has significant enzyme interactions.

How to read the evidence

Narrative review identifying theoretical and documented drug interactions. Pharmacologically sound but many specific interactions lack clinical confirmation.

When this study was published

Published in 2020. Drug interaction awareness in medical cannabis remains low despite growing patient complexity.

The bigger picture

As medical cannabis programs expand to include sicker, older patients on multiple medications, the drug interaction problem this paper describes will only grow. Cannabis has enjoyed a reputation as pharmacologically simple and safe, but this review showed it's actually a complex compound that interacts with the same enzyme systems as many prescription drugs. The medical cannabis industry has been slow to address this.

Replication

Not stated in abstract.

Funding

Not reported in abstract.

Conflicts of interest

Not reported in abstract.

Questions still open

Should pharmacogenomic testing be standard before prescribing medical cannabis?
How many adverse events from medical cannabis are actually unrecognized drug interactions?
Do frequent cannabis users who develop tolerance show different drug interaction profiles?

Common questions

Can cannabis interact with my medications?

Yes. THC is broken down by the same liver enzymes (CYP3A4, CYP2C9) that process many common medications. This can cause either drug to be stronger or weaker than expected.

Does genetics affect how cannabis works for me?

Up to 35% of Caucasians carry a gene variant that makes their body process THC differently, potentially leading to stronger effects from the same dose.

Read the original research

Potential Adverse Drug Events with Tetrahydrocannabinol (THC) Due to Drug-Drug Interactions.

Journal of clinical medicine, 9(4)

The Journal of Clinical Medicine is a peer-reviewed journal known for publishing clinical research.

View on PubMed ↗Publisher’s paper ↗

Citation

Brown, Joshua D. (2020). Potential Adverse Drug Events with Tetrahydrocannabinol (THC) Due to Drug-Drug Interactions.. Journal of clinical medicine, 9(4). https://doi.org/10.3390/jcm9040919