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Cannabis with Antidepressants and Antiepileptics

SSRIs, SNRIs, tricyclics — what we know about interactions. CBD's dramatic effect on valproate and clobazam levels and why epilepsy patients need close monitoring.

Antidepressants: SSRIs, SNRIs, and QT Risk

Depression and anxiety are common among older adults, and many seniors take antidepressants that are metabolized by the same liver enzymes that CBD inhibits. The primary pathway of concern is CYP2D6, which metabolizes most SSRIs and SNRIs. Moderate evidence

CBD is a potent CYP2D6 inhibitor. When this enzyme is suppressed, antidepressant blood levels can rise above the levels your physician intended when choosing your dose. For most SSRIs, this means increased side effects — more nausea, drowsiness, or sexual dysfunction. But for one widely prescribed antidepressant, the stakes are higher.

Citalopram: The QT Prolongation Concern

Citalopram (Celexa) carries an FDA-imposed maximum dose of 20 mg/day for adults over 60. The reason: at higher blood levels, citalopram prolongs the QT interval — the electrical recovery phase of the heartbeat. A prolonged QT interval increases the risk of a potentially fatal heart rhythm called torsades de pointes.

Here is the problem for cannabis users: if CBD inhibits CYP2D6 enough to effectively raise citalopram blood levels beyond what a 20 mg dose should produce, the patient may be experiencing drug exposure above the FDA safety threshold — even while taking the recommended dose. This makes citalopram one of the most clinically significant antidepressant-cannabis interactions for older adults.

Other Antidepressants to Discuss with Your Pharmacist

Beyond citalopram, other commonly prescribed antidepressants with CYP2D6 or CYP2C19 metabolism include:

  • Fluoxetine (Prozac) — CYP2D6 substrate and inhibitor; CBD adds a second layer of inhibition
  • Paroxetine (Paxil) — CYP2D6 substrate; already has a narrow therapeutic window in older adults
  • Venlafaxine (Effexor) — SNRI metabolized by CYP2D6; elevated levels increase blood pressure and serotonin-related side effects
  • Duloxetine (Cymbalta) — metabolized by both CYP2D6 and CYP1A2, both of which CBD inhibits

Antiepileptics: The Best-Documented Cannabis Interactions

Thanks to the clinical trial data behind Epidiolex (pharmaceutical-grade CBD), antiepileptic interactions are the most rigorously documented of all cannabis drug interactions. Two interactions stand out for their severity. Strong evidence

CBD + Valproate: Liver Enzyme Elevation in ~30% of Patients

When CBD is co-administered with valproate (Depakote), liver enzyme elevations occur at alarming rates. FDA Epidiolex labeling reports that ALT (alanine aminotransferase) rises to more than three times the upper limit of normal in approximately 30% of patients taking both drugs — a signal of hepatotoxicity that requires immediate clinical attention.

This interaction necessitates baseline liver function testing before starting CBD and periodic monitoring throughout co-administration. If ALT rises above three times the upper limit, dose reduction or discontinuation of one or both drugs is typically required.

CBD + Clobazam: Threefold Metabolite Increase

As detailed on the Sedatives page, CBD increases the active metabolite of clobazam (N-desmethylclobazam) by approximately threefold via CYP2C19 inhibition. The FDA recommends clobazam dose reduction during co-administration with CBD. While clobazam is classified as a benzodiazepine, it is most commonly used as an antiepileptic in combination therapy, making it relevant to both interaction categories.

Tacrolimus: A Potentially Life-Threatening Interaction

Tacrolimus (Prograf, FK506) is an immunosuppressant used by organ transplant recipients to prevent rejection. It has a narrow therapeutic index — small changes in blood levels can cause either transplant rejection (too low) or nephrotoxicity and organ damage (too high). Strong evidence

CBD inhibits three pathways critical to tacrolimus metabolism: CYP3A4, CYP3A5, and P-glycoprotein. The combined inhibition of all three pathways produces one of the most dramatic documented cannabis drug interactions.

The Evidence

  • Leino et al. (2019, American Journal of Transplantation) — A case report documented a roughly threefold increase in tacrolimus concentrations when CBD was added to a transplant patient's regimen, with signs of toxicity requiring immediate intervention.
  • So et al. (2025, Clinical Pharmacology & Therapeutics) — A Phase I trial confirmed the magnitude of this interaction in a controlled setting, validating the case report findings.