Medicinal cannabis and your other medicines: interactions to know

One of the most important — and most overlooked — aspects of medicinal cannabis is how it interacts with other medicines. Both CBD and THC can change how your body handles other drugs, or add to their effects. This is general education, not personalised advice: your prescriber and pharmacist must review your full medication list, because interactions depend on the specific drugs and doses involved.

How cannabinoids cause interactions

There are two broad mechanisms:

• Metabolic (how drugs are broken down). Many medicines are processed by a family of liver enzymes called CYP450. CBD inhibits several of them — notably CYP3A4, CYP2C19 and CYP2C9 — which can slow the breakdown of other drugs and raise their levels. THC is itself processed mainly by CYP2C9 and CYP3A4.
• Additive (effects stacking up). Independent of metabolism, cannabis can simply add to the effects of other drugs — most importantly, sedation.

The interactions that matter most

Sedatives and CNS depressants

Cannabis adds to the sedating effects of benzodiazepines, opioids and alcohol. Combined, they can cause excessive drowsiness, impaired coordination and slowed thinking. This is one of the most common and clinically relevant interactions, and it's a key reason not to mix cannabis with alcohol or to start it casually alongside sleeping tablets or strong painkillers.

Clobazam (the classic CBD interaction)

In patients taking the anti-seizure drug clobazam, CBD raises the level of its active metabolite roughly threefold by inhibiting CYP2C19. In trials, drowsiness was far more common when CBD and clobazam were combined (around 46%) than CBD alone (around 16%). Clobazam doses are often reduced to manage this — under specialist supervision.

Warfarin and other anticoagulants

Both THC and CBD can inhibit CYP2C9, potentially raising the effect of warfarin and increasing bleeding risk. Case reports have shown significant swings in INR (the blood-clotting measure). Importantly, this is dose- and route-dependent — a microdose case showed no meaningful change — but the safe approach is to monitor INR closely when starting or changing a cannabis product.

Valproate

Combining CBD with the anti-seizure medicine valproate increases the risk of liver-enzyme elevations. Liver-function monitoring is part of careful use here.

Hormonal contraceptives

NZ guidance notes that cannabis may possibly reduce the effectiveness of hormonal contraceptives. This is worth raising if contraception matters to you.

Why this needs professional oversight

Interactions can't be safely worked out from a website. The clinical picture depends on which exact drugs you take, their doses, the route and dose of your cannabis product, and your own liver and kidney function. NZ prescribers and pharmacists have access to the NZ Formulary's interaction checker and can assess your specific combination. The more medicines you take — and older patients often take many — the more important this review becomes.

What you can do

• Give a complete list of every prescription medicine, over-the-counter product and supplement to both your prescriber and your pharmacist.
• Flag the high-risk ones specifically: blood thinners, anti-seizure medicines, sedatives, opioids and any narrow-margin medicines.
• Don't combine cannabis with alcohol, and be cautious about combining it with other sedatives.
• Report new symptoms — unusual drowsiness, bruising or bleeding, confusion — promptly.
• Keep your GP informed if your cannabis comes from a separate clinic, so your whole medication picture stays coordinated.
• Expect monitoring for some combinations — for example INR checks with warfarin, or liver-function tests with valproate.

CBD versus THC: different interaction profiles

It's worth separating the two main cannabinoids, because their interaction risks differ. CBD is the bigger metabolic player: by inhibiting several CYP450 enzymes it can raise the blood levels of a range of other medicines, and it's the cannabinoid behind the clobazam and valproate interactions. The catch is that CBD is often dosed quite high (especially in pharmaceutical use), and the higher the dose, the more pronounced the enzyme effect. THC, by contrast, contributes more to additive effects — stacking sedation on top of opioids, benzodiazepines and alcohol — though it too is processed by, and can modestly affect, the same enzyme system. Knowing which cannabinoid dominates your product helps you and your prescriber anticipate where the risk lies.

A practical example: timing and monitoring

Consider a patient on warfarin who starts a cannabis product. The sensible approach isn't necessarily to avoid cannabis altogether, but to introduce it slowly, keep the dose modest, and check INR more frequently around the start and any dose change, watching for signs of over-anticoagulation like unusual bruising or bleeding. This illustrates the general principle: most interactions are managed, not absolute barriers — through careful product and dose choice, slow titration, and appropriate monitoring agreed with your prescriber and pharmacist.

The bottom line

Medicinal cannabis is a real medicine with real interactions. CBD's effect on liver enzymes and the additive sedation from THC are the headline concerns, with anticoagulants, clobazam, valproate and other sedatives needing particular attention. None of this means cannabis can't be used alongside other medicines — it means it should be done with a full medication review and appropriate monitoring. Be thorough about disclosing what you take, and let the professionals do the cross-checking.

Last reviewed 5 October 2026 — education, not medical advice. Always have your full medication list reviewed by your prescriber and pharmacist. Sources: CMAJ — cannabis drug interactions; LiverTox — cannabidiol; bpacnz.