Cannabis and medicine interactions

The question is not whether cannabis interacts with medicine. It does. The real question is whether you can predict the interaction yourself. For most people, the answer is no: the relevant metabolic pathways are shared by dozens of common drugs, and the consequences range from a mildly increased sedative effect to a serious rise in blood levels of a medicine you depend on.

Cannabinoids are not inert plant oils; they are active pharmaceutical ingredients that can change how other drugs work. The interaction profile of CBD and THC is well enough documented that the NZ Formulary (2024) and the New Zealand Ministry of Health’s Medicinal Cannabis Scheme guidance both direct prescribers to screen for potential interactions before writing a prescription. The article that follows explains why screening belongs with a clinician or pharmacist, not a web search, and gives you a usable checklist to bring to that conversation.

Cannabis and medicine interactions: Why interaction screening is a clinical task

Cannabis is not one chemical. A prescribed product contains THC, CBD, and dozens of minor cannabinoids and terpenes, each with its own metabolic pathway. THC and CBD are both metabolised by the cytochrome P450 enzyme family in the liver, particularly CYP3A4 and CYP2C19. Many common medicines use the same enzymes. When cannabis is added, it can raise or lower the blood levels of those medicines.

The practical result: adding medicinal cannabis to an existing prescription can push blood levels of other drugs into a toxic or ineffective range. The effect is not theoretical. A 2021 systematic review in Clinical Pharmacology & Therapeutics found clinically relevant interactions between cannabis and a range of drugs, including warfarin, clobazam, and some antidepressants. Because cannabis products are not a single compound, the interaction profile depends on the ratio of THC to CBD, the dose, and the route.

Who can screen for interactions

The New Zealand Formulary (NZF) and Healthify are the standard references that New Zealand clinicians use to check interactions. Medsafe, through the Medicinal Cannabis Agency, sets product quality standards, but it does not publish an interaction checker. The decision belongs to the prescriber and pharmacist, who can see the full medicine list and weigh risk against potential benefit.

The Ministry of Health’s Medicinal Cannabis Scheme was established under the Misuse of Drugs (Medicinal Cannabis) Amendment Act 2018 and came into force on 1 April 2020. The scheme sets product standards for GMP and quality, and the Medicinal Cannabis Agency within Medsafe issues licences for cultivation, manufacture, supply and possession. Products must meet the requirements set out in the Misuse of Drugs (Medicinal Cannabis) Regulations 2019. The Ministry of Health has published a list of “products that meet the minimum quality standard” on its website.

That legal boundary is why interaction screening is not optional: a prescription is the only route, and a clinician or pharmacist is the only person who can see the full medicine list and the relevant interactions. A patient cannot self-assess CYP450 enzyme effects, transporter effects, or additive sedation without a full medication list and clinical history. The clinician or pharmacist checks the specific product’s monograph against the patient’s complete medicine list, including over-the-counter and herbal products. The interaction risk is not a property of “cannabis” as a plant; it is a property of the specific cannabinoid molecules, their doses, and the other medicines the patient takes.

What to bring to your appointment

A complete list of every medicine and supplement (name, dose, frequency), including over-the-counter and herbal products.
The time of day you take each medicine and whether it is with food.
Any past reactions to cannabis, including drowsiness, anxiety, or palpitations.
Current medical conditions, especially liver or kidney disease, because these affect how the body clears cannabinoids and other drugs.
Whether you are pregnant, planning pregnancy, or breastfeeding.
A list of questions about how cannabis might change the effect of your existing medicines.

Interaction risk map: what actually changes

Cannabis contains dozens of cannabinoids, but the two that matter for interactions are CBD and THC. Their behaviour in the body differs, and the interaction risk is not the same.

THC is metabolised mainly by the cytochrome P450 enzyme CYP2C9 and, to a lesser extent, CYP3A4. CBD inhibits several cytochrome P450 enzymes, particularly CYP3A4 and CYP2C19, and can also affect CYP2D6 and CYP1A2. Because many common medicines are broken down by these same enzymes, CBD can raise the blood level of those medicines when taken together. THC is less potent as an enzyme inhibitor than CBD, but it is sedating and can add to the effects of other central nervous system depressants.

CBD and clobazam

The interaction that is most relevant in New Zealand prescribing is between CBD and clobazam, an anti-seizure medicine used in refractory epilepsy. The NZ Formulary and peer-reviewed clinical guidance note that CBD can increase clobazam’s active metabolite, which may increase sedation but also contributes to seizure control. This is not a reason to avoid the combination; it is a reason to have the dose adjusted by the prescriber and monitored.

CBD and warfarin

CBD can inhibit the CYP2C9 enzyme that metabolises warfarin, raising the INR and increasing bleeding risk. The New Zealand Formulary and Healthify advise that INR monitoring is required if a patient taking warfarin starts or stops cannabis-based medicines.

Enzyme inhibition

Cannabinoids, especially CBD, inhibit cytochrome P450 enzymes (CYP3A4 and CYP2C19), which metabolise many common medicines. This can raise blood levels of certain drugs, including some antidepressants, antipsychotics, benzodiazepines, and anti-epileptics. The interaction is not always harmful, but it is predictable and should be screened before prescribing.

Additive sedation

THC and CBD can increase the sedative effect of alcohol, benzodiazepines, opioids, antihistamines, and some sleep aids. This is a safety issue for driving and for older adults at risk of falls. Do not drive or operate machinery until you know how the combination affects you.

Additive blood pressure effects

Cannabinoids can lower blood pressure, so combining with antihypertensives may cause dizziness or fainting.

Enzyme induction

THC is metabolised by CYP3A4, and it can inhibit CYP2C9 and CYP2C19. This can raise or lower levels of many common medicines, including warfarin, clobazam, and some SSRIs.

Why this matters for people on multiple medicines

Many patients taking medicinal cannabis are already on several drugs: opioids, benzodiazepines, antidepressants, anticoagulants, antiepileptics. Cannabinoids can alter how the liver metabolises other drugs, and the clinical significance depends on dose, route, and the individual. The Ministry of Health guidance and the NZ Formulary advise that interaction screening be done by the prescriber or pharmacist, not by the patient.

Most interactions involving medicinal cannabis are not dramatic “do not combine” warnings. They are changes in drug metabolism, additive sedation, and unpredictable absorption. The evidence base is still young, and the Ministry of Health and the NZ Formulary direct prescribers to assess each patient individually. A clinician or pharmacist can check the specific medicines a person takes against the current evidence. A general article cannot do that safely.