Cannabis and medicine interactions

If you are taking prescribed medicines and considering medicinal cannabis in New Zealand, the first conversation you need is not about products, it is about interactions. 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. This page gives you a usable method for that screening, and explains why it belongs with a clinician or pharmacist, not a web search.

Why interaction screening belongs with a clinician or pharmacist

Cannabis contains more than 100 cannabinoids, but the two that matter for interactions are THC (delta-9-tetrahydrocannabinol) and CBD (cannabidiol). Both are metabolised in the liver by the cytochrome P450 enzyme system, mainly by the enzymes CYP3A4 and CYP2C19. THC and CBD can inhibit or induce these enzymes, which changes how other medicines are broken down. The practical result: adding medicinal cannabis to an existing prescription can raise or lower blood levels of other drugs, sometimes 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. For example, CBD is a known inhibitor of CYP3A4 and CYP2C19, enzymes that metabolise many common medicines. THC is broken down by CYP2C9 and CYP3A4, so it can raise or lower levels of drugs sharing those pathways.

This is why interaction screening belongs with a clinician or pharmacist. The NZ Formulary and Medsafe product monographs contain the official interaction data for approved medicinal cannabis products in New Zealand, but a patient cannot self-assess CYP450 enzyme interactions, 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 full 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.

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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.

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.

A second important interaction is with 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.

A third interaction class involves 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 (clobazam, for example). The interaction is not always harmful, but it is predictable and should be screened before prescribing.

A fourth interaction class is additive sedation. THC and CBD can increase the sedative effect of alcohol, benzodiazepines, opioids, and some sleep aids. This is a safety issue for driving and for older adults at risk of falls.

A fifth interaction class is enzyme induction or inhibition: CBD inhibits CYP2C19 and CYP3A4; 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.

A sixth class is additive blood pressure effects. Cannabinoids can lower blood pressure, so combining with antihypertensives may cause dizziness or fainting.

A seventh class is additive sedation with alcohol, benzodiazepines, opioids, or antihistamines.

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 under the scheme. 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.

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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, because 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.

This page explains why interaction screening belongs with a clinician or pharmacist, and gives you a usable checklist to bring to that conversation. It does not replace a professional review.

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Why interaction screening is a clinical task, not a research 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 New Zealand Formulary and the Ministry of Health advise that cannabis interactions are managed by a prescriber or pharmacist because the outcome depends on the person’s full medicine list, their liver enzyme activity, and the specific product. The interaction is not a simple “avoid this” list; it is a clinical judgement. The Ministry of Health’s Medicinal Cannabis Scheme (established under the Misuse of Drugs Act) requires a prescription from a medical practitioner, and the clinician is the person who can weigh the risk against the potential benefit. A pharmacist then checks the prescription against your other medicines, which is why the legal pathway always ends at a pharmacy.

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. The Ministry of Health (2020) guidance on the Medicinal Cannabis Scheme requires that any product supplied under the scheme be a “controlled drug” within the Misuse of Drugs Act and be manufactured to GMP standards. 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.

Why this matters for interactions: many patients taking medicinal cannabis are already on multiple medicines (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 (2020) guidance and the NZ Formulary advise that interaction screening be done by the prescriber or pharmacist, not by the patient. This page gives you a framework to use with your clinician, not a substitute for one.

Checklist before your appointment: - Bring a complete list of every medicine and supplement (name, dose, frequency), including over-the-counter and herbal products. - Note the time of day you take each medicine and whether it is with food. - Write down any past reactions to cannabis, including drowsiness, anxiety, or palpitations. - List current medical conditions, especially liver or kidney disease, because these affect how the body clears cannabinoids and other drugs. - Tell the clinician if you are pregnant, planning pregnancy, or breastfeeding. - Bring a list of questions about how cannabis might change the effect of your existing medicines.

Why interaction screening belongs with a clinician or pharmacist 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 New Zealand 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.