The risks, set out honestly
Around 80% of people taking a medicinal-cannabis product experience some adverse effect. Knowing the side effects, interactions and cautions helps you have a safer, more informed conversation with your prescriber and pharmacist.
Last reviewed 15 June 2026 · Education, not medical advice
THC side effects
Dizziness, dry mouth, sedation, faster heart rate and orthostatic hypotension, impaired concentration and short-term memory, confusion, anxiety or paranoia, red eyes and increased appetite. Per NZ guidance, around 80% of people taking a medicinal-cannabis product experience some adverse effect.
CBD side effects
At high pharmaceutical doses: somnolence, diarrhoea, fatigue, decreased appetite, and liver-enzyme elevations (ALT >3× upper limit in ~13% vs 1% on placebo, dose-dependent, worse with valproate or clobazam). Generally non-intoxicating.
Generally inappropriate
- Pregnancy, planning pregnancy, or breastfeeding
- Significant psychiatric history — especially schizophrenia or psychosis
- Unstable cardiovascular or cardiopulmonary disease
- Known hypersensitivity to a product
Use with caution
- Hepatic or renal impairment
- Children and adolescents (developing brain)
- The elderly or frail (falls risk)
What cannabis can clash with
Always give your prescriber and pharmacist a full list of your medicines. NZ clinicians can use the NZ Formulary interaction checker.
CYP450 enzymes
CBD inhibits CYP3A4, CYP2C19 and CYP2C9; narrow-therapeutic-index medicines are the main concern.
Warfarin & anticoagulants
THC and CBD can inhibit CYP2C9 and raise INR / bleeding risk (dose- and route-dependent) — monitor INR when starting or changing.
Clobazam
CBD raises the active metabolite ~3-fold; somnolence 46% with vs 16% without. Consider a clobazam dose reduction.
CNS depressants
Additive sedation with benzodiazepines, opioids and alcohol.
Valproate + CBD
Increased risk of liver-enzyme elevations.
Hormonal contraceptives
Possible reduced effectiveness — bpacnz advises caution.
Mental health — THC & psychosis
A dose-response relationship is supported: weekly use carries a relative risk around 1.35, daily use around 1.76, while infrequent use showed no significant increase. High-potency THC with daily use roughly doubles psychosis odds and is linked to earlier onset. Adolescent use and a family history markedly elevate risk.
Pregnancy & breastfeeding
THC crosses the placenta and enters breast milk. Prenatal exposure is associated with low birth weight, being small for gestational age, NICU admission, and later attention, behaviour and visual-motor deficits. ACOG and the AAP recommend abstaining; NZ guidance lists pregnancy and breastfeeding as generally inappropriate.
This is general information, not medical advice. Only a registered New Zealand doctor can decide whether medicinal cannabis is right for you.