Managing the side effects of medicinal cannabis

It is worth being honest from the outset: according to New Zealand clinical guidance, about 80% of people taking a medicinal cannabis product experience some adverse effect. Most are mild, dose-related and manageable, but they are common enough that knowing what to expect — and what to do — is part of using the medicine well.

The common, usually mild effects

From THC, the typical complaints are dizziness, dry mouth, drowsiness, a faster heart rate (and sometimes light-headedness on standing), impaired concentration or short-term memory, red eyes, increased appetite, and — at higher doses — anxiety or paranoia, headache and unsteadiness.

From CBD (mostly seen at the high doses used in pharmaceutical products), the picture is different: drowsiness, diarrhoea, fatigue, reduced appetite, and rises in liver enzymes. CBD is generally non-intoxicating.

Practical management

Most of these respond to the same core strategy — slow down and lower the dose. Many side effects are dose-related, and the "start low, go slow" approach exists precisely to keep them tolerable.

• Anxiety or paranoia (THC): this usually signals too much THC, too fast. The fix is generally a lower dose and slower titration; CBD-dominant products are less likely to cause it. If it happens acutely, a calm, safe environment helps and the effect passes as the dose wears off.
• Drowsiness or next-day grogginess: consider timing (evening dosing for sedating products), and discuss lowering the dose with your prescriber. Don't drive or operate machinery while affected.
• Dry mouth: stay hydrated; sip water, use sugar-free lozenges.
• Fast heart rate or dizziness on standing: rise slowly; this often eases as tolerance develops, but tell your prescriber, especially if you have a heart condition.
• Dizziness and fatigue generally: these are among the most common reasons people slow their titration. Patience in the first weeks usually pays off.
• Diarrhoea or appetite changes (CBD): often dose-related; raise it with your prescriber.

A simple symptom diary — dose, time, effect, side effect — makes these patterns obvious and helps your prescriber adjust.

Tolerance, dependence and stopping

Used at therapeutic doses, the risk of dependence is described as minimal, but it rises with high-dose, prolonged use. Heavy, long-term use can lead to a withdrawal syndrome on stopping — irritability, anxiety, disturbed sleep and vivid dreams, reduced appetite. If you have been using a THC product heavily and want to stop, taper with your prescriber's guidance rather than quitting abruptly. Note too that THC can disrupt REM sleep and, with frequent use, lead to rebound insomnia or (for headache sufferers) medication-overuse headache.

Interactions are a side-effect risk too

Some "side effects" are really interactions. CBD inhibits several liver enzymes and can raise levels of drugs like clobazam; both THC and CBD can affect warfarin and push up bleeding risk, so INR monitoring matters. Cannabis also adds to the sedation of benzodiazepines, opioids and alcohol. Always give your prescriber and pharmacist your full medication list.

Route matters for side effects too

How you take a product shapes which side effects you're likely to meet. Vaporised flower hits fast and fades fast, so effects (and any side effects) come on quickly but don't linger as long. Swallowed oils and capsules are slow and long-lasting, which means a side effect like grogginess can carry into the next day. Oral products are also the easiest to accidentally overdose on by redosing too soon — the classic "I didn't feel anything so I took more, then it all hit at once" mistake. Matching route to purpose, and respecting onset times, prevents a lot of avoidable discomfort.

A note on smoking versus vaporising: combustion (smoking) produces tar and harmful byproducts. Vaporising at lower temperatures substantially reduces these, which is why prescribers who allow inhaled flower favour vaporisers over smoking.

Who is at higher risk

Some groups need extra caution because side effects can be more serious for them. The frail and elderly are more vulnerable to dizziness, sedation and falls. People with a personal or family history of psychosis or schizophrenia face a real risk that THC can trigger or worsen psychotic symptoms — high-potency THC with daily use roughly doubles psychosis odds in the research. Those with unstable heart or lung disease, and anyone on interacting medicines such as warfarin or clobazam, also warrant closer monitoring. Cannabis is generally inappropriate in pregnancy and breastfeeding. If you fall into one of these groups, the risk-benefit conversation with your prescriber is especially important.

When to seek help promptly

Most side effects are a reason to adjust, not alarm. But contact your prescriber — or seek urgent care — if you experience severe or persistent vomiting, chest pain, fainting, a severe panic reaction, signs of a serious allergic reaction, or any new or worsening mental-health symptoms such as confusion, hallucinations or paranoia that don't settle. People with a personal or family history of psychosis are at particular risk and should treat new psychiatric symptoms seriously.

The takeaway

Side effects with medicinal cannabis are common but usually manageable, and most are tied to dose and speed of titration. Go slow, keep a diary, keep your prescriber informed, and know the small list of warning signs that mean "get help now." Managed well, the medicine becomes much easier to live with.

Last reviewed 20 July 2026 — education, not medical advice. Sources: bpacnz; LiverTox — cannabidiol; CMAJ — cannabinoid drug interactions.