Side effects and red flags

Side effects and red flags: a decision guide

Medicinal cannabis in New Zealand, like any prescribed medicine, can cause side effects. This page gives you a method to separate common, manageable effects from symptoms that need urgent medical help, based on guidance from Medsafe (2023), the NZ Formulary (2024), and the Ministry of Health (2023).

The 3‑question decision rule

When you or someone you care for experiences a new symptom after taking medicinal cannabis, ask these three questions. Use the answers to decide what to do.

1. Is it sudden or severe? If the symptom came on within minutes or feels extreme (e.g., collapse, seizure, severe vomiting), treat it as a red flag, seek urgent medical help.
2. Does it involve a listed red‑flag category? Check the list below. If yes, act accordingly.
3. Is it a common, mild effect that resolves within a day? If yes, it is likely a manageable side effect. If it persists beyond 24 hours or worsens, escalate.

Common, manageable side effects

These are effects that occur in a significant number of patients, are usually mild, and resolve on their own or with simple adjustments. They are not emergencies.

• Drowsiness, reported in about 30% of patients in clinical trials (systematic review, 2022). It often peaks 1-2 hours after taking an oil or 15-30 minutes after inhaling flower. If you feel drowsy, avoid driving or operating machinery. Plan to take the medicine when you can rest.
• Dizziness, common when starting or increasing a dose. It usually passes within a few days as your body adjusts. Sit or lie down if you feel lightheaded.
• Dry mouth, reported in about 20% of patients (systematic review, 2022). Drink water, chew sugar‑free gum, or use a saliva substitute.
• Increased appetite, often called “the munchies.” It is harmless and can be managed by planning healthy snacks.
• Altered perception, feeling slightly “different” or slowed. This is normal, especially when starting or with higher‑THC products. If it makes you unsafe (e.g., you feel confused and cannot follow instructions), treat it as a red flag.

These effects are dose‑dependent. Lower doses or products with a higher CBD:THC ratio (e.g., Avidekel, Eve) often cause fewer of these symptoms (NZ Formulary, 2024).

Red flags: symptoms needing urgent medical help

Any of the following requires immediate action, call 111 or go to a hospital emergency department. Do not wait to see if it passes.

1. Severe psychiatric symptoms

• Hallucinations (seeing or hearing things that are not there)
• Paranoia (extreme, irrational fear)
• Panic attacks or psychosis
• Urgency: These can escalate rapidly. Medsafe (2023) lists these as serious adverse events. Adolescents (under 25 years) are at higher risk (Ministry of Health, 2023).

2. Cardiovascular events

• Tachycardia (racing heart, palpitations)
• Hypotension (sudden drop in blood pressure, causing fainting or collapse)
• Syncope (fainting)
• Urgency: These can indicate a serious reaction. Medsafe (2023) lists them as red flags. If you faint, seek help immediately.

3. Severe nausea or vomiting

• Cyclic vomiting (vomiting, then feeling better, then vomiting again)
• Abdominal pain with vomiting
• Signs of cannabinoid hyperemesis syndrome (severe, cyclic vomiting with abdominal pain), Healthify (2024)
• Urgency: Severe vomiting can lead to dehydration and electrolyte imbalance. If vomiting is persistent and you cannot keep fluids down, get help.

4. New or worsening seizures

• Any seizure after taking medicinal cannabis, especially if you have epilepsy and the product was meant to help.
• Urgency: This is a known risk with some cannabinoid‑based medicines. The NZ Formulary (2024) lists it as a red flag. Stop the medicine and seek immediate medical assessment.

5. Signs of an allergic or severe reaction

• Difficulty breathing, swelling of the lips or throat, hives, or severe skin rash.
• Urgency: This is a medical emergency. Call 111 immediately.

Drug interaction red flags

These are not emergencies by themselves, but they require you to monitor closely and inform your prescriber.

• CNS depressants (opioids, benzodiazepines, alcohol): Combined with THC, the sedation risk multiplies. Avoid concurrent use or reduce doses under medical supervision (NZ Formulary, 2024).
• Warfarin: THC may increase your INR (a measure of blood clotting). Monitor more frequently and inform your doctor (NZ Formulary, 2024).
• CYP3A4 inhibitors (e.g., ketoconazole, clarithromycin): These can raise THC levels in your blood, increasing side effects. Tell your prescriber about all medicines you take (Medsafe, 2023).
• CYP2C9 substrates (e.g., phenytoin): CBD can raise levels of these drugs. If you take phenytoin for epilepsy, discuss with your neurologist before starting CBD (Medsafe, 2023).

Age and life‑stage red flags

• Adolescents (<25 years): Higher risk of psychiatric adverse effects (hallucinations, paranoia, psychosis). The Ministry of Health (2023) recommends extra caution and close monitoring.
• Pregnancy: Not recommended. THC may affect fetal neurodevelopment (Ministry of Health, 2023). If you are pregnant or planning pregnancy, discuss alternatives with your prescriber.
• Breastfeeding: THC is excreted in breast milk. Avoid medicinal cannabis while breastfeeding (Ministry of Health, 2023).
• Driving: THC impairs driving. New Zealand has a zero‑tolerance limit, a prescription is not a defence. Do not drive after taking any THC‑containing product (Land Transport NZ, 2024).

What to do if you see a red flag

1. Call 111 or go to a hospital emergency department.
2. Tell the paramedic or doctor that you have taken a prescribed medicinal cannabis product. Show them the container or prescription if possible.
3. Do not take another dose until a doctor tells you it is safe.
4. Report the adverse reaction to CARM (Centre for Adverse Reactions Monitoring) via your prescriber or pharmacist. This helps improve safety data for all patients (Medsafe, 2023).

What to do if you have a common side effect

1. Note it down, what product, dose, time, and symptom.
2. Wait 24 hours, most mild effects resolve within a day.
3. If it persists or bothers you, contact your prescriber or pharmacist. They may suggest a lower dose, a different product (e.g., higher CBD ratio), or a different form (e.g., oil instead of flower for slower onset).
4. Do not stop abruptly without medical advice, but do not continue if the symptom is affecting your safety.

Summary checklist

If you experience…	Action
Drowsiness, dry mouth, mild dizziness, increased appetite, altered perception (mild)	Manage at home; check again in 24 hours
Hallucinations, paranoia, panic, psychosis	Call 111, red flag
Racing heart, fainting, severe vomiting, new seizure, difficulty breathing	Call 111, red flag
Symptoms that are sudden, severe, or persist >24 hours	Seek medical help (Healthify, 2024)
Interaction with opioids, benzodiazepines, warfarin, or CYP inhibitors	Inform prescriber; do not adjust doses alone