Translated and edited from: Martin Woodbridge, A Primer to Medicinal Cannabis (Bedrocan, 2018). Supporting sources are listed under References at the end.
Some patients need extra care with medical cannabis, and a few should not use it at all. This part covers five situations: a history of substance use disorder, children and teenagers, other medicines, driving or using machinery, and pregnancy and breastfeeding. It ends with a summary table of every group, including those covered in Part 1.
Read this first. Nothing below is medical advice, and nothing below is a self-assessment tool. In Thailand, cannabis flower is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568, in force since 26 June 2025. It can be sold only against a prescription on form PT 33 (ภ.ท.33), and one prescription covers at most 30 days. The prescription must come from an authorised practitioner. This can be a medical doctor, dentist, pharmacist, Thai traditional medicine practitioner, applied Thai traditional medicine practitioner, Chinese medicine practitioner or licensed folk healer. Everything on this page is something to raise with that practitioner, not a reason to start, stop or change a treatment yourself. Stopping a prescribed medicine suddenly can be more dangerous than the risk you are trying to avoid.
This is Part 2. Part 1: Contraindications and Cautions: The Patient Groups Where Medical Cannabis Needs Extra Care — or Should Not Be Used
Three points to keep in mind
- Contraindication or precaution. A contraindication means the medicine should not be used at all, because the expected harm is greater than the expected benefit. A precaution means it may still be suitable, with changes such as a lower starting dose, closer check-ups, a different mix of cannabinoids or a different route. Most items below are precautions.
- THC and CBD carry different risks. THC, the cannabinoid that causes the high, carries most of the risk to mental state and the heart. CBD causes most interactions with other medicines. It does this by slowing the liver’s CYP enzymes, which break many medicines down. CBD has also been linked to raised liver enzymes, especially in people who also take valproate.
- Dose and route matter. Most side effects grow with the dose and change with the route, which is how the medicine enters the body. That is why guidance says to start low and go slow.
The main source, A Primer to Medicinal Cannabis (“the Primer”), is a 2018 handbook by Martin Woodbridge, a pharmacologist, policy analyst and clinical educator who wrote New Zealand’s medicinal cannabis guidelines for its Ministry of Health in 2007. It was published by Bedrocan, a Dutch company that produces medicinal cannabis. It includes interviews with a Dutch pain physician and a Dutch dispensing pharmacist, the pharmacist who hands the medicine to the patient.
History of substance use disorder
Substance use disorder means losing control over the use of alcohol, drugs or medicines, even when that use is causing harm. There is little good data on how often dependence develops when cannabis is prescribed and supervised as a medicine. Do not assume the risk is zero.
If a patient has misused substances in the past, extra care is needed. Long-term use at high doses can lead to dose escalation, where the patient needs larger and larger doses, and then to misuse and harm. In practice, the prescriber should apply the same clear rules every time on which conditions qualify for treatment. If signs of misuse appear, the prescriber and patient should talk about it openly.
Withdrawal: the part that surprises people
Stopping suddenly after long use can cause restlessness, irritability, poor sleep, vivid dreams and loss of appetite. Cannabis withdrawal syndrome is a recognised diagnosis in DSM-5, the diagnostic manual of mental disorders published by the American Psychiatric Association. It is not rare.
A meta-analysis is a study that pools the results of many earlier studies. Bahji and colleagues published one in JAMA Network Open in 2020. It pooled 47 studies with 23,518 participants who used cannabinoids regularly or were dependent on them. Overall, 47% had withdrawal symptoms.
The rate depended on where people were studied:
- 17% in population-based samples, meaning people from the general community;
- 54% in outpatient samples, meaning people attending a clinic;
- 87% in inpatient samples, meaning people admitted for treatment.
Symptoms usually start within about a week of stopping or cutting down.
What to raise with a prescriber: any history of substance misuse or dependence, including alcohol, and any period of heavy cannabis use. Before you stop, ask for a plan to reduce the dose step by step rather than stopping all at once.
Translator’s note: each PT 33 prescription covers at most 30 days. If you want to stop or cut down, plan it with the prescriber before your current supply runs out. That way you are less likely to stop suddenly just because a prescription ended.
Children and teenagers
Products containing THC are generally advised against for children and teenagers. The exceptions are specific conditions supervised by specialists. The dispensing pharmacist quoted in the Primer described the limits of current knowledge plainly: the possible long-term effects on children are still unknown.
This does not mean every cannabinoid is ruled out for children. Pharmaceutical CBD is an approved medicine for some severe childhood epilepsies, and specialists prescribe and monitor it. Either way, the decision belongs to a specialist service. It is outside what a community forum can handle. This forum does not discuss doses for children, and you should be wary of anyone online who does.
Translator’s note: we could not confirm whether Epidiolex is registered in Thailand. Ask a hospital specialist which pharmaceutical CBD products are registered and available. If your family is considering cannabinoids for a child, the first step is an appointment with a children’s neurologist at a hospital. They can say which treatments are approved and available. Do not use a CBD product sold for adults in place of a prescribed medicine.
Interactions with other medicines
CBD slows several CYP (cytochrome P450) enzymes. As a result, cannabinoids can raise the blood levels of other medicines that the same enzymes break down. Documented examples include:
- warfarin, a blood thinner. The INR rises, and so does the risk of bleeding. The INR is a blood test that shows how long blood takes to clot.
- clobazam, an anti-seizure medicine. The product labelling (the official prescribing information) reports a roughly threefold rise in its active metabolite. The active metabolite is the substance the body turns clobazam into, and it also acts on the brain.
- tacrolimus, a medicine that suppresses the immune system after an organ transplant. A published case report describes levels rising roughly threefold.
A more common problem is added drowsiness. Cannabis adds to the sedation from opioid painkillers, benzodiazepines (a group of medicines for anxiety and sleep), sleeping tablets and alcohol. Part 1 describes how this affects older patients.
The rule that matters most is simple: the prescriber and the pharmacist need your complete medicine list. This includes medicines bought without a prescription and supplements.
Translator’s note: before your appointment, put every medicine you take into one bag and bring it along. Include Thai herbal medicines, vitamins and supplements. Show the bag to the prescriber and to the pharmacist who dispenses your PT 33. This lets them check every possible interaction instead of guessing.
Driving and operating machinery
Even at treatment doses, cannabis can cause dizziness and drowsiness. It can also affect judgement and coordination. Do not drive, operate machinery or do anything else that could be dangerous while under the effect of a product containing THC.
The legal position is separate from the safety question, and in some countries it is stricter. In most Australian states and territories, it is an offence to drive with any detectable amount of THC, with few exceptions. Tasmania, for example, allows a defence for lawfully prescribed medicinal cannabis.
Tests can detect THC after the impairment has passed. In saliva and blood this can be hours to days. In urine it can be longer for frequent users. So a patient can test positive after the effect has worn off.
In Thailand, traffic law prohibits driving under the influence of intoxicating substances. A prescription is not a defence if you are impaired. We could not confirm a specific legal THC limit for drivers in Thailand.
Translator’s note for growers: “machinery” includes everyday farm work. Examples are tractors, brush cutters, chainsaws, ladders, sprayers and electrical work in a grow room. Ask your prescriber how long to wait after a dose before driving or using machinery. Arrange other transport or other workers for those hours. If your job involves any of these tasks, tell the prescriber, because it may affect which product and route they choose.
Pregnancy and breastfeeding
Not recommended. THC reaches the developing baby in the womb, and it passes into breast milk. The Primer’s position is that medical cannabis should not be prescribed during pregnancy and is not recommended while breastfeeding.
What to do: tell the prescriber if you are pregnant, may become pregnant or are breastfeeding. Do this before the first prescription and again if anything changes. They can then review whether to continue and what other options exist.
Summary table
| Group | Status | The specific concern |
|---|---|---|
| Personal or family history of psychosis or serious mental illness | Strongest caution; often stated as do-not-use for THC | THC can trigger psychotic symptoms in vulnerable people |
| Heart disease, irregular heartbeat, previous heart attack or stroke | Avoid, or use only under close supervision | Faster heart rate with lower blood pressure → heart muscle short of oxygen; smoking adds carboxyhaemoglobin |
| Liver disease | Use with monitoring and a lower dose | Slower clearance → higher, longer-lasting levels; raised liver enzymes reported mainly with CBD, especially with valproate |
| Older patients | Caution; start low, prefer low-THC | Blood pressure drop on standing and falls; stronger sedation with other brain-slowing medicines |
| History of substance use disorder | Caution; the same clear rules on which conditions qualify; prescriber review | Rising doses and misuse; withdrawal after sudden stopping (≈47% of people with regular or dependent use) |
| Children and teenagers | Specialist-supervised conditions only | Long-term effects of THC on development unknown; pharmaceutical CBD approved for some childhood epilepsies |
| Taking warfarin, clobazam, tacrolimus, valproate, opioids or sedatives | Not excluded; needs review | CYP enzymes slowed (mainly by CBD), liver enzymes, added drowsiness |
| Driving, machinery, dangerous work | Do not | Impairment; some countries also have zero-tolerance testing laws |
| Pregnancy and breastfeeding | Not recommended | Exposure of the unborn baby; THC passes into breast milk |
What a contraindication is not
A contraindication is not a judgement on the patient, and it is not necessarily permanent. A heart patient excluded from THC may still be a candidate for a CBD-predominant product, with its own interaction and liver checks. A family history of psychosis does not rule out every cannabinoid. A patient on warfarin is not excluded. They are monitored.
Every item above is information the prescriber needs, and they can only get it from you. The most common failure in this field is not a wrong prescription but an incomplete medical history. Usually the patient did not think something counted. It might be an episode from ten years ago, a grandparent’s diagnosis or a supplement.
What to do: before your appointment, write one page with your mental health history, family history, heart, liver and fall history, pregnancy status, and every medicine and supplement you take. Hand it to the prescriber, so the decision is made with the full picture.
If a prescriber has changed your treatment because of something on this list, please describe what changed and why in a reply below. Examples are a lower starting dose, a switch to a CBD-predominant product or a different route. Your account can help other readers. Please do not post doses. Post the reasoning.
This article is general information, not medical advice. It does not replace an assessment by a practitioner authorised to prescribe under PT 33.
Further reading on Asiannabis
- Medical Cannabis: What the Term Means, What the Evidence Supports, and Thailand’s PT 33
- Recognising and Managing THC Over-Intoxication: What Actually Helps, and When to Go to Hospital
- Prescribing and Dispensing Medical Cannabis: Lessons From the Dutch Doctor–Pharmacist Model for Thailand
References
- Woodbridge, Martin. A Primer to Medicinal Cannabis. Bedrocan, 2018. Includes interviews with a Dutch pain physician and a Dutch dispensing pharmacist.
- Bahji et al. Meta-analysis of cannabis withdrawal prevalence (47 studies, 23,518 participants). JAMA Network Open, 2020.
- Clobazam product labelling: roughly threefold rise in the active metabolite when taken with CBD.
- Published case report: tacrolimus levels rising roughly threefold when taken with CBD.