Most published safety discussion of medical cannabis is about what it might treat. This one is about who it is wrong for. That list is short, it is well described, and it is the part patients most often research last.
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. It can only be supplied against a prescription on form PT 33 issued by an authorised practitioner. Every item on this page is a reason to raise something with that practitioner. It is not a reason to start, stop or adjust anything on your own. Stopping a prescribed medicine abruptly can be more dangerous than the risk you are trying to avoid.
1. Two Words That Are Not Interchangeable
A contraindication is a situation where a medicine should not be used at all, because the expected harm exceeds any expected benefit. A precaution (or relative contraindication) is a situation where it may still be appropriate, but with a lower starting dose, closer monitoring, a different cannabinoid ratio or a different route.
Very little in medical cannabis sits in the first category absolutely. Most of it sits in the second, which is why the honest version of this subject is a list of conversations rather than a list of prohibitions. The published guidance is also less settled than guidance for older drug classes, because the clinical trial base is thinner. Much of what follows rests on pharmacology, observational data and prescriber consensus rather than randomised evidence.
The book cited throughout is Martin Woodbridge’s A Primer to Medicinal Cannabis (Bedrocan, 2018), referred to below as the Primer. It includes interviews with a Dutch pain physician and a Dutch dispensing pharmacist.
Two general points shape everything that follows. First, most of the psychoactive, psychiatric and cardiovascular risk attaches to THC. For that reason, guidance for higher-risk patients tends towards CBD-predominant preparations with low or no THC. CBD is not risk-free, though. It causes most cannabinoid–drug interactions through CYP enzymes, and it has been linked to raised liver enzymes, especially alongside valproate. A switch to a CBD-predominant preparation therefore also needs monitoring. Second, most adverse effects are dose-dependent and route-dependent. That is why “start low, go slow” is standard advice in prescribing guidance (see topic 1312).
2. History of Psychosis or Serious Mental Illness
This is the clearest and most consistently stated caution in the literature, and it is where sources disagree least.
THC can precipitate psychotic symptoms. The Primer states plainly that medical cannabis should not be used in patients with a personal or family history of psychosis, other serious psychiatric illness or depression, because THC can induce psychotic symptoms. The dispensing pharmacist interviewed in the Primer names schizophrenia among the known contraindications. He identifies the greatest practical risks as intoxication, triggering psychotic symptoms in psychiatric patients, and worsening existing depression.
The family history element is the part patients most often dismiss, and it is not arbitrary. The risk of a THC-precipitated psychotic episode is concentrated in people with an underlying genetic vulnerability. A family history is a crude but real marker for that vulnerability.
Depression and bipolar disorder are more contested. The Primer places depression on the do-not-use side, while other sources treat it as a caution that requires monitoring rather than exclusion. What is not contested is that the patient’s psychiatric history belongs on the table before a first prescription, not after a bad reaction.
What to raise with a prescriber: any personal history of psychosis, schizophrenia, bipolar disorder, severe depression or psychiatric admission; any family history of the same; any past bad reaction to cannabis, including one that happened years ago in a recreational context.
3. Cardiovascular and Coronary Disease
Cannabinoids have real cardiovascular effects, and they run in two directions at once. Acute use typically raises heart rate, while peripheral vasodilation lowers blood pressure by reducing systemic vascular resistance. In a heart with limited coronary reserve, that combination (higher oxygen demand, lower perfusion pressure) is how cannabis can precipitate myocardial ischaemia.
The Primer states that patients with a history of heart disease or on cardiac medication should avoid medical cannabis, or use it only under careful medical supervision. The Dutch pharmacist lists cardiac arrhythmia and other cardiac disease among the known contraindications. The Dutch pain physician interviewed in the Primer goes further about one route in particular. Smoked cannabis is associated with increased risk of myocardial infarction and stroke, partly through cardiovascular stimulation and partly through carboxyhaemoglobin from combustion. His conclusion is unambiguous: smoking is never recommended.
That last point separates the risk of the drug from the risk of the delivery method. Vaporisation removes combustion products, but it does not remove the haemodynamic effects of THC.
What to raise with a prescriber: any diagnosed heart condition, arrhythmia, previous heart attack or stroke, angina, uncontrolled hypertension, or cardiac medication.
4. Liver Disease
The liver is where cannabinoids are cleared. THC is metabolised mainly by CYP2C9, with CYP3A4 involvement; CBD is metabolised by CYP2C19 and CYP3A4. Impaired liver function means impaired clearance, so a standard dose produces higher and longer-lasting exposure than intended.
The guidance is not “do not use” but “monitor and dose accordingly.” Patients with liver disease need supervision to make sure the dose does not exceed their metabolic capacity. Abnormal liver function has also been reported as an adverse effect, mainly with CBD. In the trials of pharmaceutical CBD (Epidiolex), raised liver enzymes occurred mostly in patients also taking valproate. That is a separate reason for baseline and follow-up monitoring. CBD is not risk-free here: it causes most CYP interactions and has been linked to raised liver enzymes, especially alongside valproate, so a switch to a CBD-predominant preparation also needs monitoring.
What to raise with a prescriber: hepatitis, cirrhosis, fatty liver disease, any abnormal liver function test, and every other medicine cleared by the liver, especially valproate.
5. Older Patients
Older patients are more sensitive to the psychoactive and neurological effects of THC. They are also more sensitive to orthostatic hypotension, the drop in blood pressure on standing that causes dizziness and falls, which is the more dangerous problem. The Primer is explicit that this applies particularly to older patients who are already prone to falling and to those with dementia. It adds that if cannabis treatment is considered appropriate, older patients should begin at low doses.
Two compounding factors make this group harder than the pharmacology alone suggests. The first is polypharmacy: an older patient is more likely to be taking several medicines whose sedative effects add to those of cannabis. The second is a drug-interaction effect the Dutch pain physician observed directly. Sedation can be markedly amplified when cannabinoids are combined with other central nervous system depressants such as opioids. This amplification is most pronounced in older patients, and both heavy drowsiness and hallucinations have been reported.
A fall is not a minor adverse event in an eighty-year-old. It is often the beginning of a decline that does not reverse.
What to raise with a prescriber: age, any history of falls or dizziness on standing, cognitive impairment or dementia, and a complete list of current medicines including sleeping tablets and opioids.
6. History of Substance Use Disorder
Good data on how often dependence develops when cannabis is used as a supervised, prescribed medicine are limited, and the risk should not be assumed to be zero. Where a patient has a prior history of substance misuse, particular care is warranted. High-dose, long-term use can lead to dose escalation, misuse and harm. In practice this means consistent indication criteria, plus an open conversation if misuse becomes apparent.
Withdrawal is the part that surprises people. Stopping abruptly after prolonged use can cause restlessness, irritability, insomnia, vivid dreams and reduced appetite. Cannabis withdrawal syndrome is a recognised diagnosis in DSM-5, and it is not rare. A meta-analysis of 47 studies and 23,518 participants with regular or dependent use of cannabinoids found an overall prevalence of 47%. Rates varied by setting: 17% in population-based samples, 54% in outpatient samples and 87% in inpatient samples (Bahji et al., JAMA Network Open, 2020). Symptoms typically emerge within about a week of stopping or cutting down.
What to raise with a prescriber: any history of substance misuse or dependence, including alcohol; any period of heavy cannabis use; and, before stopping, a plan for tapering rather than stopping abruptly.
7. Children and Adolescents
THC-containing preparations are generally advised against in children and adolescents outside specific indications supervised by specialists. The dispensing pharmacist quoted in the Primer described the limits of current knowledge plainly: the potential long-term effects on children are still not known.
This does not mean every cannabinoid is off-limits for children. Pharmaceutical CBD is an approved medicine for certain severe childhood epilepsies, prescribed and monitored by specialists (see topic 1309). Either way, this is a specialist decision made inside a specialist service, and it is outside the scope of a community forum. We do not discuss paediatric dosing here, and readers should be sceptical of anyone who does.
8. Interactions With Other Medicines
CBD inhibits several cytochrome P450 enzymes, so cannabinoids can raise blood levels of other drugs cleared by the same pathways. Documented consequences include:
- raised INR and bleeding risk on warfarin;
- a roughly threefold increase in the active metabolite of clobazam (product labelling);
- increased tacrolimus levels, roughly threefold in published case reports.
Separately, and more commonly, patients meet the additive sedation with opioids, benzodiazepines, sleeping tablets and alcohol described in section 5.
The rule that matters here is the simplest one: the prescriber and the pharmacist need the complete list, including over-the-counter products and supplements.
9. Driving and Operating Machinery
At therapeutic doses cannabis can cause dizziness, drowsiness and impaired judgement and coordination. Patients should not drive, operate machinery or do anything potentially hazardous while under the influence of a THC-containing preparation.
The legal position is a separate question from the safety one, and in several jurisdictions it is stricter. In most Australian states and territories it is an offence to drive with any detectable level of THC, with limited exceptions. Tasmania, for example, has a defence for lawfully prescribed medicinal cannabis. Detection can outlast impairment by hours to days in saliva and blood, and for longer in urine for frequent users. A patient can therefore test positive after any impairment has passed.
In Thailand, traffic law prohibits driving under the influence of intoxicating substances, and a prescription is not a defence to impairment.
10. Pregnancy and Breastfeeding
Not recommended. THC reaches the developing fetus and is excreted in breast milk. The Primer’s position is that medical cannabis should not be prescribed in pregnancy and is not recommended while breastfeeding.
11. Summary
| Group | Status | The specific concern |
|---|---|---|
| Personal or family history of psychosis / serious mental illness | Strongest caution; often stated as do-not-use for THC | THC can precipitate psychotic symptoms in vulnerable individuals |
| Heart disease, arrhythmia, previous MI or stroke | Avoid, or use only under close supervision | Raised heart rate with lowered blood pressure → myocardial ischaemia; smoking adds carboxyhaemoglobin |
| Liver disease | Use with monitoring and reduced dose | Impaired clearance → higher, longer exposure; raised liver enzymes reported mainly with CBD, especially with valproate |
| Older patients | Caution; start low, prefer low-THC | Orthostatic hypotension and falls; amplified sedation with CNS depressants |
| History of substance use disorder | Caution; consistent indication criteria; prescriber review | Dose escalation and misuse; withdrawal on abrupt stopping (≈47% of people with regular or dependent use) |
| Children and adolescents | Specialist indications only | Long-term developmental effects of THC unknown; pharmaceutical CBD approved for certain childhood epilepsies |
| On warfarin, clobazam, tacrolimus, valproate, opioids, sedatives | Not excluded; requires review | CYP inhibition (mainly CBD), liver enzymes, additive sedation |
| Driving, machinery, hazardous work | Do not | Impairment; plus zero-tolerance testing laws in some jurisdictions |
| Pregnancy and breastfeeding | Not recommended | Fetal exposure; excretion in breast milk |
12. What a Contraindication Is Not
A contraindication is not a verdict on the patient, and it is not permanent by nature. A cardiac patient excluded from THC may be a candidate for a CBD-predominant preparation, with its own interaction and liver checks. A patient with a family history of psychosis is not thereby barred from every cannabinoid. A patient on warfarin is not excluded; they are monitored.
What the list does is move the decision to the right place. Every item above is information a prescriber needs and cannot get unless the patient volunteers it. The most common failure in this field is not a wrong prescription but an incomplete history. Usually the patient did not think a decade-old episode, a grandparent’s diagnosis or a supplement counted.
Related topics: 1309 (pharmaceutical CBD in childhood epilepsy), 1311, 1312 (start low, go slow) and 1358.
If a prescriber has adjusted your treatment because of something on this list, such as a lower starting dose, a switch to a CBD-predominant preparation or a change of route, describing what changed and why would help others reading this. Please do not post doses. Post the reasoning.
This article is general information, not medical advice. It is not a substitute for assessment by a practitioner authorised to prescribe under PT 33.