The same milligram dose of THC can relieve one patient and overwhelm another. Using published medical research, this guide explains why: the route, past use, body fat, genes and age all play a part. It also covers tolerance, acute bad reactions and cannabinoid hyperemesis syndrome. The later sections deal with drug interactions, the symptom diary that makes dose changes safer, and the Thai prescription rules.
The guide is for growers and patients in Thailand and Southeast Asia who want to understand a prescription, not set a dose themselves. Tetrahydrocannabinol (THC) is the main intoxicating compound in cannabis. Cannabidiol (CBD) is the main non-intoxicating one. Medical guidance follows the rule “start low, go slow”: a small starting dose, raised in small steps by a prescriber.
This is Part 2. Part 1: Start Low, Go Slow: What Published Guidance Says About Medical Cannabis Dosing
1. Why the same dose acts differently in two people
The route
Radparvar (2024) compared how fast cannabis acts by each route and how long the effect lasts. The study also compared bioavailability, which is the share of a dose that reaches the bloodstream. Topic 1411 explains bioavailability in detail.
| Route | Effect starts | Effect lasts | Share reaching the blood |
|---|---|---|---|
| Inhaled, smoked | 0–10 minutes | 2–4 hours | Up to about 25% |
| Inhaled, vaporised | 0–10 minutes | 2–4 hours | 10–35% |
| Swallowed | 1–3 hours | 6–12 hours | 4–12% |
So 10 mg swallowed and 10 mg inhaled are not the same dose in the body. They also differ in timing. For more on each route, see routes of administration.
Past use and tolerance
Past use is probably the largest single reason why people respond differently. A dose that disables someone who has never used cannabis may do almost nothing for a long-term daily user. For this reason, one published schedule, Bhaskar and colleagues’ (2021) “rapid” protocol, lists significant past cannabis use as one reason to use it.
Body fat, genes and age
THC is fat-soluble and is stored in body fat. This helps explain why frequent users clear the last traces of THC slowly. Radparvar (2024) lists body fat percentage among the factors that affect how long THC breakdown products stay in the body.
Genes also matter. CYP2C9 is a liver enzyme that breaks down THC, and some people carry a less active variant called CYP2C9*3. Sachse-Seeboth and colleagues (2009) studied 43 volunteers who took THC by mouth. People with two copies of CYP2C9*3 had about three times the total THC exposure (median area under the curve) of people with two copies of the common form, CYP2C9*1.
Age makes people more likely to become sedated and to fall. That is why several schedules start older people at 1–1.25 mg of THC. See also patient groups needing extra care.
Translator’s note: Nobody can see which enzyme variant they carry. A low starting dose protects people whose bodies clear THC slowly without knowing it.
What to do: Tell the prescriber how often you have used cannabis before and by which route. Also tell them your age and every medicine you take. Do not switch route on your own, for example from smoking to oil. Ask the prescriber how to change the dose, because the same milligrams act differently by each route.
2. Tolerance and the “tolerance break”
What brain scans show
THC acts mainly on the cannabinoid type 1 (CB1) receptor, a docking site on brain cells. See the endocannabinoid system. Hirvonen and colleagues (2012) used brain scans and found about 20% fewer CB1 receptors in the outer brain layer (cortex) of long-term daily smokers than in non-users. After about four weeks of supervised abstinence, receptor levels in the 14 people who completed follow-up had moved back toward normal.
D’Souza and colleagues (2016) studied 11 men dependent on cannabis and 19 non-users. They found receptor availability about 15% lower in the users. After two days without cannabis, the difference could no longer be detected.
Where online advice goes further than the evidence
Two days and four weeks are very different answers. Neither study measured whether patients’ symptoms were controlled. Websites recommend breaks of 48 hours, one week, 21 days or 28 days, but none of these lengths comes from clinical research. They come from wellness publishers who repeat the receptor findings without the data.
The scans show that the drop in receptors is real and that it reverses. They do not support any particular break length.
What to do: If you are thinking about a tolerance break, plan it with the prescriber. Keep your symptom diary going during the break so that you both can see what happens to the symptom.
3. The acute bad reaction
What it looks like and how it is managed
The Internet Book of Critical Care is an online reference for emergency and intensive-care doctors. It describes strong anxiety or panic, changed perception, poor coordination and memory, and sedation. It also lists heart effects: a fast heartbeat (tachycardia), and at higher exposures a slow heartbeat (bradycardia) and low blood pressure (hypotension). See also recognising and managing THC over-intoxication.
Treatment is mostly supportive: a quiet room, little stimulation and time. Calming medicines called benzodiazepines are used for severe anxiety. Cannabis alone does not usually cause fatal poisoning.
That does not make a bad reaction harmless. The same reference records a four- to eight-fold rise in the risk of heart attack (myocardial infarction) after use. See cannabis and the heart. A reaction in someone with heart disease, in a driver or in a child needs urgent medical care.
The delayed effect of edibles
The US Centers for Disease Control and Prevention (CDC) is the national public health agency of the United States. It states that edibles take 30 minutes to 2 hours to cause intoxication, and that the effects can last longer than expected. Because of this delay, some people eat too much, which can lead to poisoning or serious injury. The CDC adds that the THC content of edibles is often unknown or wrongly labelled.
The usual pattern is that a person feels nothing and eats a second portion. Both portions then take effect together. See edibles and decarboxylation.
Translator’s note: The same delay applies to any food or drink made with cannabis, including dishes cooked at home. Label such food and store it out of children’s reach.
What to do: Never take a second portion just because the first seems to do nothing. The CDC’s window runs up to 2 hours. If a child, a person with heart disease or someone who must drive has a bad reaction, take them to a hospital emergency department.
4. Cannabinoid hyperemesis syndrome
Cannabinoid hyperemesis syndrome (CHS) causes repeated attacks of severe nausea and vomiting in people who have used cannabis almost every day for a long time. Doctors often miss it. Attacks follow the same pattern each time and often come with belly pain. Many patients take hot baths or showers again and again, because heat briefly relieves the symptoms.
The Rome IV criteria are the international diagnostic criteria for gut-function disorders. They require three things for this diagnosis:
- Repeated vomiting attacks that follow the same pattern, similar to cyclic vomiting syndrome.
- Long-term cannabis use before the attacks.
- Relief when the person stops cannabis for a sustained period.
Habboushe and colleagues (2018) studied 155 hospital patients who smoked cannabis on 20 or more days a month. Of these, 32.9% reported symptoms that matched CHS. The 95% confidence interval was 25.5–40.3%, which means the true figure most likely lies in that range.
A 2015 review in the Cleveland Clinic Journal of Medicine says symptoms ease markedly after 24–48 hours of fluids, supportive care and stopping cannabis.
Stopping cannabis is the only documented cure. Emergency protocols also mention capsaicin cream (made from the compound that makes chilli hot) on the skin, and low-dose haloperidol, a medicine used here to stop vomiting. Evidence for both is limited.
Translator’s note: In Thai heat, repeated vomiting and long hot showers can dehydrate a person quickly. If someone cannot keep fluids down, take them to a hospital.
What to do: If a long-term daily user has repeated vomiting attacks that hot showers relieve, they should see a doctor. They should tell the doctor about their cannabis use.
5. Drug interactions to raise with a prescriber
Cannabinoids interact with other medicines through the cytochrome P450 enzymes. These are the liver’s main system for breaking down drugs. If cannabis slows these enzymes, another drug can build up in the blood. See the drug interaction list to take to your doctor.
Lopera, Rodríguez and Amariles (2022) report that THC slows the enzymes CYP1A2, CYP2B6, CYP2C9 and CYP2D6. They report that CBD slows CYP3A4, CYP2B6, CYP2C9, CYP2D6 and CYP2E1.
Caicedo and colleagues (2025) add that THC and CBD both bind strongly to proteins in the blood. They can push other drugs off these proteins, which raises the free, active amount of those drugs. This matters most for narrow-margin drugs, where a small rise in blood level can cause harm.
| Drug or drug group | What the research reports | Why it matters |
|---|---|---|
| Warfarin (a blood thinner) | Lopera (2022) rated this pair the highest risk. In four case reports, the INR (a blood test of how slowly blood clots) rose to about 7.6 on average, with bleeding | Narrow margin; needs blood tests |
| Clobazam (an anti-seizure medicine) | Caicedo (2025): CBD blocks the enzyme CYP2C19, and the clobazam breakdown product N-desmethylclobazam rises three- to four-fold | Risk of sedation and toxicity in epilepsy |
| Tacrolimus, everolimus (drugs that suppress the immune system, used after organ transplants) | CBD raised the peak level of tacrolimus 4.2-fold and total exposure 3.1-fold. Everolimus rose about 2.5-fold | Very narrow margins |
| Sedatives, opioid painkillers, alcohol | Their slowing effects on the brain add together. Drugs acting on the nervous system made up 13 of the 19 pairs in Lopera (2022) | More sedation and more falls |
| Other narrow-margin drugs | Anti-seizure drugs, heart-rhythm drugs, immune-suppressing drugs and buprenorphine (an opioid medicine) appear again and again as medium-to-high concern | Blood-level checks need discussion |
Translator’s note: Your medicine list should include traditional herbal remedies, supplements and products bought without a prescription. A prescriber cannot check an interaction with something they do not know about.
What to do: Before cannabis is added, give the prescriber a complete written list of everything you take. Include over-the-counter products and supplements.
6. The symptom diary
A step-up schedule only works if there is a record to judge each step against. Memory is unreliable. A diary turns impressions into data that a prescriber can use at each review.
| Column | What to write | Why the prescriber needs it |
|---|---|---|
| Date and time | Clock time of each dose | Shows timing patterns and gaps in symptom control |
| Product and route | As written on the prescription | Route decides how fast the effect starts and how long it lasts |
| Amount taken | Measured amount, in the units on the prescription | The only way to judge each step |
| Time to effect | Minutes or hours until any change | Shows whether the dose is too small or the route is just slow |
| Symptom score | The target symptom on a 0–10 scale, before the dose and at peak effect | Measures response against an agreed target |
| Sleep and daily function | Hours slept, times woken, one daily task done or not | Often the first things to change. Guidance counts better function as a response |
| Side effects | Drowsiness, dizziness, racing heart, anxiety, nausea, each with severity | The other half of every dose decision |
| Other medicines and alcohol | Everything taken that day | Without this, interactions stay hidden |
Two habits make a diary useful. First, write each entry at the time, not from memory at the end of the week. Second, use the same rating scale from start to finish. Bring the diary to every appointment.
Translator’s note: Keep the diary in the language you think in, in a notebook or a phone note. A PT 33 prescription covers at most 30 days, so each renewal visit is a natural time to review the diary.
What to do: Start the diary on the day before your first dose so you have a baseline symptom score. Fill in every column for each dose for at least the first two weeks. This shows the prescriber exactly how each step changed your symptoms and side effects.
7. The Thai legal frame
The Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568 makes cannabis flower a controlled herb. It was issued under the Thai Traditional Medicine Protection Act B.E. 2542 (1999) and came into force on 26 June 2025. The Minister of Public Health signed it on 23 June 2025.
Flower may be sold only with a PT 33 (ภ.ท.33) prescription, which covers at most 30 days. The prescription uses an official form set by the Director-General of the Department of Thai Traditional and Alternative Medicine. The notification lists these practitioners: medical doctors, dentists, pharmacists, Thai traditional medicine and applied Thai traditional medicine practitioners, folk healers (หมอพื้นบ้าน) and Chinese medicine practitioners. The notification controls the research, export, sale and processing of flower.
Advertising and online sale of flower are prohibited. Sale through vending machines is also prohibited. Selling or processing a controlled herb without a licence breaks Section 46 of the 1999 Act. Under Section 78, the penalty is up to 1 year in prison, a fine of up to 20,000 baht, or both.
Licensing became stricter in April 2026 (details). The Ministerial Regulation on Cannabis or Hemp Extracts B.E. 2569 came into force on 26 April 2026. Licences for extracts above 0.2% THC are available only to three groups:
- Thai juristic persons that are not “foreign” under the Foreign Business Act B.E. 2542.
- State agencies.
- The Thai Red Cross Society.
The Ministerial Regulation on Controlled Herbs (No. 2) B.E. 2569 came into force on 30 April 2026. It tightens the criteria for flower licences. The draft Cannabis Control Act is not yet law. The Cabinet approved the draft on 22 September 2026 and sent it to the House (status).
Setting your own dose outside this system is unwise because of the dose effects and drug interactions described above. It also falls outside the legal system that allows lawful access. The prescriber sets the product, the ratio, the route, the starting dose and every change.
What to do: If you grow, do not sell flower to patients without a licence, and do not set doses for other people. Send anyone who asks to a licensed prescriber. Supply only through licensed channels, with every batch lab-tested.
8. What the evidence does not settle
The guidelines disagree on upper limits, on the time between steps and on CBD targets. The online dose bands have no basis in research. There is almost no evidence comparing one THC:CBD ratio with another. Estimates of how fast brain receptors recover range from two days to four weeks.
The figures in this guide are the best available starting points, but they are provisional. They describe groups of patients, not individuals. They are working material for a prescriber, not instructions for a patient.
What to do: Take the figures from this guide to your prescriber as questions, not as a plan. Ask which schedule they follow and why.
If you track symptoms with a prescriber, please share how you do it below this topic. Tell us which diary columns mattered, which ones you dropped as noise, and what changed a neglected log into one that changed a consultation.
References (Part 2)
- Radparvar S. The Permanente Journal, 2024.
- Bhaskar A. and colleagues. Modified Delphi consensus. Journal of Cannabis Research, 2021.
- MacCallum C.A., Russo E.B. European Journal of Internal Medicine, 2018.
- MacCallum C.A. and colleagues. Frontiers in Pharmacology, 2021.
- Minnesota Department of Health, Office of Medical Cannabis (USA). Dosing and Chemical Composition Report, 1 December 2023.
- Sachse-Seeboth C. and colleagues. Clinical Pharmacology and Therapeutics, 2009.
- Hirvonen J. and colleagues. Molecular Psychiatry, 2012.
- D’Souza D.C. and colleagues. Biological Psychiatry: Cognitive Neuroscience and Neuroimaging, 2016.
- Internet Book of Critical Care. Entry on cannabis intoxication.
- US Centers for Disease Control and Prevention. Information on cannabis edibles.
- Rome IV diagnostic criteria for cannabinoid hyperemesis syndrome.
- Habboushe J. and colleagues. Basic and Clinical Pharmacology and Toxicology, 2018.
- Cleveland Clinic Journal of Medicine, 2015. Review of cannabinoid hyperemesis syndrome.
- Lopera V., Rodríguez A., Amariles P. Journal of Clinical Medicine, 2022.
- Caicedo and colleagues (with Pérez-Mañá C., Farré M., Papaseit E.). Pharmaceutics, 2025;17(3):319.
- Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568 (2025).
This article is part of the Asiannabis Community educational series on medical cannabis. It is general information, not medical advice. Treatment decisions belong with a licensed practitioner. Content is for education in places where medical cannabis is legally permitted.