THC does not stay in the blood. Most of it rides on blood proteins or moves into body fat, then comes back out slowly over days. Liver enzymes that many other medicines also rely on break it down. This is why a drug test can be positive days or weeks after use. It is also why levels build up with daily dosing, and why CBD can raise the blood levels of some prescription medicines.
This article draws on several published reviews and studies, which are listed under References at the end. Part 1 explained absorption. In short, inhaled THC reaches the blood within minutes. Swallowed THC passes through the liver first, so only about 4–12% reaches the blood (up to 20% in some studies). The liver also turns part of a swallowed dose into a second active compound, 11-hydroxy-THC (11-OH-THC).
This is Part 2. Part 1: Cannabinoid Pharmacokinetics: Why the Same Dose Behaves Like Two Different Drugs Depending on How It Enters the Body
5. Distribution: Where THC Goes After It Reaches the Blood
In the blood, 95–99% of THC is bound to plasma proteins. Most of it is bound to lipoproteins, the particles that carry fat in the blood. Only the small free part, 1–5%, can act on the body at any moment.
Huestis (2007), a review of human studies published in the journal Chemistry & Biodiversity, estimates the volume of distribution of THC at about 3.4 L/kg once levels have settled (at steady state). The volume of distribution is the amount of liquid the drug would have to be dissolved in to give the level measured in the blood. For a 60 kg person, 3.4 L/kg works out at about 200 litres. That is far more than all the water in the body, which shows that THC leaves the blood and is stored in tissue.
That tissue is mainly body fat (adipose tissue). THC builds up in fat, and it builds up more with frequent and long-term use. It moves back from fat into the blood only slowly, and this slow release sets how fast THC leaves the body. Slow-release fertiliser granules in a pot work in a similar way: they keep releasing a little at a time long after you stop adding more.
What you can do: remember that THC from earlier doses is still in your body when you take the next one. Tell your prescriber how often you used cannabis before the treatment began. Regular earlier use means more THC is already stored, and it also builds tolerance (section 6).
6. How Long THC Stays: In the Body, in Drug Tests and in Breast Milk
Patients often misread three results of fat storage.
THC leaves the body slowly, and more slowly with regular use. A half-life is the time it takes for the level in the blood to fall by half. The terminal half-life covers the last, slow phase. In this phase the blood level is low and is topped up by THC coming back out of fat. Huestis reports a terminal plasma half-life for THC of about 4.1 days in chronic (long-term, regular) users.
Drug tests show past use, not impairment. Drug tests look for THC-COOH, the inactive breakdown product described in Part 1. Its reported plasma half-life averages around 5–6 days, and it varies widely between people. Breakdown products can therefore stay detectable for days after use, and for weeks in frequent users. A positive test does not prove that the person was under the influence at the time.
How long a test stays positive also depends on the sample: urine, blood, saliva or hair. The sources used for this article do not give a separate time window for each type of test.
In Thailand, driving under the influence of intoxicants is an offence under traffic law.
Levels build up with repeated dosing. On a regular schedule, THC and its breakdown products build up in tissue and blood over several days. Tolerance makes it harder to predict how this build-up feels. Tolerance means a regular user feels less of the mind-altering effect from the same dose. MacCallum and Russo wrote a 2018 practical guide for doctors on medical cannabis dosing, published in the European Journal of Internal Medicine. They advise raising the dose of any cannabis preparation slowly, over as long as two weeks. This step-by-step adjustment is called titration. The prescriber decides when to judge whether a treatment plan is working.
Breast milk. Because THC is stored in fat, readers often ask how long it stays in breast milk. The sources used for this article give no figures on this. The forum topic Cannabis, Pregnancy and Breastfeeding: What the Evidence Says covers what is known.
What you can do:
- Do not drive after a dose while you feel any effect. A test can stay positive long after the effect has gone.
- Keep a copy of your PT 33 prescription and your dose log. They cannot change a test result, but they show when you used cannabis and why.
- If you take cannabis every day, agree a review date with your prescriber. Do not judge the treatment after one or two days, because blood levels and tolerance are still changing.
- If you are breastfeeding or planning to, raise it with your prescriber before you start.
7. Metabolism: Why CBD Can Change the Levels of Other Medicines
A family of liver enzymes called cytochrome P450, shortened to CYP, breaks down most of the cannabinoids. These same enzymes also break down many common medicines.
- THC is broken down by oxidation, a chemical reaction carried out by CYP2C9, CYP2C19 and CYP3A4. CYP2C9 does most of the work of making 11-OH-THC. The liver then attaches glucuronic acid, a sugar-acid group that makes the products easier to excrete. This step is called conjugation. Huestis notes that more than 100 THC breakdown products have been identified.
- CBD is broken down mainly by CYP2C19 and CYP3A4, with help from other CYP enzymes.
- CBD also slows down (inhibits) several of these enzymes, notably CYP3A4 and CYP2C19. A medicine that relies on a slowed enzyme is cleared more slowly, so its blood level rises. This is how CBD causes the interactions that have been documented.
A 2025 review by Caicedo and colleagues in the journal Pharmaceutics gives an overview of these interactions in humans. They have caused real problems in patients:
- Warfarin (a blood thinner): case reports describe a raised INR. INR is the blood-clotting test that doctors use to set the warfarin dose. A raised INR means the blood clots more slowly and the risk of bleeding is higher.
- Clobazam (an epilepsy medicine): Geffrey and colleagues published a study in the journal Epilepsia in 2015. They followed 13 children whose epilepsy had not responded to standard treatment. All 13 were taking both CBD and clobazam. On average, the level of N-desmethylclobazam, the active breakdown product of clobazam, rose by about 500%. The level of clobazam itself rose by about 60%. Most of the children needed a lower clobazam dose.
- Tacrolimus (a medicine that stops the body rejecting a transplanted organ): Leino and colleagues published a case report in the American Journal of Transplantation in 2019. It describes one patient taking high-dose CBD. The patient’s tacrolimus level, adjusted for dose, rose about three-fold.
Genes add another source of difference. CYP2C9 and CYP2C19 both have common genetic variants that make the enzyme work faster or slower in some people. So the same dose can give different blood levels in two people.
The forum topic Cannabis Drug Interactions: The List to Take to Your Doctor covers interactions in more detail. Until you have read it, follow the cautious approach in Start Low, Go Slow.
What you can do:
- Before you start, write down everything you take: prescription medicines, pharmacy medicines, herbal remedies, supplements and any CBD product. Give the list to your prescriber, so they can check each item against the enzymes above.
- If you take warfarin, ask whether your INR should be checked more often after you start or change a cannabis or CBD product. The aim is to catch a rise before it causes bleeding.
- If a child takes clobazam and is about to start CBD, ask the child’s doctor when and how the clobazam dose will be reviewed. In the Geffrey study, most children needed a lower dose.
- If you have had a transplant, tell your transplant team before you take any CBD. Ask whether your tacrolimus blood level should be checked.
Translator’s note: For a visit to a Thai hospital or clinic, bring the actual boxes, bottles or labels of everything you take. A list written from memory often misses things. People easily forget herbal and traditional remedies, and many do not count them as medicine. If a label is in a language your doctor does not read, a clear photo of it still helps the doctor or pharmacist identify the product.
8. Excretion: How THC and CBD Leave the Body
Almost no THC leaves the body unchanged; nearly all of it leaves as breakdown products. According to Huestis, 80–90% of a dose is excreted within five days. More than 65% leaves in the faeces and about 20% in the urine. The main breakdown product in urine is THC-COOH with glucuronic acid attached (THC-COOH glucuronide). CBD behaves differently: a large share of a CBD dose leaves unchanged in the faeces.
What you can do: do not read “80–90% within five days” as “clear after five days”. The figure describes a single dose. With regular use, the fat store keeps releasing THC, and tests can stay positive for weeks (section 6). Count from your last dose and take your pattern of use into account, not just how you feel.
9. What All of This Changes in Practice
| What you notice | The reason in the body | What follows for you |
|---|---|---|
| An inhaled effect arrives in minutes | Absorbed through the lung, with no first pass through the liver | Easier to adjust the dose as you go; the effect is short, so doses are repeated |
| A swallowed effect starts much later | Absorbed from the gut and partly broken down by the liver first; blood peak usually at 1–2 h, sometimes 4–6 h | Wait before a second dose; taking more too early is a common cause of a much stronger effect than intended |
| A swallowed dose needs more drug for the same effect | Oral bioavailability is about 4–12% (up to 20% in some studies), against a wide 2–56% range for smoking | A swallowed milligram is not an inhaled milligram |
| A swallowed dose feels different | Proportionally more 11-OH-THC is formed | It is not simply “stronger”; it is a different mix of active compounds |
| Levels build up with regular use | Storage in fat and a long terminal half-life | Blood levels build up over days; judge the treatment together with your prescriber |
| A drug test is positive long after use | Storage in fat and slow release back into the blood | A positive test does not show impairment, but it can still have legal consequences |
| Two patients take the same dose and get different results | Wide bioavailability ranges, CYP gene variants and differences in body build | Each patient needs their own dose adjustment; there is no shortcut |
10. The Thai Context (as of October 2026)
None of this article is dosing advice. In Thailand you could not act on it on your own anyway.
Cannabis flower (ช่อดอกกัญชา) is a controlled herb under the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568. The notification was issued under the Thai Traditional Medicine Protection Act B.E. 2542 and has been in force since 26 June 2025. Flower may be dispensed only against a PT 33 prescription from an authorised practitioner. Each prescription covers no more than 30 days. Flower may not be advertised or sold online.
In 2026, two Ministerial Regulations B.E. 2569 tightened licensing. The Ministerial Regulation on Cannabis or Hemp Extracts came into force on 26 April 2026. It allows licences for extracts above 0.2% THC only to three groups: Thai juristic persons that are not “foreign” under the Foreign Business Act B.E. 2542, state agencies, and the Thai Red Cross Society. The Ministerial Regulation on Controlled Herbs (No. 2) came into force on 30 April 2026, when it was published in the Royal Gazette. It tightens the licence criteria for cannabis flower. The forum series Thailand’s Cannabis Rules, 2022 to 2026 gives the background to these changes.
The Cannabis Control Act is still a draft, not law. The Cabinet approved the draft on 22 September 2026 and sent it to the House of Representatives.
The prescriber chooses the product, the dose and the schedule for raising it, because the prescriber knows the patient’s other medicines and conditions. Pharmacokinetics does not give a patient a dose. It helps the patient have a more useful conversation with the prescriber. The patient can describe what happened and when. They can also understand why the prescriber may ask them to wait before deciding whether a treatment works.
What you can do: bring your dose log (Part 1, section 1) and your medicine list (section 7) to every follow-up visit. Instead of just “it didn’t work”, the prescriber then has times and amounts to act on.
Translator’s note: If you work on a licensed farm or in a licensed shop, customers may ask how much to take or how long to wait. Send questions about dose to the prescribing practitioner. You can use the timing facts in this article to explain why waiting matters. The dose itself is still the prescriber’s decision.
If you have compared an inhaled and a swallowed form of the same product under a prescriber’s supervision, please post the timings you saw below this topic. Include how long the effect took to start, how long it lasted, and whether the difference matched what you were told to expect. One person’s report does not prove that a treatment works. Reports like these do help check whether the standard timings describe real patients.
References
- MacCallum CA, Russo EB. Practical considerations in medical cannabis administration and dosing. Eur J Intern Med. 2018;49:12–19. doi:10.1016/j.ejim.2018.01.004
- Huestis MA. Human cannabinoid pharmacokinetics. Chem Biodivers. 2007;4(8):1770–1804. doi:10.1002/cbdv.200790152
- Caicedo DA, Pérez-Mañá C, Farré M, Papaseit E. An Overview of the Potential for Pharmacokinetic Interactions Between Drugs and Cannabis Products in Humans. Pharmaceutics. 2025;17(3):319. doi:10.3390/pharmaceutics17030319
- Geffrey AL, Pollack SF, Bruno PL, Thiele EA. Drug-drug interaction between clobazam and cannabidiol in children with refractory epilepsy. Epilepsia. 2015;56(8):1246–1251.
- Leino AD, Emoto C, Fukuda T, Privitera M, Vinks AA, Alloway RR. Evidence of a clinically significant drug-drug interaction between cannabidiol and tacrolimus. Am J Transplant. 2019;19(10):2944–2948.
This article is part of the Asiannabis Community’s educational series on the science of medical cannabis. It is general information, not medical advice. Dosing decisions belong to a prescriber who knows your case and your other medicines.