The same quantity of the same cannabinoid can be a mild, brief experience or an overwhelming twelve-hour one, and the only thing that changed was the way it entered the body.
1. Why the Route Matters More Than the Milligram Number
A milligram figure describes what was placed in the mouth, the lungs or on the skin. It says nothing about how much reached the bloodstream, how fast, what the liver did on the way, or how long it stayed. Route of administration is the dominant variable for cannabis, because cannabinoids are strongly fat-loving, heavily metabolised by the liver and erratically absorbed from the gut.
2. Bioavailability, Onset, Peak and Duration
Bioavailability is the fraction of a quantity reaching systemic circulation unchanged; onset is when an effect becomes noticeable, peak the time to maximum concentration, duration how long it lasts. Marilyn Huestis’s 2007 review Human Cannabinoid Pharmacokinetics reports smoked tetrahydrocannabinol bioavailability of roughly 2 to 56 per cent across studies, narrowing to about 8 to 24 per cent in better-controlled work; Canadian Pharmacists Association and Health Canada material gives 15 to 50 per cent. Oral bioavailability is far lower — Huestis gives 4 to 20 per cent, most sources clustering at 6 to 10.
Why the Published Ranges Are So Wide
Inhalation technique comes first: Huestis attributes the variation to puff number, duration, spacing, breath-hold and inhalation volume. First-pass metabolism is second, since anything swallowed crosses gut wall and liver first. Food effects are third, as stomach fat alters oral absorption. Formulation is fourth — an oil, a capsule and a herbal extract with identical labelled content behave differently.
3. Route Comparison at a Glance
| Route | Bioavailability | Onset | Peak | Duration | Advantages | Risks and limits |
|---|---|---|---|---|---|---|
| Inhalation (vaporised, smoked) | 2–56%; commonly 10–35%; teaching range 15–50% | 5–10 min | 10–20 min; plasma peak ~9 min smoked | 2–4 h | Fast feedback; easy to titrate | Respiratory exposure; technique-dependent |
| Oral / edible | 4–20%; commonly 6–10% | 30–60 min, up to 3 h | 2–4 h, can exceed 6 | 4–6 h adults; 6–12 children | Long duration; standardises well | Delayed onset invites stacking; food effect |
| Sublingual / oromucosal | No robust figure; close to oral | Plasma within ~15 min | 45–120 min, licensed spray | Several hours | Partly avoids first pass | Much is swallowed; high variability |
| Topical | Negligible from a true topical | Local, gradual | Not systemic | Local | No intoxication | No systemic effect; unlike a patch |
| Rectal | Plain cannabinoid none shown; prodrug ester 13.5% in monkeys | Slow, hours | 2–8 h, prodrug studies | Extended | Bypasses the stomach | High figure unsupported |
4. Onset Time: The Most Important Safety Fact for a New Patient
If a new patient remembers one thing, it should be onset time. The reason is behavioural. When a route acts within minutes the feedback loop is tight: a person feels an effect and can stop. When it takes an hour the loop breaks, and the response to “nothing is happening” is to take more — so a second and third are added before the first arrives.
5. Inhalation: Vaporisation Versus Combustion
Combustion occurs around 230 degrees Celsius, producing what burning any organic matter does: carbon monoxide, tar and volatile aromatic hydrocarbons. Vaporisation heats material below that point so cannabinoids and terpenes volatilise without the plant matrix igniting.
In a 2007 pilot study by Donald Abrams and colleagues at the University of California, San Francisco (Clinical Pharmacology and Therapeutics), a vaporiser ran at 180 to 200 degrees Celsius. In eighteen subjects, plasma tetrahydrocannabinol was comparable between vaporisation and smoking, while smoking raised expired carbon monoxide and the vaporiser produced little or none.
Bench work by Dale Gieringer, via the Multidisciplinary Association for Psychedelic Studies, found release starting near 180 degrees and greater efficiency around 200, with benzene, toluene and naphthalene eliminated and carbon monoxide and tar reduced, though not quantified. Running hotter shifts the chemistry back toward combustion.
The Evidence on Respiratory Harm
The 2017 United States National Academies report The Health Effects of Cannabis and Cannabinoids graded evidence:
- Substantial evidence linking long-term cannabis smoking to worse respiratory symptoms and more frequent chronic bronchitis episodes.
- Moderate evidence that stopping is associated with improvement.
- Limited evidence linking occasional smoking to chronic obstructive pulmonary disease risk, controlling for tobacco.
The Honest State of Vaporiser Safety Data
Vaporisation reduces exposure to combustion products, which is not the same as safe: there are no long-term human outcome studies of dry-herb vaporisation, and the 2025 Frontiers in Toxicology review says research into health risks has lagged far behind use. The vaping-associated lung injury outbreak is the cautionary case: the United States Centers for Disease Control and Prevention recorded 2,807 hospitalised cases or deaths including 68 deaths as of 18 February 2020, with 82 per cent of 2,022 patients reporting tetrahydrocannabinol-containing products, and found vitamin E acetate strongly linked.
6. Oral and Edible Preparations
First-Pass Metabolism and 11-Hydroxy-Tetrahydrocannabinol
Swallowed cannabis reaches the liver first by the portal vein. This first-pass metabolism is why oral bioavailability is low and the oral experience chemically different: hepatic cytochrome P450 enzymes, chiefly 2C9, convert tetrahydrocannabinol into 11-hydroxy-tetrahydrocannabinol, an active metabolite that itself enters the brain. Huestis reports it at roughly 6 to 10 per cent of tetrahydrocannabinol concentrations after smoking and far higher after oral intake; she cites Nadulski and colleagues, for whom a metabolite-to-parent ratio above one within two hours indicates oral use. It has long been called at least as psychoactive as the parent compound, though the exact equivalency is unsettled.
The Delayed-Onset Pattern That Fills Emergency Departments
Andrew Monte and colleagues, in the Annals of Internal Medicine in 2019, studied acute illness by route of exposure using emergency visits at a Colorado hospital. Of cannabis-attributable visits, 238 of 2,567 were linked to edibles — while edibles were only about 0.32 per cent of tetrahydrocannabinol sold by weight in the state. Acute psychiatric and cardiovascular symptoms were more frequent among edible-related visits, cannabinoid hyperemesis among inhalation-related.
Food, Fat and Absorption
Because cannabinoids are strongly lipophilic, gut fat at dosing matters greatly — documented in regulatory labelling, not inferred. The United States Food and Drug Administration’s prescribing information for the approved cannabidiol oral solution states that a high-fat, high-calorie meal raises maximum concentration roughly five-fold and exposure roughly four-fold against the fasted state, and advises consistency with meals. The oromucosal spray’s label reports the same for its swallowed fraction: with food, maximum concentration and exposure rose about 1.6- and 2.8-fold for tetrahydrocannabinol, 3.3- and 5.1-fold for cannabidiol.
7. Sublingual and Oromucosal Delivery
The reference product is nabiximols, a licensed oromucosal spray of standardised botanical extract, each 100-microlitre actuation specified at 2.7 milligrams of tetrahydrocannabinol and 2.5 of cannabidiol. Its Summary of Product Characteristics reports cannabinoids in plasma within about 15 minutes, maximum concentration at 45 to 120 minutes, levels below those from inhalation, and coefficients of variation of 57.3 and 64.1 per cent.
Mucosa under the tongue drains straight into systemic circulation — but much of any sublingual preparation is swallowed. Erin Karschner and colleagues, in Clinical Chemistry in 2011, compared controlled oral tetrahydrocannabinol against the oromucosal extract in nine participants and found no statistically significant differences in maximum concentration, time to peak or exposure; relative bioavailability of the oral formulation was 92.6 and 98.8 per cent. Some absorption is genuinely sublingual, hence the earlier onset, but holding an oil under the tongue does not make a preparation first-pass-free.
8. Topical Preparations: Local, Not Systemic
A topical preparation acts where it is applied; a transdermal one is engineered to cross the skin into the bloodstream. A 2022 Pharmaceutics review explains why: cannabinoids sit at roughly 6 to 7 on the log P scale, whereas drugs crossing intact skin in approved transdermal products sit at 1 to 3. A cream or balm therefore acts locally, producing no meaningful systemic concentrations or intoxication; systemic delivery through skin needs engineering: permeation enhancers, patch systems.
9. Rectal Suppositories: Correcting a Widely Repeated Claim
Rectal suppositories are often claimed to deliver 50 to 80 per cent bioavailability by bypassing first-pass metabolism. This should not be repeated; it misreads a narrow literature.
The origin is work by Mahmoud ElSohly and colleagues at the University of Mississippi. Their 1991 Journal of Pharmaceutical Sciences paper on rectal bioavailability of delta-9-tetrahydrocannabinol from the hemisuccinate ester in cynomolgus monkeys reported 13.5 per cent — far below the popular figure. The same abstract records that oral administration gave low, erratic bioavailability “while the drug showed no bioavailability from various suppository formulations.” The 13.5 per cent applied to tetrahydrocannabinol hemisuccinate, a prodrug ester whose partially water-soluble group crosses the rectal mucosa; unmodified cannabinoid in a fat base was not absorbed.
Later human work in the same programme (Medical Cannabis and Cannabinoids, 2018, twelve volunteers) found the hemisuccinate suppository gave exposure 2.44-fold higher than an oral capsule (90 per cent confidence interval 1.78 to 3.35) — a comparison against oral exposure, not an absolute bioavailability figure, and for a patented prodrug rather than ordinary cannabis oil.
10. Dosage Forms in a Clinical Setting
| Form | Usual route | What it offers | What complicates it |
|---|---|---|---|
| Dried flower | Inhalation | Rapid onset; broad chemical profile | Content varies between batches |
| Oil or tincture | Oral or sublingual | Content per millilitre; quick to measure | Food alters absorption; alcohol base suits some poorly |
| Capsule | Oral | Most reproducible single unit | Slowest onset; cannot be subdivided |
| Standardised extract | Oral, sublingual, oromucosal | Defined, consistent content | Fewer products; costlier |
Why Standardisation Matters to a Patient
Standardisation means cannabinoid content is measured, declared and reproducible between batches; without it, a patient settled on a regimen with a prescriber loses that stability whenever a new batch arrives. A 2025 study in Scientific Reports on labelled tetrahydrocannabinol potency in Colorado found that of 178 flower products, 43.3 per cent fell outside a fifteen per cent margin of label — 30.3 per cent over-labelled, 12.9 under.
11. Titration and Route
Titration is adjusting a quantity gradually toward the smallest effective amount; it is prescriber-led. Inhaled, perhaps 10 to 35 per cent reaches circulation within minutes, mostly as the parent compound. Swallowed, perhaps 6 to 20 per cent arrives over hours, with far more 11-hydroxy-tetrahydrocannabinol. Held sublingually, some arrives earlier, the rest like the swallowed portion.
12. Special Populations and Route Choice
Nausea, Vomiting and Difficulty Swallowing
A patient who is actively vomiting cannot reliably retain a swallowed preparation, nor know how much was absorbed. Non-oral routes exist for these situations; the choice is a prescriber’s, informed by what is legally available. Difficulty swallowing rules out capsules; liquid and oromucosal preparations are the usual alternatives.
Children, Older Patients and Existing Lung Disease
In paediatrics the cannabinoid medicine with the strongest regulatory footing is an oral solution, allowing precise weight-adjusted measurement where inhalation is inappropriate; the Canadian Pharmacists Association material also gives a longer oral duration in children, 6 to 12 hours. In older patients, changed hepatic metabolism, cardiovascular sensitivity, polypharmacy and cytochrome P450 interactions argue for routes with modest, predictable peaks. For a patient with asthma, chronic obstructive pulmonary disease or an active respiratory infection, inhalation requires the most justification.
13. The Thai Context
Cannabis flowering buds are a controlled herb under the Protection and Promotion of Thai Traditional Medicine Wisdom Act B.E. 2542, with the Notification of the Ministry of Public Health Re: Controlled Herbs (Cannabis), B.E. 2568 effective 26 June 2025. Per analysis by Thai law firm Tilleke and Gibbins: cannabis may not be supplied without a valid prescription on the official PT 33 form; prescriptions are single-use and non-refillable; intended duration may not exceed 30 days; and seven practitioner categories may prescribe, from modern and Thai traditional medicine to pharmacy and dentistry. Therapeutic advertising is banned.
As for what is dispensed, a nationwide cohort in Medical Cannabis and Cannabinoids covered 21,284 participants across 18 provinces between September 2019 and October 2020. Overwhelmingly the preparation was a cannabis extract in 10 per cent coconut oil at 1.701 milligrams of tetrahydrocannabinol per millilitre with negligible cannabidiol, given as oral drops to about 93 per cent of participants. That predates the 2025 rules, but the point holds: the dominant Thai form is an oil taken by drops, not inhaled flower.
Inhalation in public is governed separately: a Ministry of Public Health notification in the Royal Gazette on 15 June 2022, under Section 25(5) of the Public Health Act B.E. 2535, declared cannabis or hemp smoke or odour that may harm bystanders’ health a public nuisance. Failing to abate one on official order carries up to three months’ imprisonment, a fine up to 25,000 baht, or both. A prescription authorises use of a medicine, not smoking it where others must breathe it.
14. What the Evidence Still Does Not Tell Us
There is no long-term human outcome data on dry-herb vaporisation, no robust pharmacokinetic data for the sublingual route independent of the swallowed fraction, and none for ordinary rectal cannabis preparations. None of this makes route selection arbitrary; it makes it a clinical judgement, made with a licensed practitioner who knows the patient and the law.
Which route were you started on, and did anyone explain the onset time before the first dose? We would like to hear from members across the region — in Thailand and elsewhere — how route was discussed with you, what changed when you switched forms, and whether tested, standardised preparations were available. Share your experience below, keeping to general information and leaving clinical decisions where they belong.
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 educational purposes within jurisdictions where medical cannabis is legally permitted.