Bioavailability: Why the Same Dose Hits Differently

A "5 mg THC" gummy and a "5 mg" puff from a vape pen are not the same experience, even though the number is identical. The reason is bioavailability: the fraction of an administered dose that actually reaches systemic circulation and is available to act on the body. Route of administration changes bioavailability dramatically, and so does individual metabolism, recent food intake, and inhalation technique. Understanding bioavailability is the key to predicting why the same milligram can feel like nothing or like too much.

What bioavailability actually means

Bioavailability is a pharmacokinetic concept, usually expressed as a percentage. If you consume 10 mg of a drug and 3 mg reaches your bloodstream, the bioavailability for that route is 30%. Two things determine it: how much of the dose escapes loss at the site of administration (absorption), and how much survives first-pass metabolism in the liver before reaching systemic circulation. Intravenous dosing is the reference case — 100% by definition — and every other route is measured against it.

For THC specifically, route-driven differences in bioavailability are enormous — easily 10-fold or more. The table below summarizes the typical ranges reported in the clinical literature.

| Route | Approximate THC bioavailability | Onset | Notes | |---|---|---|---| | Inhaled (smoked/vaped) | ~10–35% | Minutes | Varies heavily with technique | | Oral (edibles, capsules) | ~4–12% | 30–120 min | Hit hard by first-pass metabolism | | Sublingual / buccal | ~12–35% (estimated) | 15–45 min | Bypasses some first-pass | | Rectal (suppository) | Variable, often low | 15–60 min | Depends on base formulation | | Topical | Very low systemically | Local | Designed not to intoxicate |

These ranges are wide because they collapse a lot of individual variability, which we address below.

Inhaled THC: fast but variable

When THC is inhaled, it enters the lungs' vast alveolar surface and crosses directly into pulmonary blood, reaching the brain within minutes. Because pulmonary capillaries drain into the systemic circulation without first passing through the liver, inhaled THC largely escapes first-pass metabolism — one reason onset is fast and bioavailability is comparatively high. Reported inhaled bioavailability clusters around 10–35%, with smoking typically at the lower-mid end and vaporization often slightly higher because less THC is destroyed by combustion.

The wide spread is mostly about inhalation technique. Factors that shift the number include puff volume, breath-hold time, depth of inhalation, and the fraction of smoke or vapor that escapes uninhaled. Breath-holding, contrary to folk wisdom, has a smaller effect than people assume — most THC absorption happens in the first seconds — but it does increase deposition of tar in the case of smoking. Device characteristics (temperature, particle size for vapes) and individual lung function also matter. This is why two people hitting the same joint can have meaningfully different blood-THC peaks.

Oral THC: the first-pass effect

Oral administration has the lowest systemic bioavailability of the common routes, typically 4–12%, and the longest, most variable onset. Two physiological hurdles explain this. First, THC is lipophilic and poorly soluble in the watery environment of the gut, so absorption is incomplete and erratic — it depends heavily on whether it is delivered in a fat matrix (edibles, oils) and on what else is in the stomach. Second, and more importantly, absorbed THC travels through the portal vein directly to the liver before reaching the rest of the body. This is the first-pass effect: hepatic enzymes, chiefly CYP2C9 and CYP3A4, metabolize a large fraction of the THC on its first pass through the liver, leaving only a small portion to enter systemic circulation intact.

The first-pass metabolism is not pure loss, though. The liver converts a meaningful fraction of that THC into 11-hydroxy-THC (11-OH-THC), a metabolite that crosses the blood-brain barrier more readily than THC itself and is significantly more potent at CB1. This is the central reason edibles can feel more intense and longer-lasting than the same milligram inhaled, even though systemic bioavailability is lower: a larger share of what does reach the brain arrives as the more potent 11-OH-THC. The combination of slow, erratic absorption, heavy first-pass metabolism, and a highly potent metabolite is exactly why oral dosing produces such unpredictable peaks and why novice users frequently overshoot.

Sublingual, rectal, and topical routes

Sublingual and buccal administration (tinctures held under the tongue or in the cheek) sit between inhalation and oral in both speed and bioavailability. The cannabinoids are absorbed through the rich oral mucosa directly into systemic capillaries that bypass the portal vein, avoiding much of the first-pass effect. Estimated bioavailability is therefore higher than oral — often cited around 12–35% — with onset in the 15–45 minute range. In practice, products labeled "sublingual" often deliver only part of their dose through the mucosa, with the swallowed remainder acting more like an edible, producing a mixed pharmacokinetic profile.

Rectal administration (suppositories) is pharmacologically interesting but unreliable for THC. The lower rectum drains directly into systemic circulation, bypassing first-pass metabolism, but the upper rectum drains into the portal system — so bioavailability depends heavily on where the suppository sits and on the formulation base. THC's lipophilicity means it dissolves poorly in many suppository bases, and reported bioavailability varies widely. It is not a route consumers should assume is equivalent to oral.

Topicals — creams, balms, patches — are formulated for local effect on cannabinoid receptors in skin and underlying tissue. Systemic absorption through intact skin is very low, which is the point: topicals generally do not produce intoxication. Transdermal patches are the exception, engineered with permeation enhancers to push THC into systemic circulation, but even these are limited by THC's skin permeability.

Why the same dose hits differently

Beyond route, several factors explain person-to-person and occasion-to-occasion variability:

• Metabolism genetics. Polymorphisms in CYP2C9 and CYP3A4 change how fast individuals clear THC and generate 11-OH-THC. Slow metabolizers reach higher peaks from the same oral dose.
• Fed versus fasted state. A high-fat meal dramatically increases absorption of oral THC (the "fat helps absorption" rule), raising peak levels and shifting the time of onset. Taking an edible on an empty stomach produces a faster but often lower and more erratic peak.
• Tolerance. Chronic use downregulates and desensitizes CB1 receptors, so the same blood-THC level produces a smaller subjective effect.
• Liver enzyme competition. Drugs that inhibit or induce CYP enzymes — including some antidepressants, antifungals, and even grapefruit — can raise or lower THC exposure.
• Product variability. Edibles and tinctures can differ from their labeled dose, and the THCA-to-THC conversion in home-prepared products introduces further error (see our decarboxylation article).

The practical takeaway is that milligrams are not interchangeable across routes, products, or even days. A consumer switching from a 5 mg inhaled dose to a 5 mg edible is moving from ~35% bioavailability with fast clearance to ~4–12% bioavailability with heavy first-pass conversion into a more potent metabolite — a fundamentally different pharmacokinetic event.

Key takeaways

• Bioavailability is the fraction of a dose reaching systemic circulation; route changes it by 10-fold or more for THC.
• Inhaled THC averages ~10–35% bioavailability, varies with technique, and reaches the brain in minutes by bypassing the liver.
• Oral THC is only ~4–12% bioavailable because of incomplete gut absorption and first-pass hepatic metabolism, but the liver also produces 11-hydroxy-THC, a more potent metabolite that makes edibles feel stronger and last longer.
• Sublingual dosing partially bypasses first-pass metabolism, giving higher bioavailability and faster onset than oral.
• Individual variability — CYP genetics, fed/fasted state, tolerance, and enzyme competition — means the same milligram can produce very different effects across people and occasions.

Frequently asked questions