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Cannabinoid pharmacokinetics: inhalation versus swallowing

The same dose behaves completely differently depending on how it enters the body — and that difference, not the dose on the label, is what makes cannabis hard to prescribe.

2026-09-24

Why the route decides almost everything

Cannabinoids are strongly fat-loving and barely dissolve in water. That single property governs how they cross membranes, where they accumulate and how much of a swallowed dose ever reaches the bloodstream.

The consequence is that route of administration matters more than it does for most medicines. The same milligram inhaled and swallowed produces different peak concentrations, at different times, with different metabolites, and with different variability between people.

Swallowed: slow, low and variable

A swallowed dose passes through the liver before it reaches the general circulation. Much of it is metabolised on the way, and what survives is a small fraction of what was taken. Reported oral bioavailability is low — commonly cited figures cluster around six per cent, with ranges up to roughly twenty per cent across studies.

The liver does not simply destroy the dose. It converts THC into 11-hydroxy-THC, itself active, so the mix of compounds reaching the brain differs from what was swallowed. Onset is slow and the peak is late, which is why a swallowed dose is easy to misjudge: the effect arrives long after the decision to take more.

Inhaled: fast, and still variable

Inhalation bypasses the liver on first pass. Absorption across the lung is quick: reviews put peak blood levels within roughly six to ten minutes.

Bioavailability is higher than oral but far from consistent. The standard review of human cannabinoid pharmacokinetics reports smoked bioavailability ranging from two to fifty-six per cent, averaging around thirty. Most of that spread is not biology but behaviour — how many puffs, how deep, how long held. Smoking topography, in the literature's term, is the dominant source of uncertainty.

The dosing problem, and one attempt at solving it

A medicine whose delivered dose varies several-fold is difficult to prescribe. This is a large part of why clinicians hesitate over inhaled cannabis even where it is legal: the prescription says a number, and the patient receives something else.

One line of work approaches this with a metered-dose inhaler that heats a fixed quantity without combustion. In a randomised, double-blinded, placebo-controlled cross-over trial, twenty-seven patients with chronic pain each received a single inhalation of 0.5 mg or 1 mg of THC, or a placebo.

Peak plasma THC was 14.3 ng/ml after the 0.5 mg dose and 33.8 ng/ml after 1 mg, reached in under five minutes in both cases. Both active doses reduced pain against baseline and the effect held for the 150 minutes measured; only the 1 mg dose differed significantly from placebo.

What that trial does and does not show

It shows that a fixed inhaled dose can produce a predictable blood concentration quickly. That is a real result and the point of the device.

It does not show long-term effectiveness. Twenty-seven patients, a single inhalation each, 150 minutes of measurement. A living systematic review of cannabis for chronic pain excluded this trial from its evidence base for inadequate duration — not because the work is poor, but because a single-dose study cannot answer a question about treating pain over months.

Both things are true at once, and a reader deciding what to believe should hold both.

A note on what this page is

This is a museum page about pharmacology, not clinical guidance. Nothing here is a dose, a recommendation or a substitute for a doctor. Laws on medical cannabis differ sharply between countries and change often.

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