Vaping vs combustion vs edibles — bioavailability, onset, and the actual risk profile of each

Bioavailability is the fraction of a dose that reaches systemic circulation. Cannabinoids are notoriously variable on this dimension across delivery routes.

Inhaled (combustion or vaporization): roughly 10-35% of THC reaches circulation, depending on inhalation technique, hold time, and (importantly) experience. Naive users often have lower effective bioavailability because they don't extract the dose efficiently from the smoke or vapor. Experienced users converge toward the upper end of the range.

Oral (edibles, tinctures swallowed): 4-20%, with significant inter-individual variability. The wide range reflects first-pass hepatic metabolism — some of the dose is metabolized by the liver before reaching circulation. Food in the stomach increases bioavailability by ~3x because cannabinoids are lipophilic and absorb better with a fatty meal.

Sublingual (tinctures held under the tongue 60+ seconds): 12-35%. Partially bypasses first-pass metabolism. Onset and bioavailability profile sit between inhaled and oral.

Rectal (suppositories): rarely-discussed, but documented at 50-70% bioavailability with bypass of first-pass metabolism. Used in some pediatric and palliative contexts; not common consumer use.

Topical (skin creams, balms): essentially zero systemic bioavailability for most formulations. Topical effects are local; THC and CBD don't penetrate skin in pharmacologically meaningful amounts via standard balms (Hammell 2016 — the often-cited topical CBD anti-inflammation paper — used a transdermal gel, not a typical retail balm; commercial topicals don't replicate that delivery system).

The take-away: same milligram dose can deliver wildly different actual cannabinoid load depending on route. A 10 mg edible and a 10 mg inhaled hit are not the same drug experience, even setting aside onset timing.

Inhaled: onset 1-5 minutes; peak effect 30-60 minutes after inhalation; total duration 1-3 hours. Fast and tightly contained — easier to titrate dose because feedback is immediate.

Sublingual: onset 15-45 minutes; peak effect 90-120 minutes; duration 3-6 hours.

Oral (gummy, brownie, capsule): onset 30-120 minutes; peak effect 2-4 hours; duration 4-8 hours. Onset is slower because the dose has to be absorbed through the GI tract and processed by the liver first.

Oral on full stomach: can extend onset to 3-4 hours and duration to 6-10 hours. The fat content of the meal slows gastric emptying and improves absorption — both effects work to delay and prolong.

Rectal: onset 10-30 minutes; duration 6-8 hours. Faster than oral because of bypass of first-pass; longer than inhaled because of slower absorption.

The two clinically-relevant features:

1. Inhaled cannabis is dose-titratable. You take a hit, wait 5-10 minutes, take another if needed. The feedback loop is short enough that overshoot is unusual once you've calibrated for the strain and your tolerance.
2. Oral cannabis is dose-stack-trap territory. The most common emergency-room cannabis case in legal-state hospitals is acute over-intoxication from edibles where the user took a dose, didn't feel effects within 30-60 minutes, took more, then 90 minutes after the first dose found themselves with 2-3x the intended dose hitting at once.

The single most important harm-reduction rule for edibles: wait at least 2 hours before re-dosing. Memorize that. It prevents the most common edible mishap.

When THC is metabolized in the liver, it's converted to 11-hydroxy-THC (11-OH-THC) before being further degraded. 11-OH-THC has approximately 2-3x the CB1 affinity of THC and crosses the blood-brain barrier more readily.

This matters because oral cannabis produces substantially more 11-OH-THC than inhaled cannabis at matched parent-THC dose. After an oral dose, plasma 11-OH-THC concentrations can exceed THC concentrations themselves. After inhaled cannabis, 11-OH-THC concentrations are typically 5-10x lower than THC.

So an edible doesn't just deliver "THC delayed and slowed." It delivers a more potent, longer-lasting metabolite at higher concentrations than inhalation. The same milligram dose produces a stronger and longer experience.

Practical consequence: if someone tells you "I can handle 20 mg inhaled but 5 mg edible knocks me out," that's not a tolerance contradiction. It's the bioavailability and 11-OH-THC pharmacology working together. The inhaled 20 mg might land 2-7 mg of THC in circulation peaking quickly and clearing in 2-3 hours; the oral 5 mg might land 1-2 mg of THC plus 2-4 mg of 11-OH-THC peaking slowly and clearing in 6-8 hours. Different drugs.

Smoked cannabis carries the most established health-concern profile of the three routes:

• Combustion products. Burning plant matter at 600-1,000°F generates tar, carbon monoxide, polycyclic aromatic hydrocarbons (PAHs), and other compounds shared with tobacco smoke at varying concentrations. Cannabis-only smokers (no tobacco) have measurable elevated rates of chronic bronchitis and impaired pulmonary function (Tashkin 2013, Annals of the American Thoracic Society) but the cancer signal that's strong with tobacco doesn't replicate cleanly in cannabis-only data — the existing meta-analyses (Aldington 2008, Hashibe 2006) don't show statistically reliable lung-cancer elevation from cannabis smoke alone.
• Particulate matter. Combustion produces fine and ultrafine particulates that lodge in lung tissue. Independent of cannabinoid content, this particulate exposure is a respiratory irritant.
• Difficult dose precision. Cannabis flower dose varies by THC concentration, by how the joint or pipe was packed, by the depth and duration of inhalation. Two hits from the same joint can deliver substantially different doses depending on how they're taken.

Combustion's risk profile is real but not catastrophic in moderate use. Daily heavy smoking has well-documented respiratory consequences; occasional smoking has measurably less.

Vaporization (heating cannabis or cannabis extracts to a temperature that volatilizes cannabinoids without combustion) eliminates most combustion products. The risk profile is more favorable than smoking on respiratory dimensions, but it's not zero-risk.

Flower vaporization (volcano-style devices, dry-herb vaporizers) heats cannabis to 320-400°F. No combustion; cannabinoids volatilize, plant matter remains. This is the lowest-respiratory-risk delivery method for plant-form cannabis.

Concentrate / cartridge vaporization (pen-style devices using oil cartridges) is more complicated. The cartridge contents matter:

• Pure cannabis distillate or rosin in a known carrier (MCT, terpene-only) is reasonably clean
• Vitamin E acetate in cartridges was the cause of the 2019-2020 EVALI outbreak (E-cigarette or Vaping product use Associated Lung Injury) — over 2,800 hospitalizations and 68 deaths in the US (CDC EVALI surveillance report). The contamination was almost entirely in illicit-market THC cartridges; legal-market cartridges were rarely implicated. EVALI killed people; this isn't theoretical.
• Other thinning agents (PEG, propylene glycol, vegetable glycerin) heated to vaporization temperatures generate carbonyl compounds with their own toxicology questions; less-studied than the EVALI case but worth caution

For consumers: legal-market cartridges from brands that publish COAs are substantially safer than illicit-market. Flower vaporization is even safer. Both are safer than smoking.

Vaporized cannabis at 365°F preserves more terpene character than higher temperatures. At 410°F+ you start combusting plant matter; you're back in smoking territory on the risk profile even with a "vaporizer."

Edibles have the cleanest respiratory profile (zero) but their own distinctive risks:

• Acute over-intoxication is the dominant edibles-related ER presentation. Onset delay + 11-OH-THC potency + dose-stacking trap → users in panic states, often first-time. Not medically dangerous in healthy adults but psychologically intense.
• Pediatric exposures are a real public-health concern. Edibles look like candy. Colorado public-health surveillance shows pediatric ER visits for cannabis ingestion increased substantially after legalization, almost exclusively from accidental access to home edibles. Childproof packaging and household discipline matter.
• Drug interactions via CYP3A4/2C9. Oral cannabis runs through hepatic metabolism, which means CYP3A4 inhibitors (grapefruit juice, certain antibiotics, certain HIV medications) raise effective edible potency, and CYP3A4 inducers (St. John's Wort, certain anticonvulsants) lower it. Inhaled cannabis has the same interactions in principle but at lower magnitude because less of the dose hits the liver pre-circulation.
• Cardiovascular response. Both inhaled and oral cannabis can produce tachycardia and transient hypertension, especially in naive users at high doses. People with serious cardiovascular disease should approach cannabis cautiously regardless of route.

For most users in most contexts, edibles are a reasonable delivery method if you respect the dose-stacking rule and start low (2.5-5 mg for naive users, 5-10 mg for experienced).

If you're choosing among the three for a specific use case:

Daytime function / dose precision: inhaled or sublingual. Fast onset means tight feedback; no surprise delayed peak.

Sleep: edibles often work better because the long duration carries through the night. Inhaled cannabis can disrupt sleep architecture if the effect wears off mid-cycle.

Avoiding respiratory exposure: edibles or sublingual. Vaporized flower is a reasonable middle path with much lower respiratory exposure than smoking.

First-time use: edibles are riskier here because of dose-stacking. A naive user is almost always better off starting with a low inhaled dose where they can feel feedback fast. If they prefer edibles, start at 2.5 mg and wait two hours.

Concurrent prescriptions: the CYP-metabolism interactions are most pronounced with edibles. Consult a pharmacist if you take CYP3A4-sensitive medications.

The wider take-away: the three routes are pharmacologically different drugs sharing a name. Choose based on your goal, not on aesthetic preference.

For the underlying receptor pharmacology of THC and CBD that produces these route-dependent differences, see [Cannabinoids 101](/mushrooms/cannabis/cannabinoids-101). For a deeper read on edibles dosing specifically, [Edibles dosing science](/mushrooms/cannabis/edibles-dosing-science) covers the 11-OH-THC story and dose-stacking trap in more detail.