Definition: Bioavailability is how much of a dose, and how fast, gets absorbed into the body where it can act. The FDA defines it as the rate and extent to which the active ingredient is absorbed and becomes available at the site of action. It is often given as a percentage of the dose.
Why it matters when you track doses
It explains why the same drug can have very different doses by different routes. A shot into a vein counts as 100% by definition. A shot under the skin is usually a bit less. By mouth it can be much lower.
Label figures for injections under the skin: semaglutide 89% (Ozempic and Wegovy labels), tirzepatide 80% (Mounjaro label).
Worked example: what actually gets in
Absorbed amount = dose × bioavailability
- Semaglutide 1 mg under the skin: 1 mg × 0.89 = 0.89 mg reaches the bloodstream.
- Tirzepatide 5 mg under the skin: 5 mg × 0.80 = 4 mg.
- Route matters: the Wegovy label lists 2.4 mg once weekly as an injection, and 25 mg once daily for the tablet. That is 25 × 7 = 175 mg a week by mouth vs 2.4 mg by injection, about 73 times more. Each route has its own label dose.
You do not need to do this math to use a drug: label doses already account for it. It matters when comparing routes or products.
Common mix-ups
- Bioavailability is not purity. A pure product can still be poorly absorbed. Purity is a lab test on the vial.
- Label numbers belong to the product tested. They do not transfer to compounded or research versions.
- Switching routes is not a 1:1 swap. The Wegovy tablet and injection doses are very different numbers for this reason.
Read more on Dose Track: Wegovy pill guide · Tirzepatide guide
General education drawn from labels and published studies, not personal medical advice. Your prescriber, pharmacist and the label that came with your product are the final word.