Bioavailability

Imagine sending 100 mg of a drug on a journey through the body - some of it gets “lost” along the way (digested, broken down, or never reaching the blood at all). Bioavailability is the fraction of an administered dose that actually reaches the bloodstream unchanged. Intravenous dosing (given directly into a vein) is defined as 100%; other routes are lower.

Why peptides are usually injected

Peptides (small protein building blocks, e.g. insulin or certain hormones): swallowed, most of them would be digested in the gut - their oral bioavailability is very low. A subcutaneous injection (a shot into the fatty tissue just under the skin, e.g. in the belly) bypasses digestion and delivers a far larger share into the blood.

Relation to dose

Two substances with the same mg amount can act differently if their bioavailability differs - so the route is part of the calculation.