Bioavailability

Bioavailability is the “how much actually got in” number. You take a dose, but not all of it reaches your bloodstream in working form — some is destroyed, some never absorbed. Bioavailability is the percentage that makes it through and can go do its job. An injection into a vein is 100% by definition; everything else is measured against that.

Picture pouring a pitcher of water through a leaky funnel. The pitcher is your dose; what lands in the glass is the bioavailable part. A tight funnel wastes little; a leaky one wastes most of it.

For peptides, the funnel by mouth is extremely leaky. Peptides are made of amino acids, and your gut is built to chop amino acid chains into pieces for digestion — so swallowing most peptides destroys them before they ever reach the blood. That single fact is why BPC-157, GLP-1 drugs, and most research peptides are injected under the skin instead of taken as a pill.

It’s also why the rare peptide with real oral bioavailability (oral semaglutide took heavy engineering to get there) is a genuine achievement rather than a default option.

A concrete example is semaglutide, which comes in both subcutaneous injections and oral tablets, so the same active compound makes the route problem easy to see. The injection avoids the digestive tract, while the tablet combines semaglutide with salcaprozate sodium (SNAC), an absorption enhancer that helps it cross the stomach lining. Even with that help, DailyMed’s prescribing information estimates Rybelsus’s absolute oral bioavailability at 0.4% to 1% under tightly specified dosing conditions designed to support absorption, meaning only a small share of swallowed semaglutide reaches circulation. A 7 mg tablet therefore does not mean 7 mg reaches the blood intact, and doses across formulations cannot be compared milligram for milligram; this is pharmacokinetics in practice.