How Peptides Are Administered: Routes, Bioavailability, and Why It Matters
Most peptides are injected under the skin because the stomach digests them. Here is which ones work as nasal sprays, creams, or pills, what bioavailability means, and how to tell whether a capsule can survive digestion.
Approved peptide drugs such as semaglutide and tesamorelin are injected under the skin, and oral semaglutide only works because it is packaged with an absorption enhancer.
- Covers
- Why most peptides are injected, What bioavailability means, Which peptides work as sprays, creams, or pills
Why Administration Route Matters
Most peptides have to be injected because the gut digests them like food. A few work as nasal sprays or skin creams, and a couple survive as pills only with special help. How a peptide gets into the body decides whether it works at all.
Ordinary drugs such as aspirin are small, tough molecules. Peptides are fragile chains of amino acids, the building blocks of protein, and digestive enzymes chop them apart before they reach the blood.
This is a reference page, not a how-to for self-injection. Anyone performing a medical procedure should have a qualified provider’s guidance.
What Is Bioavailability?
Bioavailability is the share of a dose that reaches the bloodstream in working form. A drug given straight into a vein scores 100 percent. A pill with 5 percent oral bioavailability delivers a twentieth of what you swallowed. The rest is destroyed in the gut or by first-pass liver metabolism, the liver’s filtering of everything absorbed from the intestine.
For most peptides, oral bioavailability is often below 1 to 2 percent. That one number explains why they are not sold as tablets.
Why Most Peptides Cannot Be Taken Orally
The digestive tract exists to break protein into single amino acids. Good for nutrition, bad for a peptide drug. Three barriers stand in the way.
Enzymes come first. Pepsin in the stomach, then trypsin, chymotrypsin, and the enzymes lining the intestine all cut the bonds between amino acids.
Acid comes next. Stomach contents sit at a pH of 1.5 to 3.5, which can unfold a peptide and change its shape.
Size is the last barrier. Cell membranes let through small, oily molecules, and most peptides are large and attract water. Ordinary drugs are usually under 500 daltons, a unit of molecular weight; most peptides run from 500 to over 5,000.
Subcutaneous Injection: The Primary Route
Subcutaneous injection, often called subQ, puts the peptide into the fat layer just under the skin. Nearby capillaries carry it into the blood. It is the standard route for peptides in both clinics and research.
It skips digestion, absorbs at a fairly steady rate, and is simpler and safer than injecting into a vein. People with diabetes have injected insulin this way for decades. Common sites are the lower abdomen away from the navel, the outer thigh, and the upper arm, rotated so no one spot gets irritated.
Semaglutide (Ozempic and Wegovy), tesamorelin (Egrifta), and bremelanotide, covered on our PT-141 page, are all FDA-approved and all injected under the skin. Nearly every research protocol for BPC-157, ipamorelin, and TB-500 uses the same route.
Other Routes of Administration
Nasal sprays work for some peptides. Selank and Semax are approved in Russia in that form. The lining of the nose is thin and full of blood vessels, so some peptides pass through it into the blood, and possibly into the brain. For many peptides, bioavailability this way is lower than injection but higher than swallowing.
Skin creams are rarer. GHK-Cu is one of the few peptides with clinical evidence for topical use, in skincare for wound healing and aging. A cream acts where it is applied, which suits the skin and rules out whole-body effects.
Pills are the exception that tests the rule. MK-677 works by mouth because it is not really a peptide; it is a small molecule built to imitate the hunger hormone ghrelin. Rybelsus, the oral form of semaglutide, only works because it is packaged with an absorption enhancer called SNAC that shields the peptide and helps it through the gut wall.
Intravenous injection, straight into a vein, gives full bioavailability. It is used mainly in clinical research, needs medical staff, and carries a higher risk of reactions.
The Practical Implication
A peptide sold as a capsule deserves three checks: whether it is really a peptide, whether there is evidence it survives digestion, and whether that oral form has been studied.
For nearly every peptide on this site, injection under the skin is the studied route. Claims about other routes should be checked against the compound’s chemistry and evidence. See how to read peptide claims critically and what are peptides for more.
Bottom line
The gut treats peptides as food, so most of them have to be injected. Nasal sprays and skin creams work for a handful, and pills work only with special chemistry or for molecules that are not truly peptides. When a route seems too convenient, ask for the evidence.