Subcutaneous Delivery: The Science

Why under-the-skin delivery is used

Most therapeutic peptides are given by the parenteral route (bypassing the gut) because swallowing them gives poor absorption. [1] Under-the-skin dosing avoids the digestive enzymes that limit the oral route. Over the past century more than 80 peptide drugs reached the market for conditions including diabetes, cancer, osteoporosis and multiple sclerosis. [3]

What happens after the dose

Insulin is the best-studied example. A review of subcutaneous insulin absorption describes variability as a major challenge in therapy and says that variability is largely driven by how the drug is absorbed. It groups influences into three categories: the preparation, the injection site and patient, and the injection technique. [2] The review also describes the insulin absorption process and the changes manufacturers have made to the molecule to alter that process. [2]

The point for other peptides is general: the formulation, the person and the way the dose is delivered all influence how much reaches the blood and when. Results for one product cannot be assumed for another.

Label example: semaglutide injection

The US FDA label for Ozempic (semaglutide injection), read on DailyMed on 2026-10-22, reports:

• absolute bioavailability of 89%;
• peak concentration 1 to 3 days after a dose;
• an elimination half-life of about 1 week, so the drug remains in the circulation for about 5 weeks after the last dose;
• extensive albumin binding (over 99%). [4]

The label says the main reason for the long half-life is albumin binding, which reduces kidney clearance and protects against metabolic breakdown, and that the molecule is stabilized against the DPP-4 enzyme. [4] The duration is a result of drug design, not of the route alone. See lipidation, PEGylation and cyclization.

Where it goes in the body

The label says semaglutide is cleared mainly by proteolytic cleavage of the peptide backbone, that is, the body's own enzymes cut it apart. [4] So even a modified peptide is eventually broken down by the body's own enzymes. Hormone biology that illustrates the same point is in GLP-1 and GIP biology.

Immune response and quality

Peptide drugs can trigger antibodies in some people. See immunogenicity of peptide medicines. Injectables must be sterile and free of fever-causing contaminants (pyrogens). Standard sterilization does not remove pyrogens, which is why injectable products require pyrogen testing. [5] US outbreaks tied to pharmacy-compounded sterile preparations (19 outbreaks, at least 1,000 cases between 2001 and 2013) involved lapses in aseptic processing, sterilization, or sterility and endotoxin testing. [6] A certificate of analysis for a powder does not establish these properties; see online certificates of analysis for peptides and infection and injection-related risks.

What we cannot say

• Proper evidence not found that the absorption profile of any promoted research peptide (for example one sold without approval) has been measured in people to the standard seen for an approved medicine. We searched PubMed and regulator labels for absorption data and found it only for approved products and insulin. For specific compounds, see their own pages, such as BPC-157.
• Absorption numbers from an approved product do not carry over to other products, even of the same drug class.