The delivery method for a peptide drug isn't a marketing choice, it's usually dictated by basic chemistry. Here's why injections dominate, and how the alternatives manage to work at all.
Why Injection Is the Default
Peptides are vulnerable to stomach acid and digestive enzymes, which break them into individual amino acids before they can reach the bloodstream intact. Injecting a peptide, whether subcutaneously (under the skin) or intramuscularly, bypasses digestion entirely, delivering the molecule directly into tissue where it can be absorbed into circulation largely intact. This is why semaglutide, tirzepatide, and the vast majority of research and therapeutic peptides are formulated as injectables.
How Oral Peptides Get Around This
Oral semaglutide (marketed as Rybelsus) is one of the few peptide drugs available as a pill, and it only works because of a specific absorption enhancer, sodium caprylate (SNAC), that transiently increases stomach pH and helps the peptide cross the stomach lining before it's degraded. Even with this technology, oral bioavailability is low, typically under 1%, which is why the oral dose is far higher than the injectable dose for an equivalent effect, and why it has to be taken on an empty stomach with minimal water.
Nasal and Other Routes
A handful of peptides are formulated for nasal delivery, including some formulations of oxytocin and desmopressin, taking advantage of the nasal mucosa's relatively high permeability and rich blood supply. PT-141 (bremelanotide) was originally studied as a nasal spray before its FDA-approved form became a subcutaneous injection. Nasal delivery still has bioavailability limitations and works better for some peptides than others, depending on molecular size and stability.
What This Means If You're Comparing Products
When a peptide is marketed in a form that doesn't match its known pharmacology, for example an unmodified short peptide sold as an oral capsule, without the specialized formulation technology that makes oral semaglutide work, that's a legitimate reason for skepticism about whether the product can deliver a meaningful dose at all. Delivery method isn't incidental. It reflects real chemistry, and it's worth understanding before assuming a product will work the way it's described.
Storage and Handling Considerations
Delivery method also dictates storage. Most injectable peptides are supplied as a lyophilized (freeze-dried) powder that must be reconstituted with sterile water or bacteriostatic water before use, then generally refrigerated and used within a specific window, often two to four weeks, after reconstitution. Improper reconstitution, using non-sterile water, or storing a reconstituted vial at room temperature for extended periods can degrade the peptide or introduce contamination risk, independent of whatever the peptide itself is. This is a practical, often overlooked variable in whether any injectable peptide product performs as expected.