Definition: An excipient is any ingredient in a drug other than the active drug. FDA rules call it an inactive ingredient. In peptide injections, excipients do jobs such as holding the pH steady, keeping the salt level close to body fluids, protecting the peptide from breaking down and stopping bacteria from growing in multi-dose pens.
| Product | Excipients listed |
|---|---|
| Humulin R (per mL) | Glycerin 16 mg, metacresol 2.5 mg, zinc, water for injection |
| Wegovy single-dose (per mL) | Disodium phosphate dihydrate 1.42 mg, sodium chloride 8.25 mg, water for injection |
| Wegovy FlexTouch (per mL) | Disodium phosphate dihydrate 1.42 mg, phenol 5.5 mg, propylene glycol 14 mg, water for injections |
| Forteo (per mL) | Glacial acetic acid 0.41 mg, sodium acetate 0.1 mg, mannitol 45.4 mg, metacresol 3 mg, water for injection |
| Omnitrope 5.8 mg vial (per vial) | Glycine 27.6 mg and sodium phosphates; the diluent has benzyl alcohol |
All of these labels also say hydrochloric acid or sodium hydroxide may be added to adjust the pH.
FDA guidance says formulation ingredients are chosen mainly to keep a protein in its natural shape during storage and to prevent chemical breakdown such as oxidation and deamidation. Other jobs include buffering the pH, adjusting salt levels, and preserving multi-dose products against microbes (Li 2022). Sugars, surfactants and amino acids are added to protect proteins during freezing (Li 2025).
Read more on Dose Track: Peptide storage guides