Definition: pH is a measure of how acidic or basic a water-based liquid is, on a scale from 0 to 14 where 7 is neutral. Below 7 is acidic and above 7 is basic. Drug makers set the pH of an injection because a peptide can be stable in one pH range and break down faster in another.
USGS explains that pH is a logarithmic scale: each step is a tenfold change. A liquid at pH 5 is ten times more acidic than one at pH 6, and a hundred times more acidic than one at pH 7. For comparison, MedlinePlus gives normal arterial blood pH as 7.38 to 7.42.
| Product | pH on the label |
|---|---|
| Bacteriostatic Water for Injection (Hospira) | 5.7 (range 4.5 to 7.0) |
| Bacteriostatic 0.9% Sodium Chloride (Hospira) | 5.0 (range 4.5 to 7.0) |
| Humulin R U-100 | 7.0 to 7.8 |
| Wegovy injection | About 7.4 |
| Forteo | Adjusted to pH 4 |
The Humulin R, Wegovy and Forteo labels say hydrochloric acid or sodium hydroxide may be added during manufacture to adjust the pH, and the saline label says it may contain hydrochloric acid for the same reason. That is the maker setting the pH on purpose, not a contaminant.
In a lab study of a model peptide, very acidic conditions cut a bond in the chain, while above pH 6 the peptide rearranged into a different form instead (Oliyai 1993). pH and the peptide's net charge also affect how easily it clumps (Zapadka 2017).
When you mix a powder, the liquid you add helps set the final pH. The bacteriostatic water label warns that some drugs may be incompatible with a given liquid or with benzyl alcohol, and says to follow the drug maker's instructions on which liquid to use. See BAC water.
Read more on Dose Track: Bacteriostatic vs sterile water · Peptide storage guides