How to store peptides before and after mixing

Store a peptide exactly as its label or pharmacy tells you; the rules differ by product. When there’s no label, remember that peptides keep far better as a dry powder than in solution (Izutsu, 2018). Keep the unmixed powder sealed, dry and dark, and treat a mixed vial as short-lived: refrigerated unless the label says otherwise, never frozen without the label’s say-so, and used by its discard date. Once it’s mixed, the peptide calculator turns your prescribed dose into units.

Before mixing: the powder

Most injectable peptides start as a freeze-dried (lyophilized) powder. Freeze-drying is used because it significantly improves the storage stability of peptides and proteins, which are only marginally stable in water (Izutsu, 2018). How warm or cold the powder should be kept depends on the product, which is why your label comes first:

• Egrifta WR (tesamorelin) vials are stored at room temperature, 20 to 25 °C, in the box to protect them from light (label).
• Serostim vials are stored at room temperature, 15 to 30 °C, before mixing (label).

Whatever the temperature, keep the powder sealed, dry and dark until you need it.

After mixing: the vial

Once a peptide is in solution, it’s far less stable: in liquid form, therapeutic peptides and proteins often degrade chemically (Angkawinitwong et al., 2015). For medicines, the label sets the rules:

If your vial has no label instructions, ask the dispensing pharmacist. Don’t borrow another product’s numbers. For more on discard dates, see how long reconstituted peptides last.

Can peptides be frozen?

Unmixed powder is frozen in labs for long-term storage. Mixed vials are a different story. The Egrifta WR, Serostim, Zepbound and Ozempic labels all say not to freeze, and Zepbound and Ozempic add that you shouldn’t use the product if it has been frozen. Ozempic’s label also says not to store it next to the fridge’s cooling element. In practice, that means the middle of the fridge, away from the back wall and the freezer compartment.

Fridge and travel tips

• Keep it in the original box. The Zepbound label says to store it in the carton to protect it from light.
• Avoid heat. Glove boxes, sunny windowsills and bathrooms are out. Zepbound’s label says to protect it from heat and light.
• When traveling, check how long your product may stay unrefrigerated. Zepbound’s single-dose pens and vials allow up to 21 days at up to 30 °C (label). If you use a cool bag, keep the vial from freezing (for example, don’t rest it directly on a frozen ice pack), because the Zepbound and Ozempic labels say not to use a product that has been frozen.
• Bacteriostatic water doesn’t need the fridge. Its label says to store it at 20 to 25 °C (68 to 77 °F) (Bacteriostatic Water label).

Label every vial

MedlinePlus advises writing the date on any vial you’ll use more than once (MedlinePlus). For a mixed peptide, add the concentration and the discard date too. For example (numbers for illustration only):

• 10 mg + 2 mL = 5 mg/mL, so 1 unit on a U-100 syringe = 0.01 mL = 0.05 mg
• Mixed March 3; label says use within 14 days, so discard March 17

Writing “5 mg/mL” on the vial means anyone using it, including you in three weeks, can check the math in the reconstitution calculator.

Common mistakes

• Freezing a mixed vial to make it last longer.
• Leaving vials in a hot car or in direct sunlight.
• Storing next to the freezer compartment or the cooling element.
• Taking vials out of their box, which exposes them to light.
• Using a vial past its discard date because the liquid still looks clear.

Questions people ask