Does Refrigerating Insulin Keep It Safe From Bacteria?

What This Study Found

Researchers tested how well the preservatives inside insulin vials fight off bacteria at two different temperatures — room temperature and refrigerator temperature. They found that refrigerated insulin was slower to kill certain bacteria in the short term, failing stricter European safety standards at contact times under 7 days. Both refrigerated and room-temperature insulin did, however, meet U.S. Pharmacopeia (USP) safety standards.

Why This Matters for You

Most people with diabetes are told to store unopened insulin in the refrigerator. That advice is still correct — refrigeration slows insulin from breaking down and extends its shelf life. But this study raises an important question: does cold storage affect how well the preservatives inside multi-dose vials protect against bacterial contamination?

The short answer is: it might, at least in the first few days after contamination occurs. The bacteria used in this study — Serratia marcescens — is a cold-tolerant germ that can actually survive and grow at refrigerator temperatures. The preservatives in insulin were slower to knock down this bacteria when the vials were kept cold.

What Are Preservatives in Insulin For?

Multi-dose insulin vials contain small amounts of preservatives — like phenol or metacresol — to protect the contents if the vial is accidentally contaminated. Every time you insert a needle to draw a dose, there is a tiny risk of introducing germs. Preservatives are the vial's built-in defense system.

This study suggests that defense system may work a little differently depending on temperature — and that current testing methods may not fully reflect real-world refrigerator storage.

Should You Change How You Store Insulin?

No — not based on this study alone. Here's what the findings do not mean:

• They do not mean your refrigerated insulin is unsafe.
• They do not mean you should stop refrigerating unopened vials.
• They do not mean preservatives fail completely in the fridge.

What the study does suggest is that pharmaceutical testing standards may need updating to better reflect cold-storage conditions. The researchers recommend that future safety testing be done at both room and refrigerator temperatures to give a more complete picture.

Practical Tips for Handling Insulin Vials Safely

• Always use a fresh needle for each injection to reduce contamination risk.
• Store opened vials according to your insulin's specific instructions — many can be kept at room temperature for up to 28–30 days.
• Never use insulin that looks cloudy (unless it's supposed to be), discolored, or has particles floating in it.
• Check expiration dates regularly.

Proper storage supplies — like insulin coolers and travel cases — can help you maintain the right temperature whether you're at home or on the go. MDS Diabetes carries a range of insulin storage and diabetes supply products to help you manage your insulin safely. Visit mdsdiabetes.com to explore your options.

Citation

Mousavi Ghahfarrokhi, Zarei, Hamidian (2025). Preservative Efficacy Testing of Refrigerated Pharmaceuticals: Choice of Challenging Isolate and Storage Temperature. PDA Journal of Pharmaceutical Science and Technology. PMID: 40300809. DOI: 10.5731/pdajpst.2024-003021.1