Our local pharmacist told one of my diabetic dog owners that the human NPH insulin she is using for her dog would only be effective for 30 days. After that time, the pharmacist claims the vial should be discarded and replaced with a fresh vial of insulin.
The pharmacy also told the owner that the insulin should not be refrigerated.
I'm confused. I always thought that NPH insulin should be refrigerated and is good 'till the last drop!"
The pharmacist is right and wrong in his advise — it just "depends."
In general, open NPH insulin vials in human diabetic patients are kept unrefrigerated to minimize local injection site irritation, which may occur after injection of cold insulin solutions.
Human patients are also told to replace the insulin with a new vial every 4 to 6 weeks. Why? Because the insulin my lose some of its potency after the vial has been in use for >30 days when stored at room temperature.
In both humans and animals, we can recommend general guidelines for the storage and handling of NPH insulin preparations, which include Humulin N, Novolin N, or Humulin/ReliOn NPH (1).
- All opened insulin vials should be inspected daily for physical changes, such as clumping, frosting, precipitation, or discoloration, that may be accompanied by a loss of potency.
- Insulin vials should be optimally stored at refrigerated temperatures (36–46°F).
- The insulin vial should never be allowed to freeze.
- Insulin vials should never be used after the expiration date printed on the label and carton.
- Unrefrigerated insulin vials that are in use should be kept cool and away from excess heat or sunlight.
Many insulins have good potency for 6 months; however, contamination from multiple sticks can be an issue, so many veterinarians recommend switching every 3 months or so.
Whenever human insulin vials are stored under refrigeration and are used beyond 30 days, the stability of these vials may be affected by a number of factors, including:
- the number of injections per day
- the volume of insulin remaining in the vial
- exposure to light,
- degree of agitation used for dose preparation
But you are correct: in many diabetic dogs, the NPH insulin is indeed good "till the last drop" is given.
- Grajower MM, Fraser CG, Holcombe JH, et al. How long should insulin be used once a vial is started? Diabetes Care. 2003;26:2665-2666.