Unopened Humulin R should be stored in the refrigerator. Do not freeze insulin or use any insulin that has been frozen. If you have an opened vial, pen or cartridge of insulin, store it at room temperature. Once opened, you may keep using Humulin R for 31 days or until the container is empty, whichever occurs first. Once opened, you may keep using Novolin R for 42 days or until the container is empty, whichever occurs first. Monitor the expiration dates on insulin containers and throw away any insulin that is out-of-date. If you have any questions or concerns about insulin storage, ask your doctor or pharmacist.