Although it is difficult to know which products could have an extended shelf life, in general, most medicines (with a few exceptions) retain at least 70 to 80% of their potency for at least one to two years after their expiry date.

This means that Benadryl solution that has expired six months previously should work, especially if it has been stored in a dry cool space away from light in its original container. There is a possibility that it may not be quite as effective, but it is unlikely to do you any harm.

What is an expiry date?

An expiry date is the date up until which the drug manufacturer can guarantee that the medicine is fully potent and safe to take based on product testing. Expiration dates are typically conservative to make sure you get what you paid for—a fully potent and safe medicine.

Generally, solid dosage forms, such as tablets and capsules, appear to be most stable past their expiration date. Drugs that exist in solution or as a reconstituted suspension, and that require refrigeration (such as amoxicillin suspension), may not have the required potency if used when outdated. Loss of potency can be a major concern, especially if treating an infection with an antibiotic, and a substandard antibiotic increases the chance of antibiotic resistance developing.

Storing medicines properly increases the chance of them having an extended shelf life. The bathroom and medicine cabinet are not ideal places to store medications due to heat and humidity. Similarly, medications should not be left in a hot car or glovebox, or in freezing weather. Medications remain most stable in dry, cool spaces away from light. Keep the prescription bottle caps tightly closed and always keep medications out of reach of children and pets.

Since 1986, the U.S. Department of Defense (DoD) in cooperation with the U.S. Food and Drug Administration (FDA) have administered the federal Shelf Life Extension Program (SLEP) which extends the expiration dates on qualifying drugs and other materiel in federal stockpiles. Testing is undertaken on items eligible for SLEP, and products that pass testing are granted extended expiration dates but must undergo ongoing testing to monitor their continued shelf life. Products that fail testing at any time are destroyed. Products that do not receive additional extensions of their expiration dates or are not tested for SLEP are destroyed at their final expiration dates. Drug expiration extension dates on products have ranged from 12 months to 184 months (over 15 years).

There are some products that should not be used past their expiry date, such as:

• Reconstituted antibiotic suspensions
• Biologics
• Blood products
• Injectable drugs that look cloudy or which have precipitated
• Insulin
• Nitroglycerin
• Probiotics
• Tetracyclines
• Vaccines
• Any medicines that looks old, powdery or crumbly with a strong smell.

For more information see Drug Expiration Dates - Are Expired Drugs Still Safe to Take?