The term "shelf life" of a drug slightly differs from a drug's "expiration date." The shelf life generally relates to a drug's quality over a specified period of time, whereas the expiration date relates to both quality and safety of a medication at a specific point in time.
A drug that has passed its shelf life might still be safe for consumption, but its quality is no longer guaranteed. Shelf life is variably influenced by storage conditions, such as exposure to heat, light and moisture. A drug's shelf life is usually shortened if it is not stored in its original container. In manufacturing terms, shelf life can also mean the time that elapses between when the drug is made and the expiration date assigned by the manufacturer.
Expiration date and safety
The expiration date of a medication indicates the date the manufacturer or pharmacist guarantees the full potency and safety of a drug. Some medications might retain potency and might be safe briefly following the expiration date, but you must check with a pharmacist about specific medications. Generally, it is not advisable to use a drug beyond its expiration date, unless directed by a pharmacist. Some drugs, such as tetracycline antibiotics, can even be dangerous if used after their expiration date.
Which date to follow
If a bottle has both the shelf life and expiration date listed, then it is advisable to use the earlier date as the time to avoid consumption. However, better still would be handing the dosage unit to your pharmacist so he or she can give you specific advice.