The expiration date for Keflex is found on the printed package instructions included with the dispensed drug, according to MDhealth.com. As explained on Drugs.com, the expiration date for Keflex and other drugs is determined by the manufacturer and varies based on the drug's production date. Do not take expired drugs.
Unless a particular drug is known to spoil more quickly, pharmacies typically mark the expiration date as a year from when it is dispensed. However, it is never safe to assume that the drug is acceptable to take beyond the date marked. Any number of issues may result when a drug passes its expiration date. Some drugs lose their potency. Other drugs change in ways that alter their anticipated effects in potentially dangerous ways.
As the University of Utah Health Care explains, expired Keflex and other drugs containing the active ingredient cephalexin must be thrown away. Drugs.com states that manufacturers do not make any recommendations for the usage of drugs that have passed their expiration date. RxList suggests checking with a pharmacist to determine how best to dispose of expired drugs.