Drug manufacturers list an expiration date for their products, so codeine does expire in abidance by that figure. Most drugs expire within 5 years. However, according to CNN Health, drugs as old as 40 years, including codeine, may still be as potent as when they were when manufactured.
Furthermore, CNN states that drugs must fall within an acceptable range determined by the FDA when determining how much of the active ingredient is present. This range is between 90 and 110 percent. Many drugs maintain these levels even years after expiration, thus making many expired drugs reasonably safe. Opening the drug container or storing it in less than optimal conditions may alter the expiration date due to humidity, light and temperature.