I know that the manufacturer Underestimates the date for expiration.....I just don't know exactly HOW much. I know that several people (who were trustworthy sources of info to ME) have used Rx that were over one year passed their Exp dates. They describe the Rx being effective. Didn't get into the specifics of temp, quality of packaging, storage of or how the Rx looked when they used it though. I had assumed that it had been still in its sealed package and not growing anything. Depending on the medicine and my need for it....I may use it if the Risk/Benefit ratio was more B than R.