What is the shelf life o dimenhydrinate? does it become harmful, or just lose effectiveness?

Usually six to 12 M: This is also called Dramamine or Gravol. You should check on the bottle or on the card the date should be printed. On an average it's six months. Manufacturer puts expiration date on every batch of drugs based on it's potency. If you torn the part of the card showing expiration date please go to pharmacy where you got it as they should have purchase time and they can tell do not take expired drug