I keep mine in a freezer when not in use. It should last years if stored in that manner. Just remember to warm it to room temperature before removing the lid. Otherwise moisture from the air will condense on the medication and reduce its long-term stability. If you store it in a frost-free freezer, don't store it in the door and also try to keep it away from the sides. If stored improperly, it could possibly undergo a number of freeze/thaw cycles, which would also reduce its stability. If possible, store it in a non-frost-free freezer or bury it among other frozen materials so it won't experience a lot of temperature swings.