Dixie I suppose any med. can "go bad" over a long period of time especially if not refrigerated or kept in dark place if the instructions advise such. Some will only turn dark or get thicker. Meds that are in dry form used as a topdress often have an expiration date and almost anything once opened begins to deterioriate. Rather than take a chance I'd probably buy new meds if I felt mine were to old to be effective.