The specific dangers are unknown due to the nature of the drug itself.  The reason there are expiration dates on drugs because OVERTIME, THE EFFECTIVENESS OF THE DRUG MAY BE REDUCED DUE TO THE BREAKDOWN OF SPECIFIC CHEMICALS IN THE DRUG ALONG WITH EXPOSURE TO THE ELEMENTS INCLUDING AIR, MOISTURE, AND OTHER CONTAMINANTS. Therefore it is wise to dispose of expired medications by flushing down the toilet or depositing securely in the trash.  Antibiotics can naturally lose their effectiveness over time.  Although there are not many cases of harming the body due to expired effectiveness, the fact that antibiotics have to be taken over a specific amount of time(usually ten days) can give a user the false impression that they are being cured from the infection when in fact they are not.