Most drugs, including MOST antibiotics remain effective past their expiration date. There is a potiential loss of potency depending on storage conditions ( especially temp and possibly humidity{depending on packaging}) as well as the passage of time. HOWEVER TETRACYLINE clearly develops toxicity when used past expiration.
+1. When tetracycline expires, throw it out! Modern (aka post 1980) manufacturing makes that family of antibiotics generally safe past the expiration date, but don't risk it with that one.