What does a medication's expiry date mean? does it become toxic or just less potent? is it a must that i throw it away past the date?

Not necessarily: The expiration date often doesn't mean very much. If potency and side effects aren't tested beyond a certain time, that's the expiration date. It doesn't mean potency is gone or side effects increase, only that it hasn't been studied to know for sure. Almost all drugs remain safe and effective for years if not mishandled, e.g. stored in a hot car etc. Always best to replace, but usually OK if not.