My zpak is a year past expiration, is it still effective?

Probably effective: Drug expiration dates usually don't mean 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 more side effects, only that it hasn't been studied to know for sure. Most drugs remain safe and effective for years if not mishandled, e.g. stored in a hot car etc. Still, best not to treat yourself; check w/ doctor first.