Expired medications - bad?

[i3k]

Wow havent been on this forum in a long while. But here is a quick question. Is it a bad idea to use expired anti-biotics like ampicillin by aquafish. I have a fish who's tail would "rot" slowly and the top fin as well. It actually looked like the tail was burnt. I did water changes every 10 days about 30% changes and nothing helped to stop the "eating up" of the tail. I found a bottle of ampicillin i purchased long time ago and the expiration date is 11/04. I decided to use the pills and bam 5 days later the tail is about 90% regenerated. So what exactly does expiration date mean?

[theotheragentm]

Just like drugs for humans, the efficacy of drugs is much longer than listed. I always keep codeine cough syrup for a long time, because it's effective for a long time when the next illness rolls around. I think they assume it's store in less than ideal conditions, which may denature the medications.