Some people say a lot of stuff. Virtually no meds go from effective to poison on the expiration date. Drug regulators frown on that😉. Do a Google or Google Scholar search on the generic drug and for every single one there'll be studies on long term storage and efficacy. Ignore fish websites like the plague on this. I also don't think it's likely you'd get written info from a manufacturer that claims anything beyond the stamped date itself. Ignore my conclusion, too, and go for the research.
The above being said, if I were in Europe, where drugs are unavailable to the average aquarist, I'd go for it without qualm (who knows if customs might confiscate future shipments). Actually, I'd use the Maracyn without giving it a second thought. Be sure to store meds in a cool dry place. Beyond that you can extend storage of some dry meds in the fridge or freezer, taking care to bring the drug to room temp before opening to avoid condensation. Some meds, particularly liquids, can be/need to be to be refrigerated but not frozen.
And then, for what it's worth, my understanding is that some antibiotics are more sensitive to long term storage conditions and time than typical meds. For most meds a predicable decrease in potency can be compensated for by an increase in dosage, but some aquatics may be too sensitive for this course of action. I'm afraid I can't help you there.
Cheers! Steve Herje
