Aspirin is a bad actor in this regard. It tends to revert back to salicylic acid, which is ok for some specific topical uses, but has potential to be pretty harmful orally. Sniffing an old bottle of aspirin will tell you whether they're still good enough

For the record, what you're smelling for is acetic acid (vinegar). When aspirin (acetylsalicylic acid) hydrolyzes into salicylic acid, the other product of that reactions is acetic acid, which is what's in vinegar.

Thanks! It's been 25 years since my last pharmacology class, the details are all getting a bit vague.

It's not that harmful if you know what you're dealing with. Have to be careful about liver toxicity and dosage but in a pinch it will work just fine as an antipyretic. It was used before aspirin has been invented.