If someone turns to an old bottle in the medicine cabinet and notices a faint vinegar smell, it’s easy to assume the aspirin has gone bad and should be thrown away. The presence of acetic acid in a bottle of aspirin is perfectly normal. The reaction of aspirin with water to form salicylic acid and acetic acid. If that aspirin is 99.99% pure, the remaining 0.01% corresponds to roughly 3 milligrams of acetic acid, produced by aspirin hydrolysis. So, when you open an old bottle of aspirin, it is common to smell vinegar.