That's exactly what happened with aspirin. In 1897, chemists at Bayer developed acetylsalicylic acid into a medicine that was gentler on the stomach than salicylic acid.
It quickly became one of the world's most popular treatments for pain and fever. Doctors knew it worked. They just didn't know why. For decades, aspirin was prescribed millions of times every year while its molecular mechanism remained a mystery.
Global Chemical Database