I'm no expert, but I guess medicines have more than one ingredient. If you were seriously trying to avoid illegal substances, you would check everything.
The only excuse an athlete could have, IMO, is that their food/drink was laced. Even then, you could take reasonable steps to avoid this.
If you were a professional athlete, wouldn't you check each and every medication you took, before you took it? Be it for ADHD, a bad back or a headache?