The amino acid tryptophan is available as an over-the-counter supplement in the US. It's often popularly blamed for post-Thanksgiving-dinner drowsiness, as it naturally occurs in turkey meat, but I recall seeing a debunking that the levels there are probably meaningless compared to other tiring rituals/portions of that traditional dinner.
In the US, prescriptions come with a long insert with tiny print with tons of drug information, and usually a smaller single-page larger-print insert mentioning the key dosage, side-effect, and counter-indications/incompatibility considerations.
It's also common for the prescribing doctor, and the separate pharmacist who usually 'fills' prescriptions, to add oral warnings about major considerations. These quite commonly include a "don't drive after taking" clause, or something to the effect of: "this makes many people drowsy, get used to its effects for a few days before doing anything risky".
Your point is well taken -- anyone given a new drug should learn as much about it as practical before taking it, from the person delivering it and other independent sources in print or via the web.