Myth: if you can buy a drug at a drugstore without a perscription, it is not dangerous. The average American household contains about 30 different drugs. Look in the medicine cabinet, and read some of the labels.
From a non-perscription allergy medication box: "Avoid driving a motor vehicle ... while taking this product." From a non-perscription decongestant box: "Use caution when driving a motor vehicle..."
Have you ever noticed these labels before? Notice what kind of medication they are on: "harmless" drugs, drugs you would take without second thoughts. And these labels are on the drugs for good reason. Many drugs can have unwanted side effects and adversely affect driving. This is not to cause undue panic; it may be perfectly safe to drive under the effects of some over-the-counter drugs. But the lesson is this: always read the labels on a drug before using it.
The time when over-the-counter drugs become especially problematic to driving is when they are taken in excess. "More is better" is not the correct attitude to use with over-the-counter drugs. "If a little cold medicine helps, a lot can do wonders," people think. But no drug is harmless, and any drug will kill you if you take enough of it. When taken in excess, the effects of medicines are unpredictable.
Take over-the-counter drugs with caution, and make sure to read and obey the labels.