All medicines are drugs. Drugs are substances that change how our bodies work, potentially including our minds. Some are pretty safe and legal, some are extremely dangerous and illegal. But pretty much any drug can be harmful or even lethal in large enough doses. There's plenty of completely legal medicinal drugs you can buy over the counter and keep in a medicine cabinet that could kill you or cause permanent damage in the kind of amounts of Benadryl you seem to have taken.

Consider yourself lucky that you seem to have come out of this incident fine, and in future please read the label, good god, or you'll be explaining at the pearly gates that your headache was too bad to check the instructions.