Plagues and other great diseases are important parts of history and some settings. They defy man and may even defy God in their swift and terrible punishment upon the body of society.
What if potions and tonics had negative side effects? Potions of increased strength cause violent tempers, or damage joints. What if potions of dexterity caused tremors, or palsy?
What if they were addictive, and had withdrawal symptoms?
Could someone make a bad batch? No one use the brown strength potion!