Medicine. Most of the things we consider medicine are only helpful if we're already in bad condition, taking medicine while being completely fine can have terrible side effects on your body.
Democracy, it keeps away a tyrannical overlord from controlling the control, but sometimes it's impossible for different sections of government to agree on certain things, and this prolongs the process sometimes making democracy seem a little unreliable compared to the fast paced solidified decision of a single ruler.
Eating. Doing anything in excess can be harmful, but especially eating in the case of most Americans. Even with the health reforms of today, and dietary programs, many Americans are obese and suffer from type 2 diabetes simply from over-eating.
Those are just three examples I could think of.
