So in my story, my character gets a cold and their guardian is incapable/fearful of making them a Pepperup Potion, so they buy some from an apothecary. Unfortunately, the ingredients were mixed up, making them more sick. What would make a Pepperup Potion to harm someone?

Also, while we're on the subject of medicine, I'd also like to know what other potions could be used or have been mentioned. And how do wizards think of medicine and the human body, anyway?