Suppose you're in some emergency and you need medicines at the earliest. It can be pain from an injury, or a headache, or misery from indigestion or cold. You delve into your stash of over-the-counter medicines that you have stored over time and take one you've taken many times before. But, two hours later, you have a rash or stomach pain or some other odd new problem. Is it related to the medicine you've taken? But, you've taken it so many times...