Well, sometimes they figure out that if you were given a certain medicine, for instance Prozac, and you react to it in a weird way, that it meant you were NOT "A" and more like have "B". And thats the "only" way they can tell. Where a an b are bipolar or low on dopamine or low on serotonin or honestly i dont know what all.

Can a drug cause a disease? Would that be considered a (bad) side effect? Cuz otherwise why would you take the drug to begin with?

But it sounds like you are saying the drug is causing you to have reactions you didnt have before. But isnt that how it is supposed to work? Who decides if those reactions are good or bad, or have positive or negative effects on your life? For instance, helping you gain or lose weight is dependent on what the person needs. Or being more or less aggressive. Those things arent necessarily good or bad, or illness or disease.