Good question. I have no idea. Some people are just more reactive physically to things and have a tendency to react more to meds than others. I tend to have very few med reactions. My sister, on the other hand, always had pretty severe reactions to all meds. Same genetics. Different reactions. She was more allergy-prone (very much so actually), so perhaps that had something to do with it. I have no idea. I have allergies, but nothing too terribly out of the ordinary. Her general health was less stable: blood pressure, type-2 diabetes, severe allergies, several bouts with cancer, etc. I'm pretty much in good health. Is that it? I have no idea. Metabolism? Could be. Who knows?

I suspect there are a variety of causes of med side effects and actually predicting them is at this point a guessing game. I think medical science is LONG way from accurately being able to predict med reactions before actual trial.