Question: Why do some drugs affect certain people in different ways than others?

Florence McCarthy answered on 11 Nov 2015:

We are all genetically different (even if only by a very small amount) and our genes code for the make-up of our metabolism which is how we convert what we put into our bodies in to what is useful and what is waste. Our metabolism is run by the enzymes that we produce and because we are all different genetically we make different levels of enzymes. This means that a person with a high level of a certain enzyme will convert a drug into waste much faster than someone with a lower level of the same enzyme. This means that the person with the high enzyme level will convert the drug to waste faster and see less of the intended effects of the drug.