Why do some individuals require minimum dosages of pharmaceuticals?

Why do some individuals require minimum dosages of pharmaceuticals?
A minimum dosage of pharmeceuticlas for me is NONE. I avoide them like the plague. I know drug is necessary surrounded by certain applications but it have become such a big business and so commercial that people give somebody a lift them at the drop of a hat and that is to say bad.
Well the invigorating index(measure of how well a drug works) can be at variance for each human being depending on certain physiological factor e.g weight, where on earth the drug is metabolized, how well is it metabolized, splanchnic blood flow. So a minimum dose is a precaution.