I wanted to shed some light on why there is a discrepancy between how different patients respond to the same medication. Simply put, when a medication is taken orally, it gets absorbed from the gastrointestinal tract and passes through the liver where a group of enzymes called the Cytochrome P450 system metabolize the medication. Some patients have minor mutations to these enzymes (and there are different classes of enzymes that act on specific medications) that either causes them to break the medication down slower or quicker than the average person.
Those who break things down slower cause the medication to build up in their system potentially increasing the number of side effects. The solution would be to either lower the dose or change to another medication that is not metabolized by that enzyme.
For those patients who have a mutation that causes them to break down the medication more quickly than the average person, they will generally require a higher dose, or again, switch to a different medication that is broken down normally by another set of enzymes.
Thus, part of the art of medicine is knowing that everyone has the potential to have a minor or major mutation in their liver enzymes that metabolize medications. Laboratories are now coming out with various tests (such as a saliva test) that can not only find out if a patient has a mutation but allows the physician to prescribe another medication in advance to avoid any issues. I should note that these mutations are not the same as having an allergy to a medication. An allergy is a separate issue where the body views the medication as a foreign invader and sets off an allergic reaction. The mutation just determines if our body metabolizes medications correctly or not.
