In-depth article about how genetics informs how you process drugs, and also how it affects drug interactions, and the genetic tests used to determine if you will likely react badly to a drug or not. It's written by a forensic medical examiner. I've only skimmed it but seems well worth a read through.
CYP Testing to Help Prevent Dangerous Adverse Drug Reactions
January 24, 2017
Readers of this website might be aware that antidepressants can cause suicide, other violent behavior and even homicide. These can be side effects or adverse drug reactions from the medication taken. Not only can antidepressants cause these side effects, but basically every psychoactive medication can put patients at risk. Few people may know that there are DNA tests that can identify individuals who might be prone to these adverse drug reactions.1
To understand what kind of information such a test would provide, it might be helpful to explain the science behind these DNA tests.
In general, human cells contain 23 pairs of chromosomes. The father donates half of the chromosomes; the other half comes from the mother. Every chromosome contains many genes. A gene is the part of the DNA that codes for proteins, and proteins cause hereditary characteristics to be expressed. A gene can have two forms, called alleles. If someone inherits the same allele from the father and the mother, the person is called homozygous for that trait; if they are different, the person is heterozygous.
Medication needs to be metabolized to be expelled from the body. This is done by certain proteins called enzymes. Most medications that interact with brain chemistry are metabolized by an enzyme system called Cytochrome P450 (also known as CYP450 or P450). There are many different P450 enzymes, and they are divided into families and subfamilies. Cytochrome P450 family names are denoted by an Arabic number (e.g., CYP2), the subfamily by a Roman uppercase letter (e.g., CYP2D), and the individual enzymes by another Arabic number (e.g., CYP2D6). The alleles are indicated with an asterisk and a number, separated by a forward slash.
More at above link...