Man is a gene, you'd look at this statement twice, but actually every character or feature of a man defines him, these features are in turn defined by one's genes, Pharmacogenomics is the main tool for the actualization of precision medicine. Drugs are metabolized by a group of enzymes which are specialized in the breakdown of foreign substances in the body, these are known as xenobiotics. These enzymes includes the Cytochrome P450 which is the most versatile biocatalyst.

However, variant genes codes for these enzymes, the variation in the genes which codes for these enzymes is the main reason for the adoption of precision medicine, hence in the future, one's genetic make up will be studied before prescriptions,this is to meet his/her metabolic ability as regards the drug in question.
You'd ask why, but this variation classifies individuals into different hierarchy based on their ability to metabolize drugs, while some are rapid metabolizers because they have more efficient genes, many others are slow metabolizers because they posses genes which are somewhat less superior. Precision medicine is the next big thing in world medicine.
Kudos man, nice write up.