Pharmacogenomics combines the science of your personal genomic profile and the science of pharmacology to understand the genetic basis of medication response. For example, your genes that code for metabolism of a certain drug may make you eliminate that drug from your body too slowly, resulting in unpleasant or toxic side effects. However, someone else may metabolize the same drug at the same dosage at a "normal" rate, and have no adverse effects as a result.  

Conversely, your genes may code for metabolism of a certain drug that makes your body eliminate that drug too quickly. This results in an ineffective therapy, because your blood levels are not high enough to be therapeutic.

In yet another different but common example of precision medicine utility, some drugs require a specific enzyme to break down the ingested drug into an active form the body can use. In this case, the initial drug is called a prodrug. If you lack the gene that codes for that enzyme's production in your body, then you will not convert the prodrug into the active form. Your body would then get no therapeutic benefit from the medication. Increasing the dose of the medication in this example compounds the problem! Accumulation of the prodrug may cause side effects while you still get no therapeutic benefit because there is still no conversion to active drug! Precision medicine can identify if your body has or lacks the genes to produce these key enzymes, and guide your medical providers in choosing the correct therapy.