Some people should not be taking certain types of prescription drugs because of genetic variations in drug metabolism. DNA affects the safety and effectiveness of a drug.

Pharmacogenetic testing can help a doctor determine what type of drug, if any, should be prescribed for a health condition. Test results would make a psychiatrist, for example, aware of polymorphisms in the cytochrome P450 gene family that might cause someone taking an antidepressant to become suicidal, violent or homicidal.

In some countries, such as the Netherlands, pharmacogenetic testing is becoming an important part of the healthcare system. Many people in Holland carry a card with test results that they share with their doctor before a drug is prescribed.