How does pharmacogenomics work in practice?

One example is thiopurine methyltransferase (TPMT) testing for people who are candidates for thiopurine drug therapy. Thiopurine drugs are used to treat some autoimmune disorders, including Crohn's disease and rheumatoid arthritis, as well as some types of cancer, such as childhood leukemia.

The TPMT enzyme helps break down thiopurine drugs. People who are TPMT deficient don't break down and clear out these drugs quickly enough. As a result, the drug concentration in the body is too high and increases the risk of side effects, such as damage to the bone marrow (hematopoietic toxicity).

Genetic testing can identify people with TPMT deficiency so that their doctors can take steps to reduce the risk of serious side effects — by prescribing lower than usual doses of thiopurine drugs or by using other drugs instead.