Dina Fine Maron in Scientific American:
Doctors are not accustomed to making medication choices using genetics. What they have done, for decades, is to look at easily observed factors such as a patient’s age and weight and kidney or liver functions. They also considered what other medications a patient is taking and any personal preferences. If clinicians would consider genetics, here is what they could learn about prescribing the common painkiller codeine. Typically the body produces an enzyme called CYP2D6 that breaks down the drug into its active ingredient, morphine, which provides pain relief. Yet as many as 10 percent of patients have genetic variants that produce too little of the enzyme, so almost no codeine gets turned into morphine. These people get little or no help for their pain. About 2 percent of the population has the reverse problem. They have too many copies of the gene that produces the enzyme, leading to overproduction. For them, a little codeine can quickly turn into too much morphine, which can lead to a fatal overdose.
…Still, fewer than 10 hospitals around the country—including Maryland, Vanderbilt and St. Jude—are offering pharmacogenomic tests to certain patients. The other main obstacle to wider use, besides reimbursement, is the lack of a prescribing road map. Many doctors were educated in an era before such testing was available so they do not even think to order them. And a lot of physicians would likely find they are not equipped to understand the results.