updated 4/9/2009 8:30:01 AM ET 2009-04-09T12:30:01

Here's a shocker: Due to differences in DNA, up to 60 percent of the most common drugs are associated with adverse reactions.

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This includes medication used to treat common conditions like hypertension, heart failure, depression, high cholesterol, and asthma. Hence the hope being pinned on "pharmacogenetics," a field of medicine that promises to improve health care by allowing doctors to customize medical treatment to suit a person's unique genetic signature.

Though experts predict that it could be decades before personalized medicine becomes the norm, research is moving ahead: Last fall, for instance, researchers at Duke University reported that people with a specific genetic variant saw less reduction in LDL, or "bad" cholesterol, when taking statins.

But for some drugs, the future is now.

A genetic test recently approved by the FDA should help doctors determine the optimal dose of warfarin (sold as Coumadin), a blood thinner used by 1 million Americans. Determining the right dose is crucial: Too much may result in an increased risk of excessive bleeding, while too little may cause a potentially fatal blood clot. By one estimate, using DNA analysis to prescribe warfarin would prevent about 17,000 strokes and 85,000 serious bleeding incidents.

A small but growing number of doctors and hospitals are also using genetic testing to tailor treatment for these medicines:

  • Tamoxifen — DNA testing identifies the 8% of women with genetic variants that keep them from metabolizing the breast cancer drug, rendering it ineffective.
  • Painkillers like codeine — Up to 8% of whites and 2% of Asians and African Americans are poor metabolizers of these drugs and won't get relief from them; for the 1% of "ultrarapid metabolizers," risks include respiratory problems.
  • Antidepressants and antipsychotics — Some of these drugs are metabolized by the CYP2D6 and CYP2C19 genes. In 2005, the FDA approved a test that looks for these gene variations, and now companies sell consumer versions.

But experts advise against using the at-home tests without having your doctor interpret the results, notes Julie Johnson, PharmD, professor of pharmacy and medicine at the University of Florida.

The reason: These genes are involved in the metabolization of 25% of all prescription drugs, including several where they're very important. If you misinterpret the results of an at-home test (and mistakenly think you don't have the gene), you might avoid taking one or more drugs you really need.

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