“If you save one hospitalization for every 100 new warfarin users, you more than offset the cost of testing all 100,” said Dr. Robert S. Epstein, the chief medical officer of Medco Health Solutions, which manages prescription plans for employers. The test typically costs $100 to $600.

Image Dr. David Flockhart discovered how the body converted the medication tamoxifen. Credit... A.J. Mast for The New York Times

For all the potential, experts see some formidable obstacles on the path to the promised land of personalized medicine.

“It’s going to take 20 to 30 years for all this to fall into place,” said Dr. Gregory Downing, who heads efforts by the Department of Health and Human Services to spur personalized health care.

The hurdles include drug makers, which can be reluctant to develop or encourage tests that may limit the use of their drugs. Insurers may not pay for tests, which can cost up to a few thousand dollars. For makers of the tests, which hope their business becomes one of health care’s next big growth industries, a major obstacle is proving that their products are accurate and useful. While drugs must prove themselves in clinical trials before they can be sold, there is no generally recognized process for evaluating genetic tests, many of which can be marketed by laboratories without F.D.A. approval.

Genentech, a developer of cancer drugs, petitioned the F.D.A. this month to regulate such tests. It warned of “safety risks for patients, as more treatment decisions are based in whole or in part on the claims made by such test makers.”

A cautionary case is Herceptin, a Genentech breast cancer drug that is considered the archetype of personalized medicine because it works only for women whose tumors have a particular genetic characteristic. But now, 10 years after Herceptin reached the market, scientists are finding that the various tests — some approved by the F.D.A., some not — can be inaccurate.

Moreover, doctors do not always conduct the tests or follow the results. The big insurer UnitedHealthcare found in 2005 that 8 percent of the women getting the drug had tested negative for the required genetic characteristic. An additional 4 percent had not been tested at all, or their test results could not be found.