By ANDREW POLLACK


For more than two years, Jody Uslan had been taking the drug tamoxifen in hopes of preventing a recurrence of breast cancer. Then a new test suggested that because of her genetic makeup, the drug was not doing her any good.

“I was devastated,’’said Ms.Uslan, 52, who stopped taking tamoxifen and is now evaluating alternative treatments.“You find out you’ve been taking this medication for all of this time, and find out you are not getting benefit.’’

Ms.Uslan’s situation is all too common - and not just among the hundreds of thousands of women in the United States taking tamoxifen.

Experts say that most drugs, whatever the disease, work for only about half the people who take them. Not only is much of the approximately $300 billion annual drug spending in the United States wasted, but countless patients are being exposed unnecessarily to side effects.

No wonder so much hope is riding on the promise of“personalized medicine,’’in which genetic screening and other tests give doctors more evidence for tailoring treatments to patients, potentially improving care and saving money.

Many policy experts are calling for more studies to compare the effectiveness of different treatments. One drawback is that such studies tend to be“one size fits all,’’with the winning treatment recommended for everybody. Personalized medicine would go beyond that by determining which drug is best for which patient, rather than continuing to treat everyone the same in hopes of benefiting the fortunate few.

The colon cancer drugs Erbitux and Vectibix, for instance, do not work for the 40 percent of patients whose tumors have a particular genetic mutation. The Food and Drug Administration held a meeting recently to discuss whether patients should be tested to narrow use of the drugs, which cost $8,000 to $10,000 a month.

And a genetic test might help doctors determine the optimal dose of warfarin, a blood thinner used by millions of Americans. Tens of thousands of them are hospitalized each year because of internal bleeding from an overdose or a blood clot from an inadequate dose.

“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.

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. recently 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.’’

Despite all the obstacles, personalized medicine is coming. Even the drug companies, which have been worried that testing would reduce their sales, are starting to realize that their medicines might not be approved or paid for without better evidence that they work.

Last year, for instance, European regulators said Amgen’s colon cancer drug Vectibix did not provide enough benefit to patients to be approved.

So Amgen reanalyzed the data from its clinical trial.

After the results showed Vectibix worked better in patients whose tumors did not have a mutation in a gene called KRAS, the drug was approved for those patients only.