"It's like playing Russian roulette when a doctor prescribes a newly approved drug that doesn't have a big breakthrough," said Dr. Sidney Wolfe of Public Citizen Health Research Group, one of the researchers who worked on the study.
Pressure from pharmaceutical companies to approve drugs and doctors' failure to closely read warning labels are partly to blame, the researchers said. They said the findings should prompt the Food and Drug Administration to consider raising its threshold for approving new drugs when safe and effective alternatives already exist.
The findings are based on an analysis of 548 drugs approved from 1975 through 1999. Of these, 56, or more than 10 percent, were later given a serious-side-effect warning or taken off the market for safety reasons.
The number climbed to approximately 20 percent when researchers considered only drugs that were approved toward the end of the period studied.
The study, led by Dr. Karen Lasser of Cambridge Hospital and Harvard Medical School, appears in today's Journal of the American Medical Association.
An accompanying editorial by two FDA experts said the analysis overstates the problem.
Safety studies that are conducted before a drug wins approval typically involve a few thousand patients and may not detect all side effects, especially relatively rare ones, Drs. Robert Temple and Martin Himmel wrote.
"Frequent post-marketing label changes are therefore inevitable and should be anticipated," they wrote.
Temple also noted that some medications cause side effects in only certain groups of patients, such as pregnant women.
The study analyzed what are known as "black-box" warnings published in the Physicians Desk Reference, a compendium of drugs and labeling information published annually. Black-box warnings highlight the most-serious side effects.
Sixteen drugs studied were withdrawn from the market, nearly half of them more than two years after their approval.
They include the diabetes drug Rezulin, which was approved in 1997 but has been linked to dozens of cases of fatal liver damage. Lasser said doctors continued to prescribe it an unsafe manner even after it was given a black-box warning, and it was ultimately withdrawn from the market in 2000.
Two allergy drugs, Seldane and Hismanal, were linked with potentially fatal heart problems in certain patients but were not removed from the market until several years after receiving black-box warnings.
Most of the troublesome new drugs do not represent any advance in treatment and are at best no better than older, safer drugs already on the market, Wolfe said.
Unless a new drug is a breakthrough, it should be avoided until its safety record is better known, the researchers said.
Wolfe said the FDA is correct in saying doctors do not pay enough attention to warning labels, but that is "all the more reason to do the right thing on the front end. The remedy should be don't put the drug on the market unless it's a breakthrough drug."