American visitors to developing countries should not take daily doses of antibiotic drugs as a means of preventing travelers' diarrhea, a National Institutes of Health consensus-development panel said yesterday.

Although conceding the effectiveness of this increasingly popular practice, the panel concluded that because the disorder -- with its characteristic diarrhea, cramps and low-grade fever -- strikes only one in three travelers, the risk of the drugs to the two who would not become ill outweigh the benefit to the third person. The panel noted that the condition is not fatal and rarely lasts more than a week.

The panel also recommended against the use of Pepto-Bismol as a means of preventing travelers' diarrhea, though it, too, has shown effectiveness when taken in large quantities.

The two major antimicrobial agents tested as preventatives -- doxycycline and trimethoprim, taken alone or with sulfamethoxazole -- were cited by the panel as useful and effective.

Pepto-Bismol and two anti-motility drugs, diphenoxylate (Lomotil) and loperamide (Imodium), were suggested for mild cases. Travelers concerned about becoming ill are encouraged to carry these drugs and packages of a salt-sugar mixture that can be added to water for an effective rehydrating fluid in severe cases.

"Rapid institution of treatment," the panel wrote, constitutes "a more sensible approach that can shorten the disease to 30 hours or less in most people."

Most cases of travelers' diarrhea are caused by the E. coli bacterium, generally found in food or water contaminated by fecal matter.

The panel recommended dietary precautions as the first line of defense for the estimated 8 million Americans who travel annually to less-developed nations, such as Mexico and Haiti. Throughout the panel's 2 1/2 days of hearings and discussion, there were repeated references to the old travelers' admonition: "If you can't boil it, cook it or peel it, forget it."

Some international researchers asked for more flexibility -- at least recognition that because the medications work, "people will continue to use drugs prophylactically, and they should understand the risks and work it out with a physician ahead of time."

But Dr. Sherwood Gorback, a Tufts University microbiologist and panel chairman, said the group thought that few travelers visit a doctor before a trip and that "the panel didn't make rules, only recommendations."