With back-to-back outbreaks of a potentially deadly strain of E. coli – one traced to California spinach in September and now others linked to products sold by Taco Bell and Taco John's – some experts are renewing warnings against treating the symptoms with antibiotics.

For at least five years, the federal Centers for Disease Control and Prevention, the American Medical Association and other medical groups have advised against using antibiotics in treating E. coli infections because of evidence that they can make patients sicker.

But some doctors still prescribe antibiotics in E. coli cases because they are unaware of the advice, misdiagnose the still-rare E. coli O157:H7 or do not find the relationship between antibiotics and greater medical risk convincing.

The studies exist, "but I don't know how well-known they are," said Dr. David Acheson, chief medical officer in the Food and Drug Administration's food-safety center and one of several experts to warn against antibiotic use in treating the E. coli strain.

A 2000 study found that children infected by E. coli O157:H7 and given antibiotics developed a life-threatening complication called hemolytic uremic syndrome at higher rates than children who did not take antibiotics. The syndrome is the leading cause of kidney failure in children.

It is unclear whether antibiotics have contributed to the severity of the spinach- and taco restaurant-related outbreaks. A Riverside County, Calif., couple believe, however, that the drugs endangered the life of their 7-year-old son, whose infection with E. coli may be linked to unpasteurized milk from a Fresno, Calif., dairy.

When Tony and Mary Martin rushed their son, Sam, to a Riverside hospital in September, the first doctor they saw suspected E. coli and warned against using antibiotics to treat the boy's severe abdominal cramps and bloody diarrhea.

In the five days it took to get lab tests back on Sam's stool sample, the boy changed hospitals and physicians, and he was given the antibiotics Flagyl and Claforan.

Within hours, Mr. Martin said, Sam's kidneys shut down. He spent two months in the hospital.

"This could have been prevented with a 3-cent band on his wrist that said, 'No antibiotics,' " Mr. Martin said.

Ten states are taking part in a study by the CDC to help settle the antibiotic question. Preliminary results are expected by next summer.

"No one knows for sure whether antibiotics are a risk factor," said Dr. Linda Demma, a CDC epidemiologist. "Different doctors will do different things, since the literature is inconclusive so far. We're hoping to prove it one way or the other."

Scientists who have studied the link are not waiting for further confirmation to preach restraint.

"It's kind of like aspirin and Reye's syndrome," said Dr. Phillip Tarr, professor of pediatric gastroenterology at Washington University in St. Louis and an author of the 2000 study. "No one ever did a control study that proved aspirin causes Reye's syndrome. But none of us will give aspirin for chicken pox."

Patients suspected of being infected with E. coli should be hospitalized and given intravenous hydration, he said.

The CDC discourages giving antibiotics for diarrhea in general because it promotes antibiotic resistance and most people recover without treatment. Some doctors, however, prescribe antibiotics for a less dangerous Shigella bacteria because antibiotics reduce the amount of bacteria shed in stools and allow children to return to day care with less chance of infecting others.

Two elderly women and a 2-year-old child died in the spinach outbreak in September, which sickened more than 200 people in 26 states.