That, in turn, raised the risk of shortages of the drugs if more people had needed them, while increasing the likelihood of germs building resistance to the drugs because of overuse, says the U.S. Food and Drug Administration (FDA) study.
The new figures suggest many more people got antibiotics in response to the anthrax attacks than was reported in November 2001, when the U.S. Centers for Disease Control and Prevention (CDC) said 32,000 Americans had been given preventive doses.
Anthrax was confirmed or suspected in 22 people. Eleven cases involved the skin form of the infection, while 11 -- including five deaths -- were the much more lethal inhaled form.
Prescriptions for Cipro soared by some 160,000, or more than 40 percent, in October 2001 compared with the previous year, says the study, published in the April/May issue of Pharmacoepidemiology and Drug Safety.
After the CDC confirmed a second drug, doxycycline, could treat anthrax exposure, prescriptions for that drug rose by 120,000, or about 30 percent, by early November 2001, compared with the previous year, the study found.
"This was really our first biological terrorism event, and people responded, I think, in a predictable manner: with fear," says Dr. Dianne Murphy, a co-author of the study.
"We understand that people are going to be afraid and want to get antibiotics, but people should understand that they should not start taking these antibiotics just because someone else has been exposed," adds Murphy, director of the Office of Counterterrorism and Pediatric Drug Development in the FDA's Center for Drug Evaluation and Research.
FDA researchers also warn of potential side effects from the drugs, including breathing difficulties, swelling of the face, throat or lips; severe diarrhea, nausea, stomach pain, dizziness and headaches.
The study did not give a geographic breakdown on the prescriptions. And researchers, who based their analysis on a national prescription database, could not say how many people actually took the antibiotics and how many stockpiled them.
The CDC ultimately recommended that 10,000 people potentially exposed to anthrax take antibiotics for 60 days. Those drugs came from a CDC supply and were not included in the figures cited in the FDA study.
The study says the CDC is conducting an extensive evaluation of these 10,000 patients, looking at how well the drugs worked and possible side effects.
The CDC says it has determined that less than half of those 10,000 patients took the drugs for the full 60 days.
Still, experts expressed concern about over-prescription contributing to overuse of the antibiotics, thus raising the resistance of bacteria to the drugs.
Even those who stockpiled the drugs for anthrax may end up taking them for something else -- even something as simple as a bad cold (for which the drugs are useless), says Dr. Peter Houck, who has researched over-prescription of Cipro and other antibiotics.
"As is the case with every other antibiotic, the more you use them, the more the resistance tends to develop," says Houck, chief medical officer of the Centers for Medicare and Medicaid Services in Seattle. "So there's been a lot of concern about overusing these drugs and that the drugs will become less effective."
Already, many germs are becoming resistant to Cipro because of overuse, according to a February study published in the Journal of the American Medical Association.
Cipro, for example, has become much less effective in fighting serious infections patients pick up in hospitals, says Dr. Joseph Guglielmo, a professor and vice chairman of the clinical pharmacy department at the University of California at San Francisco.
"Cipro used to be a wonderful drug for treatment of serious infections in hospitals," he says. "Because so much Cipro and similar drugs have been used, what has happened is we are losing that drug and its value in the hospital very rapidly."
Dr. Richard Besser, director of the CDC's Campaign for Appropriate Antibiotic Use, says fear, anxiety and uncertainty likely contributed to the surge in antibiotic prescriptions after the anthrax attacks.
Besser says state, federal and local governments have devised plans to ensure rapid delivery of antibiotics in response to possible anthrax attacks, and doctors now know more about when to prescribe -- and not prescribe -- antibiotics.
"The more faith that the public has that they'll be able to get the medications when they need them," he says, "the less stockpiling they'll do."
For more on anthrax, its symptoms and treatment,visit the U.S. Centers for Disease Control and Prevention. To learn more about antibiotic resistance, check with the Alliance for the Prudent Use of Antibiotics.