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Long-term use of antibiotics appears safe to treat a large-scale exposure to anthrax in a bioterrorism attack

The intentional release of anthrax bacteria in a bioterrorism attack would require large-scale use of antibiotics to prevent anthrax infection in the affected communities. The U.S. Centers for Disease Control and the Infectious Diseases Society of America recommend 60 days of the antibiotics ciprofloxacin, doxycycline, or amoxicillin to prevent post-exposure anthrax infection. Most antibiotics are typically taken up to 14 days for acute infections, and there are concerns about the safety of taking these antibiotics for 60 days. However, a new study eases those concerns. It found limited evidence of cumulative dose-related adverse events (ADEs) from taking any of these antibiotics for more than 28 days versus taking them for 28 days or less.

Researchers at the University of Pennsylvania Center for Education and Research on Therapeutics and their colleagues estimated risks of ADEs associated with taking these three antibiotics for more than 28 days by examining office visit, hospital admission, and prescription data from an electronic medical record database and two insurance claims databases from January 1999 until July 2001. They were looking in particular for hospitalization for the following serious ADEs: anaphylactic shock, phototoxicity, liver toxicity, kidney toxicity, seizures, ventricular arrhythmia, or infectious colitis. A total of 33,183 amoxicillin, 15,250 ciprofloxacin, and 50,171 doxycycline prescriptions continued for 30 or more days.

Long-term doxycycline use was associated with 0.9 ADEs per 100,000 antibiotic patient-days in one database only. Long-term amoxicillin use resulted in 1.2 ADEs per 100,000 person-days in only one database. Ciprofloxacin showed the highest risk of ADEs per 100,000 person days of antibiotic use (3.5 and 5.7 ADEs per 100,000 person-days, found in two databases), but the risk did not appear to increase after 28 days of taking the antibiotic.

The study was supported in part by the Agency for Healthcare Research and Quality (HS10399).

More details are in "Adverse events associated with prolonged antibiotic use," by Sharon B. Meropol, M.D., M.S.C.E., K. Arnold Chan, M.D., Sc.D., Zhen Chen, Ph.D., and others, in the May 2008 Pharmacoepidemiology and Drug Safety, 17(5), pp. 523-532.

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