Giving antibiotics to large numbers of people who are at risk of being exposed to anthrax will lead to resistance, according to researchers from Liverpool in the British Medical Journal. It is essential that they are used carefully and according to national guidelines.

Additionally, although considered generally safe, the antimicrobial drug ciprofloxacin, which has been purchased in large amounts by the U.S. and several other national governments, is associated with tendon ruptures and neuropsychiatric disorders, and in most countries it is not licensed for use in pregnancy or for children, say the authors.

Experts say its value has already been compromised by the development of resistance following over-use. The researchers say they cannot exclude the possibility that the anthrax bacterium itself could develop a resistance to the drug, the way other bacteria has to more common antibiotics.

"The important thing is to ensure that prophylaxis is given only to those who really need it and discourage mass usage by an understandably alarmed public. To induce antimicrobial resistance on a mass scale would be an even greater triumph for the terrorists," they concluded.