Fusidic Acid Monotherapy

Abstract

Dr Biswas and colleagues (February 2002 JRSM1) present two cases of hypocalcaemia arising during fusidic acid therapy. We agree that this could represent a rare side-effect of fusidic acid. However, we would like to focus on a different aspect—that of antimicrobial resistance selection. In both patients fusidic acid was used as monotherapy (for more than one month in case 2, unspecified for case 1). The general view is that fusidic acid should not be used alone because natural mutants with an alteration in the elongation factor G are harboured even at low rates of 106 staphylococci2. This leads to a rapid spontaneous mutation rate, seen when the organism is grown in increasing concentrations of the drug. Therefore, combination with another antistaphylococcal antibiotic such as flucloxacillin or vancomycin will decrease the risk of resistance emergence1,2,3, particularly if the infection is with methicillin-resistant Staphylococcus aureus or chronic4. The British National Formulary (BNF) recommends ‘clindamycin alone or flucloxacillin+fusidic acid’ for the treatment of osteomyelitis5. Both patients were taking aspirin during treatment with fusidic acid. There is some evidence that this too could promote the emergence of resistance6, and one might consider replacing aspirin with a different antiplatelet agent. In case 1 the patient was concurrently taking aspirin and ciprofloxacin. According to the BNF such a combination can increase the risk of convulsions even in patients with no previous history5 and ciprofloxacin resistance might also increase in the presence of salicylates7. In conclusion, fusidic acid should be used only in combination with another antistaphylococcal agent, and interactions with even as ‘benign’ a drug as aspirin should be considered.