Limitations of and indications for the use of co-trimoxazole.

Co-trimoxazole is still widely used for indications where trimethoprim alone is equally effective. Microbiological and pharmacokinetic considerations reveal that trimethoprim alone provides adequate anti-microbial activity for treatment of conditions for which co-trimoxazole is often given. Synergy may be shown in vitro, but in clinical practice is an unusual occurrence. There is no evidence from clinical studies that the sulphonamide moiety fo co-trimoxazole prevents the development of resistance to trimethoprim. The adverse event profile of co-trimoxazole is a summation of that of sulphonamide and of trimethoprim. Thus, using trimethoprim alone should reduce both the incidence and potential severity of adverse events seen when co-trimoxazole is used. Clinical trials have shown trimethoprim to be as effective as co-trimoxazole in many of the common bacterial infections of the urinary and respiratory tracts. However, there are a few specific varieties of infection for which co-trimoxazole can be shown to be superior to trimethoprim: these include toxoplasmosis, brucellosis, nocardiosis, chancroid and pneumonia caused by Pneumocystis carinii. For many common infections, scientific, rational, economic and clinical reasons dictate that trimethoprim is preferable to co-trimoxazole.