Sulfonamides interfere in the synthesis of folic acid. They inhibit the production of folic acid by inhibiting the enzyme dihydropteroate synthetase. Actually sulfonamides compete for this enzyme together with the usual substrate PABA. Sulfonamides are used in the treatment of toxoplasmosis. Sulfonamides are always used in combination with other drugs. Co-trimoxazole with the addition of trimethoprim is most frequently applied in this co-therapy.

Resistance (against e.g. Neisseria, Shigella, E. coli) is increasing problem with sulphonamides. The drugs have a rapid oral absorption (70-100%) and a half-life of 6-11 hours.

Bacteria strains, which are sensitive to sulphonamides, are Streptococcus, Haemophilus, and Chlamydia. The following infections can be treated with sulphonamides: urinary tract infections, Nocardiosis, and toxoplasmosis.

In combination with trimethropim urinary tract infections, bacterial respiratory tract infections, GI infections, infections by Pneumocystis carinii, and Nocardia infections can be treated.


The combination of sulfonamides and trimethoprim is active in Pneumocystis carinii infections.