Classification And Uses

Sulphonamides may be classified as follows: Systemic use

Sulphonamide-trimethoprim combination. Co-

trimoxazole (sulfamethoxazole plus trimethoprim); the optimum synergistic in vitro effect against most susceptible bacteria is achieved with 5:1 ratio of sulfamethoxazole to trimethoprim, although concentrations achieved in the tissues vary considerably. Each drug is well absorbed from the gut, has a t{/2 of 10 h and is 80% excreted by the kidney; consequently, the dose of co-trimoxazole should be reduced when renal function is impaired.

Co-trimoxazole, at first, very largely replaced the use of a sulphonamide alone. In turn, trimethoprim on its own is now used in many conditions for which the combination was originally recommended, and it may cause fewer adverse reactions (see below). The combination is, however, retained for:

• Prevention and treatment of pneumonia due to Pneumocystis carinii, a life-threatening infection in immunosuppressed patients

• Prevention and treatment of toxoplasmosis, and treatment of nocardiasis

Sulfadiazine (tV2 10 h), sulfametopyrazine (t\ 38 h) and sulfadimidine (sulfamethazine) (t^ approx. 6 h, dose dependent) are available in some countries for urinary tract infections, meningococcal meningitis and other indications, but are not drugs of first choice (resistance rates are high).

Topical application

Silver sulfadiazine is used for prophylaxis and treatment of infected burns, leg ulcers and pressure sores because of its wide antibacterial spectrum (which includes pseudomonads).


Sulfasalazine (salicylazosulfapyridine) is used in inflammatory bowel disease (see p. 649); in effect the sulfapyridine component acts as a carrier to release the active 5-aminosalicylic acid in the colon (see also rheumatoid arthritis, p. 292).

Adverse effects of sulphonamides include malaise, diarrhoea, mental depression and rarely cyanosis, which latter is due to methaemoglobinaemia. These may all be transient and are not necessarily indications for stopping the drug. Crystalluria may rarely occur.

Allergic reactions include: rash, fever, hepatitis, agranulocytosis, purpura, aplastic anaemia, peripheral neuritis and polyarteritis nodosa. Rarely, severe skin reactions including erythema multiforme bullosa (Stevens-Johnson syndrome) and toxic epidermal necrolysis (Lyell's syndrome) occur.

Haemolysis may occur in glucose-6-phosphate dehydrogenase deficient subjects. Co-trimoxazole in high dose may cause macrocytic anaemia due to interference with conversion of DHF to THF. Patients with AIDS have a high rate of allergic systemic reactions (fever, rash) to co-trimoxazole used for treatment of Pneumocystis carinii pneumonia. Co-trimoxazole should not be used in pregnancy because of the possible teratogenic effects of inducing folate deficiency


Subsequent to its extensive use in combination with sulphonamides, trimethoprim (t'/2 10 h) has emerged as a useful broad spectrum antimicrobial on its own. It is active against many Gram-positive and Gram-negative aerobic organisms excepting the enterococci and Pseudomonas aeruginosa; the emergence of resistant organisms is becoming a problem. The drug is rapidly and completely absorbed from the gastrointestinal tract and is largely excreted unchanged in the urine. Trimethoprim is effective as sole therapy in treating urinary and respiratory tract infections due to susceptible organisms and for prophylaxis of urinary tract infections.

Adverse effects are fewer than with co-trimoxazole and include: skin rash, anorexia, nausea, vomiting, abdominal pain and diarrhoea.

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