The systemic sulfonamides are bacteriostatic agents having a similar spectrum of activity. Sulfonamides competitively inhibit bacterial synthesis of folic acid (pteroylglutamic acid) from aminobenzoic acid. Resistant strains are capable of utilizing folic acid precursors or preformed folic acid.

Sulfonamides exist in the blood in 3 forms – free, conjugated (acetylated and possibly others) and protein bound. The free form is considered to be the therapeutically active one.

Sulfadiazine given orally is readily absorbed from the gastrointestinal tract. After a single 2 g oral dose, a peak of 6.04 mg/100 mL is reached in 4 hours; of this, 4.65 mg/100 mL is free drug.

When a dose of 100 mg/kg of body weight is given initially and followed by 50 mg/kg every 6 hours, blood levels of free sulfadiazine are about 7 mg/100mL. Protein binding is 38% to 48%. Sulfadiazine diffuses into the cerebrospinal fluid; free drug reaches 32% to 65% of blood levels and total drug 40% to 60%.

Sulfadiazine is excreted largely in the urine, where concentrations are 10 to 25 times greater than serum levels. Approximately 10% of a single oral dose is excreted in the first 6 hours, 50% within 24 hours and 60% to 85% in 48 to 72 hours. Of the amount excreted in the urine, 15% to 40% is in the acetyl form.^[1]

Adapted from the FDA Package Insert.