Sulfadiazine is a short-acting sulfonamide antibiotic that acts as a competitive antagonist of para-aminobenzoic acid (PABA), inhibiting bacterial synthesis of dihydrofolic acid. It is primarily used in combination with pyrimethamine for the treatment of toxoplasmosis and as a component of silver sulfadiazine for burn wound prophylaxis. In India, its systemic use is limited due to the availability of newer antibiotics and the risk of severe adverse effects like Stevens-Johnson syndrome.
Adult: For toxoplasmosis: 2-4 g initially, followed by 2-4 g daily in 4-6 divided doses (with pyrimethamine and folinic acid). For other infections: 2-4 g/day in 3-6 divided doses.
Note: Take with or after food with a full glass of water. Maintain adequate fluid intake (at least 2-3 L/day) to prevent crystalluria. Do not crush or chew the tablet. Complete the full prescribed course.
Sulfadiazine is a structural analogue of para-aminobenzoic acid (PABA). It competitively inhibits the bacterial enzyme dihydropteroate synthase. This prevents the incorporation of PABA into dihydrofolic acid, a precursor of tetrahydrofolic acid, which is essential for the synthesis of purines, thymidine, and ultimately bacterial DNA and RNA.
Pregnancy: Category C (US FDA). Teratogenic in animals. Contraindicated at term due to risk of kernicterus in the newborn. Use only if potential benefit justifies the fetal risk, primarily for life-threatening toxoplasmosis with pyrimethamine (which is Category C/X).
Driving: May cause dizziness or headache. Patients should not drive or operate machinery until they know how the drug affects them.
| Warfarin | Increased anticoagulant effect; risk of bleeding | Major |
| Methotrexate | Increased methotrexate toxicity (displacement from protein binding) | Major |
| Phenytoin | Increased phenytoin levels and toxicity | Major |
| Sulfonylureas (e.g., Glimepiride) | Enhanced hypoglycemic effect | Moderate |
| Cyclosporine | Decreased cyclosporine levels; risk of transplant rejection | Moderate |
| Pyrimethamine | Synergistic effect against Toxoplasma; increased risk of bone marrow suppression | Moderate (requires folinic acid) |