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# Sulfadiazine

A sulfonamide that reaches the brain well; with pyrimethamine it is the classic treatment of toxoplasmosis, including in HIV.

Updated 2026-11-01

- Class: [Sulfonamides and trimethoprim](https://durgical.com/en/pharmacology/classes/sulfonamides-and-trimethoprim/)
- Enzyme: Dihydropteroate synthase (bacterial and Toxoplasma folate pathway)
- ATC code: J01EC02
- WHO Model List of Essential Medicines

✂**Dihydropteroate synthase (bacterial and Toxoplasma folate pathway)**Sulfadiazine

ⓘ The figure shows sulfamethoxazole, a closely related sulfonamide that occupies the same PABA pocket, in dihydropteroate synthase of Yersinia pestis.

**Target in 3D:** Dihydropteroate synthase (bacterial and Toxoplasma folate pathway). PDB structure 3TZF (RCSB PDB, public domain). [View the structure at RCSB PDB](https://www.rcsb.org/structure/3TZF) · Drag to rotate · pinch to zoom

## Mechanism of action

Sulfadiazine competes with para-aminobenzoic acid at dihydropteroate synthase, so the microbe makes less dihydrofolate. Pyrimethamine blocks the next enzyme, dihydrofolate reductase, in Toxoplasma, giving a sequential block of folate synthesis.

- Diffuses into tissues and the cerebrospinal fluid
- Competes with PABA at dihydropteroate synthase
- Folate synthesis falls (more with pyrimethamine)
- The parasite or bacterium cannot make DNA

## Pharmacokinetics

**Absorption**
Well absorbed by mouth.

**Distribution**
Enters the cerebrospinal fluid well; crosses the placenta and reaches breast milk.

**Elimination**
Urine; crystals can form in acid, concentrated urine.

## Indications

- Toxoplasmic encephalitis in people with and without HIV, with pyrimethamine [HIV infection and AIDS →](https://durgical.com/en/conditions/hiv-infection-and-aids/)
- Prevention of rheumatic fever recurrences as an alternative to penicillin [Rheumatic heart disease →](https://durgical.com/en/conditions/rheumatic-heart-disease/)
- Congenital toxoplasmosis, with pyrimethamine

## Adverse effects

### Common

- Nausea, vomiting and loss of appetite
- Rash and fever
- Photosensitivity

### Serious (rare, important)

- Crystalluria, kidney stones and kidney injury
- Stevens–Johnson syndrome and toxic epidermal necrolysis
- Blood disorders (agranulocytosis, haemolysis in G6PD deficiency)
- Kernicterus in newborns

## Contraindications

- Allergy to sulfonamides
- Infants under 2 months, except with pyrimethamine for congenital toxoplasmosis
- Pregnancy at term and breastfeeding (kernicterus risk)

## Key interactions

**Pyrimethamine**
Synergy against Toxoplasma, but more folate depletion and marrow suppression.

**Oral anticoagulants and methotrexate**
Their effects may increase, as the sulfonamide displaces them from albumin.

## What is monitored

- Urine output and urinalysis for crystals
- Full blood count
- Skin for rash

## Administration and safety

- Plenty of fluid is given to keep urine output high and avoid crystals
- Given by mouth

## Nursing care

- Encourage fluids and measure urine output
- Teach to report rash, sore throat, fever or blood in the urine
- Ask about sulfa allergy

## Pregnancy and breastfeeding

Its safe use in pregnancy is not established, and it is avoided at term because it can cause kernicterus.

## Memory aid

«Sulfadiazine + pyrimethamine: the toxo twins»: two blocks of the parasite's folate, one partner each.

## Test yourself

Think of the answer, then tap the question to see it.

## Which drug is sulfadiazine combined with for toxoplasmosis?

Pyrimethamine.

## Why is high fluid intake important with sulfadiazine?

It can crystallise in concentrated urine and cause stones and kidney injury.

## Why is it avoided at term?

Sulfonamides displace bilirubin and can cause kernicterus in the newborn.

[Add to my cards (3)](https://durgical.app/estudiar/?add=/en/pharmacology/sulfadiazine/)

## Related drugs

- [Sulfamethoxazole + trimethoprim (co-trimoxazole)](https://durgical.com/en/pharmacology/sulfamethoxazole-trimethoprim/)
- [Trimethoprim](https://durgical.com/en/pharmacology/trimethoprim/)

## Related conditions

- [HIV infection and AIDS](https://durgical.com/en/conditions/hiv-infection-and-aids/)
- [Rheumatic heart disease](https://durgical.com/en/conditions/rheumatic-heart-disease/)

[← Back to the class: Sulfonamides and trimethoprim](https://durgical.com/en/pharmacology/classes/sulfonamides-and-trimethoprim/)

Educational content on how drugs work; not a prescribing guide and no substitute for a health professional's advice. No doses are given.

Sources: [DailyMed - SULFADIAZINE tablet](https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=8993651b-fff6-4e3f-9b60-78587bc9bcee) (DailyMed (U.S. National Library of Medicine), 2026); [Sulfadiazine: MedlinePlus Drug Information](https://medlineplus.gov/druginfo/meds/a682130.html) (MedlinePlus (U.S. National Library of Medicine), 2026); [eEML - Electronic Essential Medicines List](https://list.essentialmeds.org/?query=sulfadiazine) (World Health Organization, 2025).

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Representative model, for educational purposes only.

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