Dapsone in Dermatology
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Direct answer
Dapsone in dermatology is a sulphone used mainly in leprosy, dermatitis herpetiformis and some neutrophilic dermatoses, with anti-inflammatory effects alongside its antibacterial action. Its key safety issues are G6PD-related haemolysis and dose-related methaemoglobinaemia, so G6PD status is checked before prescribing. Rare dapsone hypersensitivity syndrome causes fever, rash and hepatitis and requires the drug to be stopped.
What you must remember
- Mechanism: inhibits bacterial folate synthesis and suppresses neutrophil activation, giving anti-inflammatory effects.
- Leprosy: dapsone forms part of multidrug therapy, given daily at 100 mg in the WHO regimens for both paucibacillary and multibacillary disease.
- Dermatitis herpetiformis: dapsone controls itch and blistering at low doses, although a gluten-free diet is still required.
- G6PD screening: deficiency predisposes red cells to oxidative haemolysis, so testing is done before starting.
- Methaemoglobinaemia: dapsone oxidises haemoglobin iron, causing cyanosis and low pulse oximetry readings.
- Dapsone hypersensitivity syndrome: fever, rash and hepatitis, linked to HLA-B*13:01 and reported mainly in Asian populations.
- Treatment and monitoring: full blood count and reticulocytes are checked, and methylene blue treats symptomatic methaemoglobinaemia, with caution in G6PD deficiency because it can itself cause haemolysis.
Recognising dapsone toxicity by mechanism and clue
Dapsone toxicity is predictable once the mechanism is known, and each complication has a classic bedside clue. The table sets these out with the action to take.
| Complication | Mechanism | Clue | Action |
|---|---|---|---|
| Haemolysis | Oxidative damage in G6PD deficiency | Falling haemoglobin, dark urine | Stop drug, supportive care |
| Methaemoglobinaemia | Haemoglobin iron oxidised to ferric state | Cyanosis, low pulse oximetry | Stop drug, methylene blue if symptomatic |
| Hypersensitivity syndrome | Immune reaction linked to HLA-B*13:01 | Fever, rash, hepatitis | Stop drug, do not rechallenge |
| Agranulocytosis | Marrow suppression, uncommon | Sore throat, fever | Urgent blood count, stop drug |
A cyanosed patient with a low pulse oximetry reading and normal arterial oxygen tension suggests methaemoglobinaemia rather than cardiorespiratory failure. Falling haemoglobin in a G6PD-deficient patient on dapsone points to haemolysis and should be investigated before it is attributed to leprosy or anaemia.
Clinical vigilance matters most in the first weeks of therapy, because haemolysis and methaemoglobinaemia can appear early and need prompt action.
Where the exam tries to catch you
Dapsone questions reward the link between each adverse effect and its mechanism.
- Trap: attributing a falling haemoglobin to leprosy alone Correct: G6PD status should be checked, since dapsone haemolysis in deficiency is a classic exam point.
- Trap: reassuring a cyanosed patient whose oximeter reads low Correct: a low reading with normal arterial oxygen suggests methaemoglobinaemia, which is treated with methylene blue when symptomatic.
- Trap: treating dapsone fever and rash as a simple drug rash Correct: dapsone hypersensitivity syndrome causes hepatitis and the drug should not be rechallenged.
Frequently asked questions
Why is G6PD testing needed before dapsone?
G6PD testing is needed before dapsone because G6PD deficiency leaves red cells vulnerable to oxidative damage. Dapsone can precipitate haemolysis in deficient individuals. Testing before starting identifies patients who need an alternative or close monitoring.
What is dapsone-induced methaemoglobinaemia?
Dapsone-induced methaemoglobinaemia occurs when dapsone or its metabolites oxidise haemoglobin iron to the ferric state. Patients may show cyanosis and low pulse oximetry readings despite a normal arterial oxygen tension. Symptomatic cases are treated by stopping dapsone and giving methylene blue.
Which HLA allele is linked to dapsone hypersensitivity syndrome?
HLA-B*13:01 is the allele linked to dapsone hypersensitivity syndrome, which presents with fever, rash and hepatitis. The association has been reported mainly in Asian populations. Dapsone should be stopped and not restarted after this reaction.
Which skin conditions are treated with dapsone?
Dapsone is used in leprosy as part of multidrug therapy, in dermatitis herpetiformis, and for some neutrophilic dermatoses. Topical dapsone gel is used for acne. Each use depends on the drug's antibacterial and anti-inflammatory actions.