Dapsone — Veterinary Vademecum Drug Reference

Dosage & Administration Guidelines

Species	Route	Dose	Frequency	Duration & Clinical Notes
Dog	PO	1-2 mg/kg	q8h to q12h	Duration: Variable; often 3-6 months for mycobacterial infections Notes: Start at lower end and titrate up; monitor for hemolysis and methemoglobinemia.
Cat	PO	1-2 mg/kg	q24h	Duration: Variable; often 3-6 months for mycobacterial infections Notes: Cats are more sensitive to adverse effects; use with caution.
Horse	PO	3-5 mg/kg	q24h	Duration: Variable; often 90 days for EPM Notes: Off-label use; monitor for hemolytic anemia.

Clinical Indications & Species Uses

General Indications

Treatment of leprosy and other mycobacterial infections
Treatment of dermatological conditions with inflammatory component (e.g., vasculitis, pyoderma gangrenosum)

Dog (Canine)

Treatment of leproid granuloma (canine leprosy)
Treatment of dermatophytosis (in combination with other antifungals)
Treatment of sterile pyogranuloma syndrome
Treatment of vasculitis
Treatment of pemphigus complex (adjunctive therapy)
Treatment of nocardiosis
Treatment of actinomycetoma

Cat (Feline)

Treatment of feline leprosy (Mycobacterium lepraemurium)
Treatment of nocardiosis
Treatment of atypical mycobacterial infections

Pharmacology & Mechanism of Action

Drug Class: Sulfone antibiotic | Pharmacological Group: Antibacterial (leprostatic)

Mechanism of Action: Dapsone is a sulfone antibiotic that inhibits bacterial synthesis of dihydrofolic acid by competitively binding to dihydropteroate synthase, thereby blocking the conversion of para-aminobenzoic acid (PABA) to dihydropteroate. This action is bacteriostatic and is effective against a range of gram-positive and gram-negative organisms, as well as some protozoa. It also has anti-inflammatory properties, particularly in dermatological conditions, by inhibiting neutrophil chemotaxis and the myeloperoxidase-peroxide-halide system, which reduces tissue damage.

Pharmacodynamics: Dapsone exhibits a slow onset of action and is primarily bacteriostatic. It is effective against Mycobacterium leprae, Mycobacterium tuberculosis, and some strains of Staphylococcus, Streptococcus, and Nocardia. In veterinary medicine, it is used for its anti-inflammatory effects in dermatological conditions such as pemphigus and vasculitis. The drug accumulates in skin and mucous membranes, where it exerts its therapeutic effects. It also has activity against Pneumocystis jirovecii and Toxoplasma gondii.

⚡ Pharmacokinetics Summary

Absorption: Dapsone is well absorbed after oral administration, with peak plasma concentrations occurring within 2-8 hours in most species. Absorption is nearly complete, with bioavailability approaching 100% in dogs and cats.

Distribution: Dapsone is widely distributed throughout the body, with high concentrations in the skin, liver, kidneys, and erythrocytes. It crosses the blood-brain barrier and placenta. Protein binding is approximately 70-90% in dogs and cats.

Metabolism: Dapsone is metabolized in the liver primarily by N-acetylation and N-hydroxylation. The acetylation rate varies among species; dogs are slow acetylators, while cats are intermediate. The N-hydroxy metabolite is responsible for some of the drug's adverse effects, such as methemoglobinemia.

Excretion: Dapsone and its metabolites are excreted primarily in the urine, with a small amount in the feces. Biliary excretion and enterohepatic recirculation occur. In dogs, the elimination half-life is approximately 10-15 hours; in cats, it is approximately 20-30 hours.

Half-Life: Dogs: 10-15 hours; Cats: 20-30 hours; Humans: 20-30 hours

Bioavailability: Nearly 100% after oral administration

Protein Binding: 70-90% in dogs and cats

Available Formulations & Strengths

Oral Tablet 25 mg, 50 mg, 100 mg (PO)

Contraindications & Clinical Warnings

Contraindications:

Hypersensitivity to dapsone or other sulfonamides
Severe anemia or glucose-6-phosphate dehydrogenase (G6PD) deficiency
Severe hepatic or renal impairment
Pregnancy (especially in the first trimester) unless benefits outweigh risks
Lactation (use with caution)

Warnings & Clinical Precautions:

Monitor complete blood count (CBC) and methemoglobin levels regularly, especially during the first few weeks of therapy.
Use with caution in animals with pre-existing cardiac, hepatic, or renal disease.
Dapsone may cause hemolytic anemia, methemoglobinemia, and agranulocytosis; discontinue if severe adverse effects occur.
In cats, use with caution due to increased risk of methemoglobinemia.
Avoid concurrent use with other drugs that may cause hemolysis or methemoglobinemia (e.g., acetaminophen, sulfonamides).
May cause photosensitivity; avoid excessive sun exposure during treatment.
Use with caution in breeding animals; may affect fertility.

Adverse Effects & Reactions

Common:

Anorexia
Vomiting
Diarrhea
Lethargy
Methemoglobinemia (cyanosis, weakness)
Hemolytic anemia

Serious / Severe:

Agranulocytosis
Aplastic anemia
Peripheral neuropathy
Hepatotoxicity
Severe skin reactions (e.g., Stevens-Johnson syndrome)
Renal toxicity

Rare:

Hypoalbuminemia
Psychosis
Pulmonary eosinophilia
Exfoliative dermatitis

Key Drug Interactions

Interacting Drug / Class	Clinical Effect & Mechanism	Severity
Trimethoprim-sulfonamide combinations	Increased risk of methemoglobinemia and hemolysis due to additive oxidative stress.	High
Acetaminophen (paracetamol)	Increased risk of methemoglobinemia and hepatotoxicity.	High
Probenecid	Decreased renal excretion of dapsone, leading to increased plasma levels and toxicity.	Moderate
Rifampin	Decreased dapsone levels due to increased hepatic metabolism; may reduce efficacy.	Moderate
Antacids (aluminum/magnesium)	Reduced absorption of dapsone; separate administration by at least 2 hours.	Mild
Cimetidine	May increase dapsone levels by inhibiting metabolism; monitor for toxicity.	Moderate

Overdose & Toxicity Management

Signs of Toxicity:

Severe methemoglobinemia (cyanosis, dyspnea, lethargy)
Hemolytic anemia
Nausea, vomiting
Seizures
Coma
Hepatotoxicity

Emergency Treatment Protocol: Induce emesis or perform gastric lavage if recent ingestion. Administer activated charcoal. Provide supportive care: oxygen therapy, intravenous fluids, and blood transfusions if severe anemia. Methylene blue (1-2 mg/kg IV) may be used to treat methemoglobinemia, but use with caution in G6PD-deficient animals. Monitor CBC, methemoglobin levels, liver and renal function. In severe cases, consider hemodialysis.

Food Animal Withdrawal Times

Dapsone is not approved for use in food animals in the United States. Extra-label use in food animals is prohibited due to lack of established withdrawal times and potential for residues. Do not use in animals intended for human consumption.

Storage, Handling & Regulatory Information

Storage Temperature: Store at controlled room temperature (20-25°C, excursions permitted to 15-30°C).

Light Sensitivity: Light-sensitive — protect from direct exposure.

Handling & Special Conditions: Protect from light and moisture. Keep in a tightly closed container.

Dispensing Status: Prescription Required (Rx Only)

Approval Status: Not FDA-approved for veterinary use; approved for human use (leprosy, dermatitis herpetiformis).

Extra-Label (Off-Label) Use: In the United States, dapsone is not FDA-approved for veterinary use. Its use in animals is extra-label and must comply with AMDUCA regulations. For food animals, extra-label use is prohibited if the drug is not approved for use in that species and if there are no established withdrawal times. Use in companion animals is permitted with a valid veterinarian-client-patient relationship.

Clinical Pearls & Practice Notes

💡 Clinical Insights: Dapsone is a valuable therapeutic option for certain infectious and inflammatory dermatological conditions in dogs and cats, particularly those involving mycobacteria or immune-mediated processes. Its use requires careful monitoring due to the risk of hematological adverse effects. Baseline CBC, serum chemistry, and urinalysis should be obtained before starting therapy, and periodic monitoring (every 2-4 weeks initially) is recommended. Methemoglobin levels should be checked if clinical signs of cyanosis or weakness develop. Dapsone should be used with caution in breeds with known G6PD deficiency (e.g., some collie breeds) and in cats. In horses, it is occasionally used off-label for EPM, but evidence is limited. Due to its slow onset of action, combination therapy may be necessary for severe infections. Always consider drug interactions and adjust dosing in patients with hepatic or renal impairment. Client education is essential to ensure compliance and early recognition of adverse effects.

References & Literature

📚 Plumb's Veterinary Drug Handbook
📚 Papich Veterinary Pharmacology and Therapeutics
📚 Compendium of Veterinary Products (CVP)