Dapsone (DDS)

What is this medication for?

• Second-line mycobacteriosis; brown recluse spider envenomation adjunct (leukocyte inhibition limits dermonecrosis); cutaneous vasculitis; pemphigus alternative (with sulfasalazine); Pneumocystis (potential)
• Relatively contraindicated (toxicity)

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• stomach/intestinal upset, neuropathies, cutaneous drug eruptions, photosensitivity (Additional)

Worth a phone call to your veterinarian

• Hepatotoxicity, methemoglobinemia, hemolytic low red blood cell counts (The toxicity triad — cats particularly vulnerable)
• Thrombocytopenia, low infection-fighting cell counts (Marrow effects)

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Cats; preexisting hematologic/hepatic disease; G6PD-type hemolysis risk (human analog)

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Other hematotoxic/liver damage drugs
  • Trimethoprim-sulfa
  • Rifampin (human)

Storage and handling

Room temperature; inexpensive generic.

Good to know

• The brown-recluse trick is the exam-worthy use: dermonecrotic loxoscelism is neutrophil-driven → dapsone's neutrophil inhibition blunts the expanding lesion (early use).
• Toxicity profile (MetHb + hemolysis + liver) makes it a monitoring-heavy drug — "rarely used due to severe adverse-effect potential" is the honest label.
• Cats: relatively contraindicated — same methemoglobinemia machinery as acetaminophen sensitivity.