Melarsomine dihydrochloride

What is this medication for?

• Heartworm disease treatment — stabilized Class I–III (stage L5 immature to adult D. immitis)
• NOT recommended — supportive/steroid management instead (worms few but reaction severe)

How is it given?

This medication is normally given by your veterinary team (in clinic or hospital).

• The one drug in the library dosed into the lumbar muscles only, never into a vein/under the skin — technique IS the safety profile (back-leak into subcutis = worst local reactions).

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

• Pulmonary thromboembolism (dead-worm emboli, 7–30 day window) (The feared complication — coughing/gagging, fever, lung congestion, depression)
• Anorexia, vomiting, salivation, low energy/tiredness (Common label events)

Worth a phone call to your veterinarian

• Injection-site reactions (pain, swelling, firm nodules — may persist indefinitely) (~⅓ of dogs)

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

• Death (severe reactions, caval syndrome dogs) (Class into a vein risk)

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

• has or has had: Class into a vein disease (caval syndrome — worms in vena cava/right atrium → mechanical extraction first); cats; severe renal/hepatic/pulmonary disease; heartworm antigen use: confirm diagnosis
• has or has had: Older dogs (>8 y) more susceptible; STRICT CAGE REST through the embolic weeks is part of the prescription

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Doxycycline/Wolbachia pre-treatment
  • Corticosteroids/antihistamines

Storage and handling

Human exposure hazard (arsenical cytotoxin) — reconstitute carefully, gloves; deep into a muscle into lumbar epaxial muscles ONLY — NEVER into a vein or under the skin (fatal toxicity/tissue damage); firm post-injection pressure to prevent back-leak.

Good to know

• The one drug in the library dosed into the lumbar muscles only, never into a vein/under the skin — technique IS the safety profile (back-leak into subcutis = worst local reactions).
• Three-way exam trio: dogs only (cats = steroid support), Class into a vein excluded (caval = surgery), cage rest = medicine (exercise kills via emboli).
• Modern layering (AHS-era): doxycycline + macrocyclic lactone before/rearound melarsomine, staged 3-dose protocol for heavy worm burdens — the arsenical remains the only adulticide.