Melarsomine
Melarsomine dihydrochloride
Also known as: Immiticide · Diroban · Melarsomine dihydrochloride
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Dosing by species
🌎 NA — North America🌍 EU — Europe
Dogs
| Indication | Dose | Route | Frequency | Duration | Region |
|---|---|---|---|---|---|
| Heartworm disease (Class I & II) | 2.5 mg/kg deep IM as directed (lumbar epaxial muscles (L3-L5) twice 24 hours apart | IM | q24h for 2 doses | 2 days | 🌎 NA |
| Heartworm disease (Class III) | 2.5 mg/kg deep IM as directed (lumbar epaxial muscles (L3-L5). Strict rest and give all necessary systemic treatment. One month later, give 2.5 mg/kg deep IM as directed (lumbar epaxial muscles (L3-L5) twice 24 hours apart. | IM | Once, then q24h for 2 doses 1 month later | 1 month + 2 days | 🌎 NA |
| Heartworm disease (Alternative Protocol / AHS Treatment of Choice) | 2.5 mg/kg deep IM as directed (lumbar epaxial muscles; L3-L5). Strict rest and give all necessary systemic treatment. One month later, give 2.5 mg/kg deep IM as directed (lumbar epaxial muscles; L3-L5) twice 24 hours apart | IM | Once, then q24h for 2 doses 1 month later | 1 month + 2 days | 🌎 NA |
- Heartworm disease (Class I & II): Use alternating sides with each administration. In 4 months, the regimen may be repeated.
- Heartworm disease (Class III): Requires strict rest.
- Heartworm disease (Alternative Protocol / AHS Treatment of Choice): Treatment of choice regardless of stage of disease due to increased safety and efficacy.
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Melarsomine is an organic arsenical chemotherapeutic agent used exclusively as an adulticide for canine heartworm disease (Dirofilaria immitis). It is highly effective against mature adult worms and immature (4-month-old, stage L5) larvae.
Clinical Pearl: While melarsomine is the only approved adulticide, current American Heartworm Society (AHS) guidelines strongly recommend a multimodal approach. Pre-treatment with a macrocyclic lactone (e.g., ivermectin) and doxycycline for 1-2 months prior to melarsomine administration is advised. Doxycycline eliminates Wolbachia (a symbiotic bacterium essential to the heartworm's survival and reproduction), which weakens the worms, reduces worm biomass, and significantly decreases the inflammatory response and risk of severe pulmonary thromboembolism when the worms die.
Melarsomine is significantly safer and more efficacious than its predecessor, thiacetarsamide, but still carries a narrow margin of safety and requires precise dosing and administration techniques.
Mechanism of action
While the exact mechanism of action is not definitively established, as an arsenical compound, melarsomine is believed to exert its parasiticidal effect by binding to sulfhydryl (-SH) groups of essential enzymes within the parasite.
- Enzyme Inhibition → Disrupts cellular respiration and vital metabolic pathways in the parasite.
- Target → Highly effective (90-99%) at killing adult and L5 larval stages of D. immitis in dogs.
- Clearance → Dead worms degrade and are swept into the pulmonary vasculature, where they are eventually cleared by the host's immune system (macrophages).
Safety & warnings
Contraindications
- Class IV (very severe) heartworm disease / Caval syndrome
- Cats (reportedly very toxic)
- Intravenous (IV) administration
- Subcutaneous (SC) administration
Adverse effects
- Injection site reactions (pain, swelling, tenderness, firm nodules)
- Coughing/gagging
- Depression/lethargy
- Anorexia/inappetence
- Excessive salivation
- Fever
- Lung congestion
- Vomiting
- Pulmonary thromboembolism (can be fatal)
- Paresis and paralysis (rare)
Precautions
Low margin of safety; calculate dosages very carefully (do not confuse mg/lb with mg/kg). Administer ONLY via deep IM injection in the lumbar epaxial muscles (L3-L5). Do NOT give IV or SC, as significant toxicity or tissue necrosis may occur. Strict cage rest (2-4 weeks) is mandatory to prevent fatal pulmonary thromboembolism. Older dogs (>8 years) and those with severe pulmonary artery disease are at higher risk for adverse effects. Avoid human exposure; wash hands after use or wear gloves. If human exposure occurs, contact a physician.
Drug interactions
Has been shown not to reduce adverse effects and may complicate therapy; use is not recommended.
Drugs that have similar adverse effects (e.g., depression) may cause additive adverse effects or increase their incidence when used with melarsomine.
Monitoring
- Clinical efficacy
- Adverse effects (observe dogs for 24 hours after the last injection)
- Signs of pulmonary thromboembolism (coughing, hemoptysis, tachypnea, dyspnea)
- Injection site reactions
Pharmacokinetics
Half-life
Absorption
Rapidly absorbed after IM injection in dogs; time to peak plasma concentration is about 11 minutes.
Distribution
The apparent volume of distribution is about 0.7 L/kg.
Elimination
Terminal half-life is approximately 3 hours.
Overdose
There is a low margin of safety with melarsomine.
- 3X dose (7.5 mg/kg) in healthy dogs causes respiratory inflammation and distress, excessive salivation, restlessness, panting, vomiting, edema, tremors, lethargy, ataxia, cyanosis, stupor, and death.
- 2X dose in infected dogs has caused diarrhea, excessive salivation, restlessness, panting, vomiting, and fever.
- Treatment: Dimercaprol (BAL in Oil) may be considered to treat melarsomine overdoses, though it may reduce the clinical efficacy of the melarsomine.
Available products
Formulations
- Powder for injection
Veterinary
- Melarsomine Dihydrochloride Powder for Injection: 50 mg/vial (Immiticide)
Regulatory status
No regulatory data for: 🇺🇸 US · 🇪🇺 EU · 🇬🇧 UK · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
The unreconstituted powder should be stored upright at room temperature. Reconstitute with 2 mL of the provided sterile water for injection (yields 25 mg/mL). Once reconstituted, keep in the original container and refrigerate for up to 24 hours. Do not freeze. Do not mix with any other drug.
Client information
CRITICAL WARNING: STRICT CAGE REST IS MANDATORY.
- Exercise Restriction: As the adult heartworms die, they break apart and are carried into the lungs. Any increase in heart rate (from walking, playing, or excitement) forces these worm fragments deeper into the small blood vessels of the lungs, causing a potentially fatal blockage (pulmonary thromboembolism). Your dog must be kept strictly confined to a cage or small room for several weeks after treatment.
- Injection Site Pain: The medication is given via a deep injection into the back muscles. It is normal for your dog to be sore, reluctant to move, or have a small lump at the injection site.
- What to Watch For: Monitor your dog closely for coughing, gagging, fever, lethargy, or loss of appetite.
- Emergency Signs: If your dog experiences severe difficulty breathing, pale or blue gums, or collapses, seek emergency veterinary care immediately.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.