Melarsomine (Immiticide) for Dogs: Heartworm Treatment

This information is for educational purposes only. Never give your pet any medication without your veterinarian's guidance. Dosing, frequency, and safety depend on your pet's specific health profile.

Melarsomine dihydrochloride, sold as Immiticide and historically Diroban, is an injectable medication used by your vet to kill adult heartworms in dogs. It is an arsenical antiparasitic drug, which means it is specifically designed to target mature Dirofilaria immitis worms living in the heart and pulmonary arteries.

This medication is not a monthly heartworm preventive and it is not something pet parents give at home. Melarsomine is given as a deep intramuscular injection in the lumbar epaxial muscles by veterinary professionals. Because treatment can trigger inflammation and complications as worms die, dogs usually need a full treatment plan that also includes testing, exercise restriction, and supportive medications.

For most dogs, melarsomine is part of a broader heartworm protocol rather than a stand-alone drug. Your vet may first use a heartworm preventive, doxycycline, and sometimes prednisone before the injections begin. That step helps reduce circulating larvae, weaken the bacteria that live inside heartworms, and lower the risk of treatment-related complications.

Melarsomine is used to treat adult heartworm infection in dogs. It is the only FDA-approved adulticide for this purpose. In practical terms, that means it is the medication your vet uses when a dog has tested positive for heartworm disease and is considered a candidate for adult worm treatment.

Most veterinarians follow the 3-dose protocol recommended by the American Heartworm Society for dogs in all disease stages when possible. This approach is typically paired with a macrocyclic lactone heartworm preventive and doxycycline before melarsomine is started. The goal is to improve safety, reduce worm burden over time, and increase the chance of clearing the infection.

Melarsomine is not used to prevent heartworm disease, and it does not replace year-round prevention after treatment. Even after successful therapy, dogs can become infected again. Your vet will usually recommend continued prevention and follow-up antigen testing months after treatment is complete.

Melarsomine dosing is determined by your vet based on your dog’s body weight, heartworm stage, overall health, and treatment plan. The commonly referenced dose is 2.5 mg/kg by deep intramuscular injection into the lumbar epaxial muscles. Pet parents should never attempt to give this medication themselves.

The most widely recommended schedule is the 3-dose protocol: one injection, then two more injections given about 30 days later on consecutive days. In many practices, this is described as injections on approximately day 60, day 90, and day 91 of the full treatment timeline, because dogs are often pretreated first with a heartworm preventive and doxycycline.

Exercise restriction is a major part of dosing safety. Even when the injections go smoothly, dead and dying worms can lodge in the lungs and cause serious inflammation or embolic complications. That is why your vet may recommend crate rest or very strict leash-only activity during treatment and for 6 to 8 weeks after the final injection.

If your dog has advanced heartworm disease, breathing problems, caval syndrome, or other medical concerns, your vet may adjust the plan, stabilize first, or discuss alternatives. The right protocol depends on the whole patient, not only the drug label.

The most common side effects after melarsomine are injection-site pain, soreness, swelling, stiffness, and reluctance to move. Some dogs also seem tired for a day or two after treatment. Because the injections are placed deep in the back muscles, temporary discomfort is common and your vet may send home pain-control or anti-inflammatory medications.

More serious problems can happen because adult heartworms are dying inside the body. Watch closely for coughing, rapid or difficult breathing, fever, weakness, loss of appetite, collapse, or coughing up blood. These signs can suggest inflammation in the lungs or a thromboembolic complication, and they need prompt veterinary attention.

See your vet immediately if your dog has trouble breathing, faints, becomes suddenly weak, develops severe lethargy, or seems much worse after an injection. Strict activity restriction matters here. Even a dog that feels normal can be at higher risk if they run, jump, or play hard during recovery.

Many dogs complete treatment successfully, but heartworm therapy is never casual care. Close follow-up with your vet, careful monitoring at home, and sticking to the rest plan are a big part of keeping treatment as safe as possible.

Melarsomine is usually used with other medications rather than in isolation. Common companion drugs in a heartworm protocol include a monthly macrocyclic lactone preventive, doxycycline, and sometimes prednisone or pain medication. These combinations are standard parts of many treatment plans, but they still need veterinary oversight because each dog’s heartworm stage and medical history are different.

There is no long public list of classic household drug interactions for melarsomine the way there is for some oral medications. The bigger concern is whether your dog is healthy enough for treatment and whether other drugs could affect sedation, pain control, liver function, kidney function, or recovery. Dogs with severe lung disease, heart failure, or caval syndrome may need a modified plan.

Tell your vet about every medication and supplement your dog receives, including heartworm prevention, flea and tick products, steroids, NSAIDs, antibiotics, calming aids, and herbal products. That helps your vet avoid overlapping side effects, choose the safest supportive medications, and time treatment appropriately.

If your dog is taking anything new and you are not sure it is compatible with heartworm treatment, ask before giving the next dose. With melarsomine, coordination matters as much as the medication itself.