Melarsomine dihydrochloride (Immiticide; generic Diroban) is the only drug the FDA has ever approved to kill adult heartworms in dogs, and it has held that position since 1995. It is an arsenical, administered by deep intramuscular injection into the epaxial lumbar muscles between L3 and L5 — never intravenously, never into another muscle group. It replaced thiacetarsamide, an older arsenical that had to be given IV and could slough skin at the catheter site. The treatment works by killing the adult Dirofilaria immitis worms living in the pulmonary arteries, and that mechanism is the source of essentially every safety concern the drug carries: dead worms dislodge, travel to the lungs, and cause pulmonary thromboembolism.

To put numbers on what three decades of use have produced, VetMedGuide analyzed the FDA Center for Veterinary Medicine (CVM) public adverse-event database, a public extract dated June 2026 containing 1.34 million reports. We identified 23,567 reports naming melarsomine and tabulated their reactions, outcomes, and reporting history.

The headline is that the spontaneous data reproduces the two risks every veterinarian already counsels owners about — injection-site reactions and the post-treatment thromboembolic complication — and adds a third, less-discussed signal: a large block of treatment-failure reports. A meaningful number of reports end in death, and the pattern of those deaths (sudden death outpacing euthanasia) is consistent with acute pulmonary thromboembolism rather than gradual decline. Read with the standard caveat that these are old, sick, heartworm-positive dogs, the dataset explains exactly why the American Heartworm Society protocol is built around exercise restriction and adjunct medication.

How we identified reports

The FDA's public database masks brand names — every product is recorded as "MSK" — so a drug must be identified by its active ingredient. Melarsomine is unique to Immiticide and its generic Diroban, so we matched reports whose active-ingredient field named it. Because heartworm treatment routinely combines melarsomine with a heartworm preventive (a macrocyclic lactone such as ivermectin or moxidectin) and with doxycycline and prednisone, many reports name several drugs; a report is counted whenever melarsomine appears, which is the same convention used across this database. We worked from the event-level file (one record per unique adverse-event report) so each report is counted once. The FDA explicitly cautions that spontaneous reports do not establish causation or true event rates.

Two clusters dominate, and they are the ones the label predicts

The first cluster is injection-site toxicity. Swelling, pain, and a palpable lump at the injection site together account for the largest non-efficacy block — roughly 4,650 mentions — and they are exactly what the label lists as the most commonly reported adverse events. Melarsomine is an irritant injected into muscle; local soreness for a day or two is expected, and the reports largely reflect that predictable local reaction rather than something unexpected. Pain management (an NSAID or other analgesic) after each injection is standard practice, and the data shows why.

The second cluster is the post-adulticide thromboembolic picture. Cough (928), fever (988), lethargy (1,128), depression (1,096), anorexia (1,217), vomiting (2,718), and a block of respiratory signs sit together in exactly the combination the label warns can follow worm death — typically within five to ten days of an injection. The label and the American Heartworm Society both state that some degree of pulmonary thromboembolism is virtually guaranteed after adulticide therapy; the clinical question is only whether it is symptomatic, and strict exercise restriction through the entire treatment and recovery period is the single most effective way to keep it from becoming severe.

The deaths are sudden, not gradual

Outcomes are recorded where known, and the melarsomine pattern differs in an instructive way from the injected monoclonal antibodies analyzed in the same database. Of 23,567 reports, 1,129 list the dog as died and 179 as euthanized — 1,308 reports with a fatal outcome — while 1,687 list the dog as recovered or normal and 724 as recovered with sequela. The disproportion matters: for bedinvetmab (Librela), euthanasia outnumbers death roughly two to one, reflecting gradual decline in old dogs. For melarsomine, death outnumbers euthanasia by roughly six to one. That is the signature of an acute event — a pulmonary thromboembolism or anaphylactoid reaction that kills before a decision to euthanize is on the table — and it is consistent with the mechanism and the known risk window in the days after an injection.

The honest caveats apply. These are heartworm-positive dogs; untreated heartworm disease is itself fatal, and the alternative to melarsomine is not "no risk" but progressive cardiopulmonary damage. A death in a report involving melarsomine does not establish that the injection caused it — worm burden, disease severity (Class 1–4), concurrent pulmonary arterial disease, and owner adherence to exercise restriction all drive outcome, and none of them are fully separable in spontaneous data. Severity matters at the extreme, too: the label contraindicates melarsomine in Class 4 disease (very severe, including caval syndrome), where the risk of catastrophic cardiopulmonary collapse is highest and surgical worm extraction is the indicated route instead. And when melarsomine is unavailable or contraindicated, a non-arsenical "slow-kill" protocol (a macrocyclic lactone plus doxycycline, with months of exercise restriction) is the fallback — slower, and with its own risk of ongoing lung damage, but it is the option the literature turns to when adulticide is off the table. What the data does corroborate is the label's central point: the period after each injection is the window that decides whether treatment is uncomplicated or catastrophic, and the dog's activity level during that window is the variable an owner actually controls.

A treatment-failure signal worth noting

The efficacy terms — ineffective against heartworm adults (4,219), ineffective against larvae (1,123), and lack of efficacy (2,658) — together exceed 8,000 reports. Some of this reflects the known imperfect efficacy of any single treatment course (the American Heartworm Society protocol uses three injections precisely because a single course does not clear every dog), and some reflects the documented emergence of macrocyclic lactone-resistant heartworm strains that leave more worms to be killed by the adulticide. The 2025 reporting volume, while lower than the mid-2010s peak, remains substantial. For owners, the practical implication is that a dog is not "clear" the day the last injection is given; antigen testing months later is how treatment success is actually confirmed, and a dog still antigen-positive at four months needs rechecking before more treatment.

A reporting curve that spans thirty years

Melarsomine has generated a remarkably steady reporting volume — roughly a thousand reports a year for two decades — which is what you would expect for a drug with no competitor and a stable indication. The recent decline (from a 2016 peak near 1,220 to under 700 in 2025) coincides with both the post-pandemic normalization of veterinary visits and a genuine epidemiological question: whether better prevention, the slow-kill alternative used during historical drug shortages, and shifting heartworm prevalence are reducing adulticide use, or whether reporting is simply falling. The data cannot separate those.

What to take into the exam room

• Exercise restriction is the whole game. The thromboembolic cluster and the death pattern in the data are the reason the label and the American Heartworm Society insist on strict cage rest through treatment and the weeks after. The variable that most changes outcome is the dog's activity level, and that is an owner variable.
• Expect injection-site soreness; treat it. Local swelling and pain are the most common reactions and are manageable with analgesia. A dog that is painful or lame at the injection site after melarsomine should be made comfortable, not ignored.
• Treat the thromboembolic window as the risk window. Cough, fever, lethargy, anorexia, or respiratory signs in the days after an injection are reasons to call the clinic the same day, not wait.
• Plan to confirm clearance. A single treatment course does not clear every dog, and the efficacy reports in the dataset are the reason a follow-up antigen test — not just the last injection — is what defines success.

Sources

• FDA CVM, animal adverse-event database (public extract, June 2026); analysis by VetMedGuide. The FDA cautions that spontaneous reports do not establish causation or true event rates. https://www.fda.gov/animal-veterinary/safety-health/reporting-animal-drug-and-device-side-effects-and-product-problems
• Immiticide (melarsomine dihydrochloride) sterile powder prescribing information — adverse reactions, pulmonary thromboembolism warning, injection-site administration (epaxial L3–L5), cage-rest recommendation. https://www.zoetisus.com/content/_assets/docs/vmips/package-inserts/immiticide-prescribing-information.pdf
• DailyMed, DIROBAN (melarsomine dihydrochloride and water kit) label — clinical field-trial adverse-event prevalence and post-approval paresis/paralysis reports. https://dailymed.nlm.nih.gov/dailymed/drugInfo.cfm?setid=8e49e345-ef33-45b9-acb1-8abb94f0060f
• American Heartworm Society, Current Canine Guidelines — three-dose melarsomine protocol, doxycycline (Wolbachia), exercise restriction, and the role of antigen testing to confirm clearance. https://www.heartwormsociety.org/veterinary-resources/american-heartworm-society-guidelines
• MSD Veterinary Manual, "Heartworm Disease in Dogs, Cats, and Ferrets" — melarsomine as the only FDA-approved adulticide, post-treatment adverse effects, and non-arsenical alternatives. https://www.msdvetmanual.com/circulatory-system/heartworm-disease/heartworm-disease-in-dogs-cats-and-ferrets
• dvm360, "IMMITICIDE (melarsomine dihydrochloride) Sterile Powder: Approved by the FDA as a canine heartworm treatment" — protocol background, adverse-event profile, and historical context. https://www.dvm360.com/view/immiticide-melarsomine-dihydrochloride-sterile-powder-approved-by-the-fda-as-a-canine-heartworm-treatment