Reported sign · VeDDRA term
Lack of efficacy (endoparasite) - heartworm: reports listing this term
In plain language: the product did not have the expected effect: heartworm
52,940 reports in FDA's animal adverse event database list the term "Lack of efficacy (endoparasite) - heartworm" (the product did not have the expected effect: heartworm): 3.9% of all reports. It is a VeDDRA term, chosen by the person who coded the report to match what was described.
5,628 reports listed it in the 12 months to March 2026, close to the 5,716 of the 12 months before. FDA posts reports quarterly with a delay of three to six months, so the newest months are incomplete and usually grow in later releases.
By active ingredient, the drugs named most often in those reports are ivermectin + pyrantel pamoate, moxidectin and milbemycin oxime + praziquantel. That order mostly follows how many reports each has overall; the table gives the term's share within each drug's own reports as well.
99.7% of the reports concern dogs, 0.3% of the reports concern cats and 0% of the reports concern other canids. Of reports listing this term that state seriousness, 37% were classed as serious; 0.5% record a death or euthanasia among the outcomes.
Drugs named most often in these reports, by active ingredient
| Active ingredients recorded for the drug | Reports listing the term | Share of that drug's own reports |
|---|---|---|
| Ivermectin + Pyrantel Pamoate | 22,378 | 13.8% |
| Moxidectin | 8,622 | 15% |
| Milbemycin Oxime + Praziquantel | 7,936 | 24% |
| Pyrantel Pamoate + Sarolaner | 5,531 | 10.6% |
| Milbemycin Oxime + Spinosad | 5,079 | 3.6% |
| Melarsomine Dihydrochloride | 2,742 | 11.6% |
| Lufenuron + Milbemycin Oxime | 1,979 | 7.2% |
| Ivermectin | 1,842 | 9.2% |
| Lufenuron + Milbemycin Oxime + Praziquantel | 1,650 | 24.6% |
| Doxycycline | 1,194 | 28.5% |
| Milbemycin Oxime | 1,163 | 3.6% |
| Afoxolaner + Moxidectin + Pyrantel Pamoate | 1,021 | 11.6% |
| Imidacloprid + Moxidectin | 865 | 3.7% |
| Selamectin | 829 | 1.3% |
| Afoxolaner | 748 | 1.2% |
| Fluralaner | 564 | 0.7% |
| Prednisone | 491 | 5.2% |
| Lotilaner | 468 | 4.1% |
| Ivermectin + Praziquantel + Pyrantel Pamoate | 423 | 4.7% |
| Moxidectin + Pyrantel Pamoate + Sarolaner | 310 | 28.4% |
| Sarolaner | 273 | 1.6% |
| Gabapentin | 148 | 1.4% |
| Carprofen | 119 | 0.2% |
| Oclacitinib | 103 | 0.3% |
| Imidacloprid | 96 | 1.4% |
Ordered by number of reports, which mostly tracks how many reports a drug has overall. Across the whole database the term appears in 3.9% of reports. A drug's presence in a report means the animal received it, not that it produced the sign; FDA states that these data cannot be used to compare one product with another. openFDA masks brand and manufacturer names in adverse event records, so the reports counted here cannot be attributed to a particular brand.
Single ingredients, pooled across combinations
| Active ingredient | Reports listing the term | Share of the ingredient's own reports |
|---|---|---|
| Pyrantel Pamoate | 28,956 | 11.6% |
| Ivermectin | 24,394 | 12.5% |
| Milbemycin Oxime | 16,514 | 7% |
| Moxidectin | 10,774 | 11.1% |
| Praziquantel | 9,981 | 12% |
| Sarolaner | 6,178 | 7.8% |
| Spinosad | 5,093 | 2.4% |
| Lufenuron | 3,523 | 8.9% |
| Melarsomine Dihydrochloride | 2,742 | 11.6% |
| Afoxolaner | 1,760 | 2.5% |
| Doxycycline | 1,194 | 28.5% |
| Imidacloprid | 953 | 3.3% |
| Selamectin | 893 | 1.2% |
| Fluralaner | 574 | 0.6% |
| Lotilaner | 538 | 4% |
| Prednisone | 491 | 5.2% |
| Gabapentin | 148 | 1.4% |
| Carprofen | 119 | 0.2% |
| Oclacitinib | 103 | 0.3% |
| Prednisolone | 82 | 1.5% |
| Trazodone | 72 | 4.5% |
| Dexamethasone | 67 | 1.3% |
| Tramadol | 52 | 1.4% |
| Fenbendazole | 51 | 1.2% |
| Diphenhydramine | 41 | 0.8% |
Species split
Coloured bar: share of reports listing this term. Grey bar: the species' share of all reports. The term appears in 5.4% of all dog reports; The term appears in 0.1% of all cat reports.
Trend
Reports by month FDA received them, five years to March 2026. The newest months are incomplete because FDA posts reports with a delay of three to six months.
Outcomes and ages in these reports
Outcome recorded in the report
Age of the animal
| Measure | Reports listing this term | All reports |
|---|---|---|
| Reports | 52,940 | 1,357,245 |
| Classed serious, of reports stating it | 37% | 38.6% |
| Death or euthanasia recorded, per 1,000 reports | 5 | 63.8 |
Grey bars and the right-hand column are the whole database. Outcomes are as stated by the reporter and describe the report as a whole, not this term alone.
Questions about "Lack of efficacy (endoparasite) - heartworm"
What does "Lack of efficacy (endoparasite) - heartworm" mean in an adverse event report?
In plain language: the product did not have the expected effect: heartworm. It is a VeDDRA dictionary term picked to code what the reporter described. The report's free-text description is not published by FDA and is not shown here.
How many reports list lack of efficacy (endoparasite) - heartworm?
52,940 reports through March 2026 (3.9% of all reports), 5,628 of them in the latest 12 months.
Which drugs are named most often alongside this term?
By active ingredient: ivermectin + pyrantel pamoate (22,378), moxidectin (8,622), milbemycin oxime + praziquantel (7,936), pyrantel pamoate + sarolaner (5,531) and milbemycin oxime + spinosad (5,079). Drugs with many reports overall lead almost every term's list. Brands cannot be identified: openFDA masks them.
Which species are involved?
dog (99.7%), cat (0.3%), other Canids (0%) and other (0%).
Does a drug appearing here mean it produced this sign?
No such conclusion can be drawn. FDA states that "there is no certainty that the reported drug or device caused the adverse event" and that the event "may have been related to an underlying disease, other drugs used at the same time or other non-drug related sources."
My animal shows this sign. What now?
This site cannot assess an animal. A veterinarian can; for an emergency, contact a veterinary emergency clinic. See the guide on when to call a vet.
Where this comes from. Counts are built from the openFDA Animal & Veterinary adverse event bulk files released 28 Sep 2026, which hold reports FDA's Center for Veterinary Medicine received through 31 Mar 2026. openFDA masks brand and manufacturer names in these files, so drugs are counted by the active ingredients each record discloses and no figure is attributed to a brand, application or company. Ingredient names, and the separate reference lists of approved products, follow the FDA Green Book as of 30 Sep 2026. Spelling variants of the same VeDDRA term (for example with and without a cross-reference note) are pooled. No report text, lot numbers or reporter details are published. See the methodology and sources; FDA's record is authoritative, and corrections are handled within five working days.