Lack of efficacy (endoparasite) - heartworm: in adverse event reports for animal drugs

Clinical sign, a VeDDRA term

In plain words: the product did not have the expected effect: heartworm.

Read this first. FDA says this about the data:

An API dataset query result cannot be used to estimate the risk associated with the product or compare one drug or device product with another. When looking at the results, be aware that there is no certainty that the reported drug or device caused the adverse event. This dataset should not be the sole source of information for clinical decision-making or other assumptions about the safety or effectiveness of a product. The adverse event may have been related to an underlying disease, other drugs used at the same time or other non-drug related sources.
FDA Center for Veterinary Medicine publishes these words. Source: Adverse Event Reports for Animal Drugs and Devices, read on 2 Oct 2026. The data have the reports that FDA received through 26 Jun 2026.

This page is not veterinary advice. For an animal that is ill, speak to a veterinarian.

Contacting a veterinarian How to report to FDA

From May 2002 to June 2026, FDA received 54,242 adverse event reports for animal drugs that list this sign. That is 3.9% of the 1,378,761 reports that FDA received from January 1987 to June 2026. In the 12 months to June 2026, FDA received 5,433 reports that list this sign.

Of the 54,242 reports, 99.7% are about dogs. A report can list more than one sign and more than one drug. A report is not proof that a drug caused a sign.

By number of reports in all years, this sign is number 9 of 1,149 clinical signs with a page. Reports that list this sign fell by 5.2% in the 12 months to June 2026, compared with the 12 months before. Reports for all animal drugs fell by 4.6% in the same periods. The newest months can get more reports in a later update. Reports about dogs are 99.7% of the reports that list this sign, and 72.6% of all reports.

Drug pages with reports that list this sign

The table has every drug page with 20 or more reports that list this sign, in order of name. No count decides which drug is in the table. The table has 47 drug pages.

Show the 47 drug pages

Drug pages with 20 or more reports that list this sign, in order of name
Active ingredients	Reports that list the sign
(S) - methoprene	27
Acepromazine Maleate	31
Afoxolaner	772
Afoxolaner + Moxidectin + Pyrantel Pamoate	1,168
Buprenorphine	30
Butorphanol Tartrate	31
Carprofen	121
Cephalexin	20
Deracoxib	21
Dexamethasone	64
Diphenhydramine	44
Doxycycline	1,244
Febantel + Praziquantel + Pyrantel Pamoate	34
Fenbendazole	51
Fluoxetine hydrochloride	36
Fluralaner	580
Furosemide	23
Gabapentin	157
Imidacloprid	96
Imidacloprid + Moxidectin	887
Ivermectin	1,860
Ivermectin + Praziquantel + Pyrantel Pamoate	430
Ivermectin + Pyrantel Pamoate	22,771
Lotilaner	484
Lotilaner + Moxidectin + Praziquantel + Pyrantel Pamoate	126
Lufenuron + Milbemycin Oxime	1,994
Lufenuron + Milbemycin Oxime + Praziquantel	1,703
Maropitant	38
Melarsomine Dihydrochloride	2,840
Meloxicam	22
Metronidazole	27
Milbemycin Oxime	1,189
Milbemycin Oxime + Praziquantel	8,054
Milbemycin Oxime + Spinosad	5,117
Moxidectin	8,908
Moxidectin + Pyrantel Pamoate + Sarolaner	319
Oclacitinib	107
Prednisolone	83
Prednisone	521
Pyrantel Pamoate	58
Pyrantel Pamoate + Sarolaner	5,803
Sarolaner	278
Sarolaner + Selamectin	83
Selamectin	844
Spinosad	27
Tramadol	53
Trazodone	78

The openFDA records give the value "MSK" for the brand name and for the manufacturer name of each drug. FDA says that MSK means "masked". So the site cannot count a report for a brand. A report can name more than one drug.

An API dataset query result cannot be used to estimate the risk associated with the product or compare one drug or device product with another.
The adverse event may have been related to an underlying disease, other drugs used at the same time or other non-drug related sources.
FDA Center for Veterinary Medicine. Source: Adverse Event Reports for Animal Drugs and Devices

Ingredients, all combinations

The list has every active ingredient with 20 or more reports that list this sign, in order of name. It has 38 ingredients.

Show the 38 ingredients

Reports by species

Dog99.7%54,086
Cat0.3%146
Other Canidsunder 0.1%4
Otherunder 0.1%3
Wolfunder 0.1%2
Cattleunder 0.1%1

The share is of the 54,242 reports that list this sign. The sign is in 5.4% of all reports about the species dog. The sign is in 0.1% of all reports about the species cat.

Reports by month and by year

Reports for each month, Jul 2021 to Jun 2026. The month is the month in which FDA received the report.

FDA says that a report can take 3 to 6 months to reach the public data. The newest months can get more reports in a later update.

Reports that list Lack of efficacy (endoparasite) - heartworm, by the year in which FDA received them
Year	Reports
2026	2,560
2025	5,678
2024	5,896
2023	6,301
2022	7,073
2021	7,548
2020	7,586
2019	6,909
2018	3,497
2017	626
2016	267
2015	12
2014	110
2013	3
2012	5
2004	2
2002	169

What these reports record

Outcome at the time of the report

Reports with this sign
All reports

Recovered/Normal1.8%964, All reports: 20.7%
Recovered with Sequela[recovered, with a lasting effect]10.9%5,920, All reports: 2.3%
Ongoing42.7%23,176, All reports: 17.4%
Died0.3%148, All reports: 4.9%
Euthanized0.2%121, All reports: 1.5%
Outcome Unknown44.1%23,922, All reports: 22.7%

An outcome in a report is not proof that the drug caused it. The person who sends the report gives the outcome. Each share is of the 54,242 reports that list this sign.

Age of the animal

Reports with this sign
All reports

Under 1 year1.4%760, All reports: 9.5%
1 to 3 years54.8%29,714, All reports: 23.9%
4 to 7 years27.3%14,808, All reports: 20.2%
8 to 11 years11.9%6,439, All reports: 15.4%
12 years and over2.7%1,480, All reports: 10.7%
Age not given1.9%1,041, All reports: 20.3%

The outcome is for the full report, not for this sign only.

Next in the A to Z list of signs

Lack of efficacy (ectoparasite) - mite NOS[the product did not have the expected effect: mite NOS]
Lack of efficacy (ectoparasite) - NOS[the product did not have the expected effect: target not specified]
Lack of efficacy (ectoparasite) - Sarcoptes[the product did not have the expected effect: Sarcoptes]
Lack of efficacy (ectoparasite) - tick NOS[the product did not have the expected effect: ticks]
Lack of efficacy (endoparasite) - hookworm[the product did not have the expected effect: hookworms]
Lack of efficacy (endoparasite) - NOS[the product did not have the expected effect: target not specified]
Lack of efficacy (endoparasite) - roundworm NOS[the product did not have the expected effect: roundworms]
Lack of efficacy (endoparasite) - tapeworm[the product did not have the expected effect: tapeworms]

All clinical signs

Cite this page

Pet Drug Reports. Lack of efficacy (endoparasite) - heartworm: adverse event reports for animal drugs. https://petdrugreports.com/reaction/lack-of-efficacy-endoparasite-heartworm/ Source: FDA Center for Veterinary Medicine, openFDA Animal and Veterinary adverse event reports, export of 30 Oct 2026. Reports received through 26 Jun 2026.

Where the data come from

The counts come from the openFDA Animal and Veterinary adverse event files, export of 30 Oct 2026. The files have the reports that the FDA Center for Veterinary Medicine received through 26 Jun 2026. The files mask brand and manufacturer names. The site counts a drug by the active ingredients that each record gives, and gives no count to a brand, an application or a company. Ingredient names and the lists of approved products follow the FDA Green Book of 2 Nov 2026. The site counts the different spellings of one VeDDRA term together.

The site does not publish report text, lot numbers or details of a reporter. FDA's record is the authority. See how the site counts reports, the data sources and the corrections page.

Questions about this sign

What does this term mean in a report?: In plain words: the product did not have the expected effect: heartworm. It is a term from VeDDRA, the list of terms that FDA and drug companies use to code a report. The person who coded the report selected the term.
How does the count of the latest 12 months compare with the 12 months before?: FDA received 5,433 reports in the 12 months to June 2026. It received 5,729 reports in the 12 months before. FDA says that a report can take 3 to 6 months to reach the public data. The newest months can get more reports in a later update.
How many of these reports have a serious flag?: Of the 53,959 reports that say if the event was serious, 37.5% have the serious flag. The person who sends a report sets the flag.
Does a drug in the table cause this sign?: The reports do not show that. A report can name more than one drug. FDA states this about the data.
When looking at the results, be aware that there is no certainty that the reported drug or device caused the adverse event.
The adverse event may have been related to an underlying disease, other drugs used at the same time or other non-drug related sources.
FDA Center for Veterinary Medicine. Source: Adverse Event Reports for Animal Drugs and Devices