Didn't prevent heartworm after Fenbendazole in dogs

The FDA has received 65 reports of didn't prevent heartworm in dogs after Fenbendazole. That is 1.1% of all reports for this medicine, compared with 13% across reports for all dog medicines, so it is reported less often than average with this medicine.

65Reports
25%Marked serious
1.5%Recovered
3.1%Died or euthanised
178 daysTypical onset
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How soon it started

Same day
1 2.6%
1–2 days
0 0.0%
3–7 days
1 2.6%
1–4 weeks
3 7.9%
Over a month
33 87%

How the animals did

Recovered
1 1.5%
Still unwell when reported
13 20%
Euthanised
2 3.1%
Not known
49 75%

Problems reported at the same time

Didn't stop worms
14 22%
Weight gain
7 11%
Didn't work as expected
6 9.2%
Behaviour change
6 9.2%
Vomiting
5 7.7%
Diarrhea
5 7.7%
Underdose
4 6.2%
Drug dose omission
4 6.2%

Age of the animals

Under 1 year
6
1–3 years
41
4–7 years
12
8–11 years
4
12+ years
0

About this medicine

Fenbendazole treats intestinal worms and some other parasites.

Questions owners ask

Can Fenbendazole cause didn't prevent heartworm in dogs?
65 reports link the two, but a report alone does not prove the medicine was the cause: other illnesses, other medicines or coincidence can be involved. Your vet can judge it for your animal.
How soon after a dose was it reported?
In half of the reports with dates, it started within 178 days of the first dose.
What should I do?
Don't give another dose until you've spoken to your vet, and keep the packaging so you can tell them the product and dose. Your vet can also report it to the FDA (or you can, through the manufacturer or FDA form 1932a).

Didn't prevent heartworm with similar medicines

All reported side effects of Fenbendazole in dogs · Medicines most reported with didn't prevent heartworm