Lethargy after Ivermectin (Heartgard) in cats

The FDA has received 178 reports of lethargy in cats after Ivermectin. That is 18% of all reports for this medicine, compared with 14% across reports for all cat medicines.

178Reports

18%Marked serious

19%Recovered

15%Died or euthanised

1 daysTypical onset

How soon it started

Same day

21 50%

1–2 days

5 12%

3–7 days

3 7.1%

1–4 weeks

6 14%

Over a month

7 17%

How the animals did

Recovered

26 15%

Recovered with lasting effects

7 3.9%

Still unwell when reported

4 2.2%

Died

26 15%

Euthanised

1 0.6%

Not known

114 64%

Problems reported at the same time

Wobbliness (ataxia)

57 32%

Loss of appetite

47 26%

Eye or vision problems

38 21%

37 21%

27 15%

25 14%

15 8.4%

14 7.9%

Age of the animals

Under 1 year

1–3 years

4–7 years

8–11 years

12+ years

About this medicine

Ivermectin prevents heartworm disease (monthly).

Products with this ingredient include Heartgard. The FDA hides brand names in its public data, so these reports cover every product with the same active ingredients.

Some herding breeds (for example Collies) carry the MDR1 gene variant and can be more sensitive to this drug class at high doses.

Questions owners ask

Can Ivermectin cause lethargy in cats?: 178 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 1 day 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).

Lethargy with similar medicines

Lethargy after Selamectin (Revolution) 3,126 reports · 14% of its reports
Lethargy after Eprinomectin + Esafoxolaner + Praziquantel (NexGard Combo) 533 reports · 16% of its reports
Lethargy after Eprinomectin + Praziquantel 96 reports · 16% of its reports
Lethargy after Milbemycin Oxime (Interceptor) 79 reports · 20% of its reports
Lethargy after Milbemycin Oxime + Spinosad (Trifexis) 91 reports · 30% of its reports

All reported side effects of Ivermectin in cats · Medicines most reported with lethargy