Lethargy after Milbemycin Oxime (Interceptor) in cats

The FDA has received 79 reports of lethargy in cats after Milbemycin Oxime. That is 20% of all reports for this medicine, compared with 14% across reports for all cat medicines.

79Reports

11%Marked serious

24%Recovered

2.5%Died or euthanised

1 daysTypical onset

How soon it started

Same day

12 43%

1–2 days

7 25%

3–7 days

2 7.1%

1–4 weeks

2 7.1%

Over a month

5 18%

How the animals did

Recovered

18 23%

Recovered with lasting effects

1 1.3%

Still unwell when reported

8 10%

Died

1 1.3%

Euthanised

1 1.3%

Not known

50 63%

Problems reported at the same time

28 35%

20 25%

19 24%

15 19%

Fever

11 14%

Tremors

10 13%

Drooling

6 7.6%

Liver problems

5 6.3%

Age of the animals

Under 1 year

1–3 years

4–7 years

8–11 years

12+ years

About this medicine

Milbemycin Oxime prevents heartworm and controls some intestinal worms (monthly).

Products with this ingredient include Interceptor. 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 Milbemycin Oxime cause lethargy in cats?: 79 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 Ivermectin (Heartgard) 178 reports · 18% of its reports
Lethargy after Eprinomectin + Praziquantel 96 reports · 16% of its reports
Lethargy after Milbemycin Oxime + Spinosad (Trifexis) 91 reports · 30% of its reports

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