Behaviour change after Methimazole in cats

The FDA has received 97 reports of behaviour change in cats after Methimazole. That is 5.4% of all reports for this medicine, compared with 5.4% across reports for all cat medicines.

97Reports

47%Marked serious

30%Recovered

9.3%Died or euthanised

19 daysTypical onset

How soon it started

Same day

13 17%

1–2 days

6 7.7%

3–7 days

12 15%

1–4 weeks

20 26%

Over a month

27 35%

How the animals did

Recovered

27 28%

Recovered with lasting effects

2 2.1%

Still unwell when reported

30 31%

Died

2 2.1%

Euthanised

7 7.2%

Not known

29 30%

Problems reported at the same time

27 28%

26 27%

22 23%

17 18%

17 18%

16 16%

Restlessness or agitation

13 13%

Kidney problems

11 11%

Age of the animals

Under 1 year

1–3 years

4–7 years

8–11 years

12+ years

About this medicine

Methimazole treats an overactive thyroid in cats.

Questions owners ask

Can Methimazole cause behaviour change in cats?: 97 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 19 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).

Behaviour change with similar medicines

Behaviour change after Velagliflozin (Senvelgo) 42 reports · 0.9% of its reports
Behaviour change after Bexagliflozin (Bexacat) 230 reports · 5.7% of its reports
Behaviour change after Insulin 112 reports · 3.0% of its reports

All reported side effects of Methimazole in cats · Medicines most reported with behaviour change