Decreased haematocrit after Methimazole in cats

The FDA has received 34 reports of decreased haematocrit in cats after Methimazole. That is 1.9% of all reports for this medicine, compared with 0.2% across reports for all cat medicines, so it is reported unusually often with this medicine.

34Reports

68%Marked serious

0.0%Recovered

21%Died or euthanised

41 daysTypical onset

How soon it started

Same day

2 6.9%

1–2 days

0 0.0%

3–7 days

0 0.0%

1–4 weeks

10 34%

Over a month

17 59%

How the animals did

Still unwell when reported

12 35%

Died

5 15%

Euthanised

2 5.9%

Not known

15 44%

Problems reported at the same time

17 50%

17 50%

14 41%

14 41%

13 38%

11 32%

10 29%

Decreased haemoglobin

10 29%

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 decreased haematocrit in cats?: 34 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 41 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).

Decreased haematocrit with similar medicines

Decreased haematocrit after Velagliflozin (Senvelgo) 20 reports · 0.4% of its reports
Decreased haematocrit after Bexagliflozin (Bexacat) 52 reports · 1.3% of its reports
Decreased haematocrit after Insulin 22 reports · 0.6% of its reports

All reported side effects of Methimazole in cats · Medicines most reported with decreased haematocrit