Hyperphosphataemia after Methimazole in cats

20 reports to the FDA describe hyperphosphataemia in cats given Methimazole. That is 1.1% of all reports for this medicine, compared with 1.0% across reports for all cat medicines.

20Reports

85%Marked serious

0.0%Recovered

35%Died or euthanised

36 daysTypical onset

How soon it started

Same day

1 6.3%

1–2 days

0 0.0%

3–7 days

2 13%

1–4 weeks

4 25%

Over a month

9 56%

How the animals did

Still unwell when reported

10 50%

Died

2 10%

Euthanised

5 25%

Not known

3 15%

Problems reported at the same time

19 95%

12 60%

11 55%

7 35%

7 35%

7 35%

Hypernatremia

6 30%

Elevated symmetrical dimethylarginine (sdma)

6 30%

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 hyperphosphataemia in cats?: 20 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 36 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).

Hyperphosphataemia with similar medicines

Hyperphosphataemia after Velagliflozin (Senvelgo) 79 reports · 1.6% of its reports
Hyperphosphataemia after Bexagliflozin (Bexacat) 73 reports · 1.8% of its reports
Hyperphosphataemia after Insulin 25 reports · 0.7% of its reports

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