Methimazole

What is this medication for?

• Hyperthyroidism (medical management, or pre-surgical/pre-radioiodine stabilization)

How is it given?

Give exactly as your veterinarian prescribed — never change the dose or stop on your own unless your veterinarian has told you to.

• Human handlers, especially pregnant women, should wear gloves when handling tablets, cat litter, or bodily fluids of treated cats — significant human absorption risk, particularly with transdermal formulation.

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Vomiting, loss of appetite, low energy/tiredness (Most common, first 3 months, ~10%)
• Eosinophilia, low white blood cell counts, low platelet counts, lymphocytosis (~15% in first 8 weeks, usually transient)

Worth a phone call to your veterinarian

• Self-induced facial excoriation, bleeding, hepatopathy, agranulocytosis, positive ANA (Rare (1.5–2.7% each) but serious — require withdrawal)
• Acquired myasthenia gravis (Rare)
• Unmasking of underlying renal disease (Documented — reducing T4 lowers GFR)

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Trouble breathing, collapse, seizures, severe vomiting or diarrhea, facial swelling, or pale gums

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Hypersensitivity to methimazole/carbimazole/polyethylene glycol; autoimmune disease, primary liver disease, renal failure, coagulopathy, pregnant/lactating queens (label)
• has or has had: History of hematologic abnormalities

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Benzimidazole antiparasitics
  • Digoxin, theophylline, beta-blockers
  • Warfarin

Storage and handling

Room temperature, light-resistant, protect from moisture. Compounded transdermal gel: potency assured only ~2 weeks.

Good to know

• Human handlers, especially pregnant women, should wear gloves when handling tablets, cat litter, or bodily fluids of treated cats — significant human absorption risk, particularly with transdermal formulation.
• Twice-daily dosing is more effective than once daily — don't consolidate to simplify owner compliance without a good reason.
• This drug controls but does not cure hyperthyroidism — set owner expectations for lifelong therapy or definitive treatment (surgery/radioiodine).