Your cat has been on methimazole for two weeks, and this morning she brought her breakfast back up. Or she has scratched a raw patch behind her ear. Or the recheck bloodwork came back with a liver value flagged, and nobody has phoned you yet.
So you searched, and you got a list: vomiting, anorexia, lethargy, itching, liver disease, low platelets, agranulocytosis. All of it true, all of it in one undifferentiated column, none of it telling you the one thing you actually want to know — is this the kind of thing that passes, or the kind of thing that ends the drug?
Those are not one category. They are three, and they behave completely differently. Below: what the reported reactions are, when each kind shows up, which ones stop the prescription, what the transdermal gel does and does not fix, and why a worsening kidney value after the drug starts working is usually not the drug at all.
Key takeaways
- Most reactions arrive early. In the largest published series, clinical side effects hit 48 of 262 cats (18.3%), "usually within the first month of therapy," and the blood-count changes followed inside two months.⁵
- The drug-stopping reactions are a small, named set. Merck: "Serious adverse effects develop in < 5% of treated cats and include agranulocytosis, thrombocytopenia, hepatopathy, or severe facial pruritus."²
- The transdermal gel fixes one category only. A randomised trial found significantly fewer gastrointestinal effects on the gel, but "no differences … in the incidence of neutropenia, hepatotoxicity, or facial excoriations."⁶
- Worsening kidney values after the drug works is usually not the drug. Merck: "Establishing a euthyroid state can unmask renal disease, even in patients with normal renal marker parameters."²
What are the side effects of methimazole in cats?
Common ones are digestive and dermatological; rare ones are haematological and hepatic. The FDA's list of the most frequently reported reactions runs: "decreased appetite; vomiting; decreased activity level; skin reactions, such as itchy skin (especially on the face, head, and neck), hair loss, and other skin lesions; weight loss; elevated liver values; decreased red blood cell level (anemia); behavioral changes (for example, vocalizing or hiding); elevated kidney values; and diarrhea or loose stool."¹
That list is accurate and, on its own, close to useless — because it puts a cat who threw up once next to a cat whose white cell count has collapsed. What a veterinarian is actually doing when you phone is sorting the event into one of three boxes, and the box decides everything that follows.
| Category | What it looks like | What usually happens to the prescription |
|---|---|---|
| Self-limiting upset | Vomiting, reduced or absent appetite, diarrhoea, lethargy⁴¹ | Often survives it. Today's Veterinary Practice: pause, supportive care, then "the medication can be restarted at a lower dose"⁴ |
| Drug-stopping reactions | Agranulocytosis, thrombocytopenia, hepatopathy, severe facial pruritus² and self-induced excoriation of the face and neck⁵ | Ends it. Merck puts these at "< 5% of treated cats"; carbimazole is not a substitute because it converts to methimazole² |
| Not an adverse drug reaction | Kidney values rising once thyroid levels normalise¹² | Not toxicity. Merck: "Establishing a euthyroid state can unmask renal disease"² |
Read the third row twice. It is the one that most often gets an owner to the wrong conclusion, because "her kidney numbers got worse after we started the tablets" sounds exactly like a drug injuring a kidney, and generally it is not.
When do methimazole side effects start, and how long do they last?
Early, and the early window is the whole design of the monitoring schedule. Peterson and colleagues followed 262 hyperthyroid cats and found clinical side effects in 48 of them — 18.3% — "usually within the first month of therapy." Mild blood-count changes appeared in another 43 cats (16.4%), "usually within the first 2 months of treatment," and consisted of eosinophilia, lymphocytosis and slight leukopenia.⁵
The serious reactions run a little later but still inside the same season. Today's Veterinary Practice places lymphadenopathy, hepatopathies, aplastic anaemia, thrombocytopenia and agranulocytosis as "generally manifested within the first few months of treatment."⁴ VCA puts the common effects "within the first three months" too.³
Going the other way is quicker than most owners expect. Merck states that these adverse reactions "should resolve within 2 weeks" once the drug stops.² Peterson's blood abnormalities were faster still: they "resolved within 1 week after cessation of methimazole treatment."⁵ That reversibility is the reason methimazole is so often the first treatment tried rather than radioiodine — a tablet you can stop buys information a permanent treatment cannot.
One consequence worth holding onto: a cat steady on methimazole for a year is not in the same risk position as a cat three weeks in. That is why the FDA's testing interval widens rather than staying flat.¹
Which methimazole side effects mean stopping the drug?
A short, specific list. Merck's Veterinary Manual names it outright: "Serious adverse effects develop in < 5% of treated cats and include agranulocytosis, thrombocytopenia, hepatopathy, or severe facial pruritus."² Those four are the reactions that generally end the prescription rather than adjust it.
Peterson's series gives the number a shape. Of 262 cats, ten (3.8%) developed "more serious hematologic reactions … including agranulocytosis and thrombocytopenia (associated with bleeding)."⁵ Agranulocytosis means the white cells that fight bacterial infection have effectively gone; thrombocytopenia means the platelets that stop bleeding have. Neither announces itself the way vomiting does, which is why the bloodwork is scheduled rather than symptom-triggered.
Today's Veterinary Practice extends the severe list to lymphadenopathy, hepatopathies and aplastic anaemia, and adds less frequent events: "facial pruritus, exfoliative dermatitis, myasthenia gravis, and a bleeding disorder secondary to vitamin K antagonism."⁴ The facial itching is the one owners see first, and it is not ordinary scratching — a small number of cats excoriate the face and neck badly enough to draw blood. That presentation sits on the drug-stopping side of the line.
There is one substitution that looks obvious and is not. Carbimazole is a related antithyroid drug, and it is tempting to read a methimazole reaction as a reason to switch. Merck closes that door: carbimazole "exerts its effects through immediate conversion to methimazole after administration and therefore should not be administered to cats that do not tolerate methimazole."² It is the same drug wearing a different name by the time it reaches the thyroid.
The FDA adds a watch list that is easy to miss because none of its items sound like a drug reaction: veterinarians "should monitor cats on methimazole for signs of illness, including fever, enlarged lymph nodes, and anemia."¹ A cat who is suddenly feverish three weeks into treatment is a phone call, not a wait-until-Monday.
My cat is vomiting on methimazole — do we have to stop?
Probably not for good. Gastrointestinal upset is both the most common reaction and the one with the most workarounds. Today's Veterinary Practice sets out the standard path: "Most of the gastrointestinal side effects can be controlled by discontinuation of the medication and supportive care. Once the signs have resolved, the medication can be restarted at a lower dose."⁴
Merck offers the second lever — route: gastrointestinal upset "may be mitigated by changing to a transdermal formulation."² Between a lower dose, a pause and a different route, the stomach is the problem the prescription most often survives.
What does not work is the intuitive fix. If twice a day upsets her, why not give the whole thing once? Trepanier and colleagues ran exactly that comparison in 40 newly diagnosed cats, 5 mg once daily against 2.5 mg twice daily. Adverse effects — "primarily gastrointestinal tract upset and facial pruritus" — were not significantly different between the groups. What did differ was control: 54% of the once-daily cats were euthyroid at two weeks against 87% of the twice-daily cats, and the authors concluded once-daily dosing "cannot be recommended for routine use."⁷ You give up efficacy and get no tolerability back.
Two practical notes while the stomach is unsettled. Do not double up: VCA's instruction on a missed dose is to give it when you remember and resume the schedule — "Do not give the pet two doses at once."³ A dose that came back up ten minutes later is a question for your vet, not a reason to re-dose, and where two people share the tablets our guide to keeping a double dose from happening covers the rest. And if the tablet itself is the fight rather than the drug, our walkthrough on giving a cat a pill without the wrestling match may save the route change.
Does the transdermal gel really have fewer side effects?
It has fewer of one kind, and that qualifier is the whole answer. Sartor and colleagues randomised 47 newly diagnosed hyperthyroid cats to transdermal methimazole in pluronic lecithin organogel, applied to the inner ear flap, or to oral methimazole at 2.5 mg every 12 hours by either route. Cats on the oral route had gastrointestinal adverse effects in 4 of 17 cases, against 1 of 27 on the gel — a significant difference at P = .04.⁶
Then comes the sentence the pharmacy pages leave out. In the same trial, "no differences were found between groups in the incidence of neutropenia, hepatotoxicity, or facial excoriations."⁶ The three categories that end a prescription were unchanged by the route. The gel is a genuinely good answer to a cat who vomits her tablets; it is not protection against the reactions that matter most.
The trade is on the other side of the ledger. Significantly more oral cats had thyroid levels inside the reference range at two weeks — 14 of 16 against 14 of 25 — though that gap was no longer significant by four weeks.⁶ Slower control, gentler on the stomach, same risk of the serious reactions. That is the honest shape of the choice, and it is a conversation with your vet rather than a preference to state.
Why did my cat's kidney values get worse after starting methimazole?
Because the thyroid disease was hiding them. This is the single most misread event in feline hyperthyroidism treatment, and both the FDA and Merck explain it in almost the same words. The FDA: "In cats with hyperthyroidism, the high level of thyroid hormones increases the blood flow to the kidneys, which helps their kidneys and may even hide kidney disease." Once the drug lowers those hormones, "the blood flow to the kidneys returns to normal and the kidney function in some cats will worsen."¹
Merck states the same conclusion flatly: "Establishing a euthyroid state can unmask renal disease, even in patients with normal renal marker parameters."² The kidney disease was there before the first tablet. The tablet removed the thing that was flattering the numbers.
The distinction matters because the two readings lead to opposite actions. Read as toxicity, the answer is to stop the drug. Read as unmasking, your vet now has information no other test could have given them, and can weigh how tightly to control the thyroid against what the kidneys will tolerate. The FDA calls treating these cats "a delicate balancing act," and notes that older cats often have both diseases at once.¹ A reversible tablet is what lets that balance be tested before anything permanent happens. If you are watching the kidney side, our guide to which signs actually point at the kidneys covers what shows up at home.
Which is also why the dose is not simply driven as low as it will go. Merck's target is a landing zone rather than a floor: "The dosage is adjusted at 2-week intervals to achieve a T4 concentration in the low- to mid-reference interval."²
Is methimazole safe for the people in the house?
It requires handling precautions that almost no owner-facing page leads with. The FDA is explicit: "Methimazole can cause birth defects in people." The drug "crosses the placenta and concentrates in the thyroid gland of the fetus," and it "is also found in breast milk."¹
Exposure is not limited to the tablet. "Trace amounts of methimazole can be found in a treated cat's feces (stool) and bodily fluids," which is why the FDA advises gloves "when you scoop your cat's litter or if you have contact with your cat's feces, urine, or vomit," gloves when handling the drug itself, and hand-washing with soap and water after every dose.¹ Pregnant women, women who may become pregnant and nursing mothers "should use extra caution." VCA adds gloves specifically for applying the transdermal gel.³
In a one-person household that is a habit to build once. In a household where several people share the dosing rota — and hyperthyroid cats are twice-a-day cats for life, so most households end up sharing it — the precaution has to travel with the task. The person who covers a Tuesday evening because someone else is out needs the same glove and the same hand-wash, and needs to know why.
How do you tell a side effect from the disease?
By having a record of what she was like before the drug started — which is harder than it sounds, because hyperthyroidism and its treatment share almost every symptom. Vomiting, poor appetite, weight loss and lethargy all appear on the FDA's list of methimazole side effects.¹ Vomiting, diarrhoea and weight loss are also signs of untreated hyperthyroidism itself.¹ Cornell's list of possible medication effects — "vomiting, anorexia, fever, anemia, and lethargy" — overlaps the disease almost completely.⁸
So "she's been off her food this week" is not, by itself, an answer to anything. Off her food compared with when? That is the question the recheck turns on, and it is answerable only from something written down at the time.
| When | What the FDA advises | What it is looking for |
|---|---|---|
| Before the first dose | Complete blood count, chemistry panel, thyroid hormone levels¹ | The baseline every later result is read against |
| 3 weeks | Repeat bloodwork¹ | Inside the window where clinical side effects usually appear⁵ |
| 6 weeks | Repeat bloodwork¹ | Inside the window for blood-count changes⁵ |
| Every 3 months thereafter | Repeat for as long as the cat is on the drug¹ | Long-term liver, kidney and blood-cell surveillance |
| Above 10 mg per day | Test more often than the schedule above¹ | Higher dose, tighter watch |
What to bring to each of those appointments, in roughly the order a vet will ask for it:
- Every dose given, and every dose missed — a cat unresponsive at 70% adherence is a different problem from one unresponsive at 100%.
- Vomiting, with dates — which days, and whether it clusters near dosing.
- Appetite, judged the same rough way every day rather than precisely but occasionally.
- Weight, on the same scales — the number that settles whether treatment is working.
- Any scratching at the face or neck, with the date it started and whether the skin is broken.
- Anything from the FDA's watch list — fever, swollen glands, pale gums, yellowing.¹
Cornell is blunt that this is a long commitment: "Lifelong treatment, usually involving twice-daily oral dosage, will be required, and for some owners and cats, this dosage schedule may be difficult to maintain."⁸ Our companion guide to methimazole and the twice-daily routine covers the dosing side, and our vet visit checklist covers what to have ready. Bring a record rather than an impression, and the recheck becomes a decision instead of a guess.
Veterinary medical disclaimer: SteadyTails is a logging and coordination tool, not a substitute for veterinary care. Nothing on this page is a diagnosis, a prescription or a dosing instruction. Methimazole is prescription-only, and any figure quoted here is a number recorded in a veterinary reference or a published study, not an amount to give your cat. Never start, stop, pause, lower or double a dose on your own judgement — including after a vomited tablet. If your cat develops a fever, swollen glands, pale or yellow gums, unexplained bruising or bleeding, stops eating, or scratches her face or neck badly enough to break the skin while taking methimazole, contact your veterinarian promptly. If she collapses, bleeds without stopping, or has been given a double dose, contact your veterinarian or an emergency animal hospital straight away; in the US, ASPCA Animal Poison Control is available around the clock on 888-426-4435.
"Has she been sick more this week, or does it just feel that way?"
Methimazole is a twice-a-day tablet for life, usually shared between people, and the recheck turns on details nobody can hold in their head — which days she vomited, when the scratching started, what she actually ate. SteadyTails keeps doses, refusals, weight and symptom notes on one timeline everyone in the household writes to, so the next appointment starts from a record. Coming soon to iOS and Android.
Join the SteadyTails waitlist →References
- U.S. Food and Drug Administration. Hyperthyroidism in Cats—Two FDA-Approved Drugs Available to Treat It — "Two drugs, Felimazole oral tablets and Felanorm oral solution, are FDA-approved to treat hyperthyroidism in cats." Most common side effects: "decreased appetite; vomiting; decreased activity level; skin reactions, such as itchy skin (especially on the face, head, and neck), hair loss, and other skin lesions; weight loss; elevated liver values; decreased red blood cell level (anemia); behavioral changes (for example, vocalizing or hiding); elevated kidney values; and diarrhea or loose stool." Serious: "liver disease; immune-mediated anemia …; low platelets; and severely low level of neutrophils in the blood." "Blood tests should be repeated 3 weeks and 6 weeks after starting methimazole and then every 3 months for as long as your cat is on the drug." "Blood tests should be done more often in cats receiving high doses of methimazole (more than 10 milligrams per day)." "Veterinarians should monitor cats on methimazole for signs of illness, including fever, enlarged lymph nodes, and anemia." "In cats with hyperthyroidism, the high level of thyroid hormones increases the blood flow to the kidneys, which helps their kidneys and may even hide kidney disease." "…the blood flow to the kidneys returns to normal and the kidney function in some cats will worsen." "Older cats often have both hyperthyroidism and kidney disease, and treating these cats is a delicate balancing act." "Methimazole can cause birth defects in people." "The drug crosses the placenta and concentrates in the thyroid gland of the fetus." "The drug is also found in breast milk." "Trace amounts of methimazole can be found in a treated cat's feces (stool) and bodily fluids." "…wear gloves when you scoop your cat's litter or if you have contact with your cat's feces, urine, or vomit." "Wash your hands with soap and water after giving your cat methimazole…" "Pregnant women, women who may become pregnant, and nursing mothers should use extra caution when handling or administering Felimazole or Felanorm." "Both Felimazole and Felanorm are only available with a veterinarian's prescription." fda.gov Retrieved 2026-09-30.
- Merck Veterinary Manual. Hyperthyroidism in Animals — "Serious adverse effects develop in < 5% of treated cats and include agranulocytosis, thrombocytopenia, hepatopathy, or severe facial pruritus." "These adverse reactions should resolve within 2 weeks." "The most common side effect of methimazole is GI upset, which may be mitigated by changing to a transdermal formulation." "The initial dose of methimazole is 1.25–2.5 mg/cat, PO or transdermal, every 12 hours." "The dosage is adjusted at 2-week intervals to achieve a T4 concentration in the low- to mid-reference interval." "CBC, serum biochemical analysis, and measurement of serum T4 concentration should be repeated at 2-week intervals, with the drug dosage adjusted as needed." Carbimazole "exerts its effects through immediate conversion to methimazole after administration and therefore should not be administered to cats that do not tolerate methimazole." "Establishing a euthyroid state can unmask renal disease, even in patients with normal renal marker parameters." merckvetmanual.com Retrieved 2026-09-30.
- VCA Animal Hospitals. Methimazole — administered "by mouth in the form of an oral tablet or … compounded as an oral liquid," or "applied to hairless skin (usually on the inside of the ear flap) in the form of a transdermal gel." Common side effects "vomiting, anorexia, tiredness," usually within the first three months. Rare side effects: "itchiness, facial scratches, severe changes in blood cell counts, liver disease, immune-mediated disorders, myasthenia gravis." Missed dose: "give it as soon as you remember, and then continue with the regular dosing schedule… Do not give the pet two doses at once." Monitoring "every 2–3 weeks for the first 3 months," then "every 3-6 months." Handling: "Pregnant and nursing women … should wear gloves when handling"; "Wear gloves when applying the transdermal gel." vcahospitals.com Retrieved 2026-09-30.
- Today's Veterinary Practice. Focus on Pharmacology: Methimazole — Management of Feline Hyperthyroidism — "the most common side effects of methimazole/carbimazole administration are gastrointestinal (hyporexia to anorexia, vomiting, and diarrhea)." "Most of the gastrointestinal side effects can be controlled by discontinuation of the medication and supportive care. Once the signs have resolved, the medication can be restarted at a lower dose." "More severe side effects include lymphadenopathy, hepatopathies, aplastic anemia, thrombocytopenia, and agranulocytosis, which are generally manifested within the first few months of treatment." "Less frequent events include facial pruritus, exfoliative dermatitis, myasthenia gravis, and a bleeding disorder secondary to vitamin K antagonism." todaysveterinarypractice.com Retrieved 2026-09-30.
- Peterson ME, Kintzer PP, Hurvitz AI. Methimazole treatment of 262 cats with hyperthyroidism. Journal of Veterinary Internal Medicine. 1988;2(3):150–7 — "Clinical side effects developed in 48 (18.3%) cats (usually within the first month of therapy), which included anorexia, vomiting, lethargy, self-induced excoriation of the face and neck, bleeding diathesis, and icterus caused by hepatopathy." "Mild hematologic abnormalities developed in 43 (16.4%) cats (usually within the first 2 months of treatment), which included eosinophilia, lymphocytosis, and slight leukopenia." "In ten (3.8%) cats, more serious hematologic reactions developed including agranulocytosis and thrombocytopenia (associated with bleeding)." "These hematologic abnormalities resolved within 1 week after cessation of methimazole treatment." Antinuclear antibodies developed "in 52 of 238 (21.8%) cats tested." PMID 3265728. Abstract via NCBI E-utilities: eutils.ncbi.nlm.nih.gov Retrieved 2026-09-30.
- Sartor LL, Trepanier LA, Kroll MM, Rodan I, Challoner L. Efficacy and safety of transdermal methimazole in the treatment of cats with hyperthyroidism. Journal of Veterinary Internal Medicine. 2004;18(5):651–5 — "Forty-seven cats with newly diagnosed hyperthyroidism were randomized to receive either transdermal methimazole in pluronic lecithin organogel (PLO; applied to the inner pinna), or oral methimazole (2.5 mg q12h for either route)." "Cats treated with oral methimazole had a higher incidence of gastrointestinal (GI) adverse effects (4 of 17 cats) compared to the cats treated with transdermal methimazole (1 of 27; P = .04), but no differences were found between groups in the incidence of neutropenia, hepatotoxicity, or facial excoriations." "Significantly more cats treated with oral methimazole had serum T4 concentrations within the reference range after 2 weeks (14 of 16 cats) compared to those treated by the transdermal route (14 of 25; P = .027). This difference was no longer significant by 4 weeks of treatment." PMID 15515580. Abstract via NCBI E-utilities: eutils.ncbi.nlm.nih.gov Retrieved 2026-09-30.
- Trepanier LA, Hoffman SB, Kroll M, Rodan I, Challoner L. Efficacy and safety of once versus twice daily administration of methimazole in cats with hyperthyroidism. Journal of the American Veterinary Medical Association. 2003;222(7):954–8 — "Cats were randomly assigned to receive 5 mg of methimazole, PO, once daily (n = 25) or 2.5 mg of methimazole, PO, twice daily (15)." "…the proportion of cats that were euthyroid after 2 weeks of treatment was lower for cats receiving methimazole once daily (54%) than for cats receiving methimazole twice daily (87%)." "Percentages of cats with adverse effects (primarily gastrointestinal tract upset and facial pruritus) were not significantly different between groups." "Results suggest that once daily administration of methimazole was not as effective as twice daily administration in cats with hyperthyroidism and cannot be recommended for routine use." PMID 12685785. Abstract via NCBI E-utilities: eutils.ncbi.nlm.nih.gov Retrieved 2026-09-30.
- Cornell Feline Health Center, Cornell University College of Veterinary Medicine. Hyperthyroidism in Cats — "Some cats may experience side effects from medication, however, including vomiting, anorexia, fever, anemia, and lethargy." "Regardless of the medication, blood tests should be conducted periodically during treatment to evaluate whether the therapy is effective and to monitor kidney function and for potential side effects." "Lifelong treatment, usually involving twice-daily oral dosage, will be required, and for some owners and cats, this dosage schedule may be difficult to maintain." vet.cornell.edu Retrieved 2026-09-30.