Methimazole
Pronunciation: meth-IM-a-zole.
- Generic name
- methimazole
- Brand names
- Tapazole
- Drug class
- Thioamide antithyroid agent (thyroid peroxidase inhibitor)
- Form
- Tablet (5 mg and 10 mg in the United States)
- Route
- Oral
- Legal status
- Prescription-only medicine
Supply Rules Vary
Trade names by country: United States
Australia: · Full directory
What methimazole is and how it works
Methimazole is an oral antithyroid medicine used to treat hyperthyroidism (an overactive thyroid gland), including hyperthyroidism caused by Graves' disease or toxic multinodular goiter. It reduces the thyroid gland's ability to make thyroid hormone; it does not destroy hormone already made or stored. (1) (3)
Methimazole is prescribed when surgery or radioactive iodine therapy is not an appropriate option, or to control symptoms of hyperthyroidism before thyroid surgery or radioactive iodine treatment. It requires regular blood tests and close medical supervision because of rare but serious risks to the liver and blood cell counts.
Mechanism of action: how methimazole works
Methimazole inhibits thyroid peroxidase-mediated synthesis of thyroid hormones. It does not inactivate thyroxine (T4) or triiodothyronine (T3) already stored in the thyroid or circulating, nor does it interfere with exogenously administered thyroid hormone. (1)
What methimazole is used for
Graves' disease with hyperthyroidism or toxic multinodular goiter
Used in patients for whom surgery or radioactive iodine therapy is not an appropriate treatment option. (1)
Preparation for thyroidectomy or radioactive iodine therapy
Used to ameliorate (improve) symptoms of hyperthyroidism before thyroid surgery or radioactive iodine therapy. (1)
Before taking methimazole
Do not take / not appropriate for
- Do not take methimazole if you have had a hypersensitivity reaction to methimazole or any component of the product. (all patients) (1)
Tell your clinician about
- Any history of liver disease, abnormal liver tests, or a previous liver reaction to an antithyroid medicine.
- Any history of blood disorders, low blood cell counts, or a previous reaction of fever/sore throat/mouth sores to any medicine.
- Pregnancy, plans for pregnancy, or breastfeeding — methimazole is not recommended during pregnancy or breastfeeding without specialist advice, and there are rare reports of birth defects associated with first-trimester use.
- All other medicines, especially blood thinners (such as warfarin), beta-blockers, digoxin/digitalis medicines, and theophylline, because doses of these may need adjustment as thyroid levels change.
- Any planned surgery, since methimazole can affect blood clotting time (prothrombin time).
How to take methimazole
- Methimazole is taken by mouth, usually divided into doses roughly every 8 hours (three times a day), often with food to reduce stomach upset.
- Take it at evenly spaced times and continue for as long as the prescriber directs, even if you begin to feel well.
- Do not stop taking methimazole or change your dose on your own, including reducing or increasing how much you take, without talking to the prescriber first — hyperthyroidism can worsen and thyroid hormone levels need laboratory-guided adjustment.
Procedures and treatment changes
Tell the treating team before surgery, procedures, or any medicine change. Do not alter treatment without the responsible clinician. (1) (3) (4)
Missed dose
Take the missed dose as soon as you remember. However, if it is almost time for the next dose, skip the missed dose and continue the regular dosing schedule. Do not take a double dose to make up for a missed one. If you are unsure, ask your pharmacist or prescriber.
Overdose
If an overdose is suspected, call a poison control center (in the United States, 1-800-222-1222) or seek emergency medical care immediately, even if there are no symptoms. Do not attempt to manage a suspected overdose without professional guidance. (3) (1)
Side effects
Most people tolerate methimazole, but it carries FDA-labeled warnings for rare, serious effects on the liver and blood cell counts that require prompt medical attention. Report any warning symptom to the prescriber immediately rather than deciding on your own whether to continue or stop the medicine.
Skin rash, itching, or hives
What to notice: Mild rash, itching, or urticaria, sometimes with hair loss.
What to do: Tell the prescriber; mild skin reactions are common but should still be reported. (1) (3)
Nausea, vomiting, stomach discomfort, or altered taste
What to notice: Gastrointestinal upset, taste changes, joint or muscle pain, headache, drowsiness, dizziness, or numbness/tingling.
What to do: Discuss persistent or bothersome symptoms with the prescriber rather than stopping the medicine yourself. (1) (3)
Possible liver problem
What to notice: Loss of appetite, itching, pain in the upper right side of the abdomen, unusual tiredness, dark urine, or yellowing of the skin or eyes.
What to do: Contact the prescriber promptly for liver-function blood tests. Do not wait to see if symptoms resolve on their own. (1)
Possible low blood cell counts
What to notice: Fever, sore throat, mouth sores, unusual bruising or bleeding, or general feeling of illness.
What to do: Contact the prescriber immediately for a blood count check; these can signal agranulocytosis or other blood disorders that require prompt evaluation. (1)
Serious warnings
Agranulocytosis and other blood cell disorders — emergency
Symptoms: Fever, sore throat, mouth sores, chills, or general malaise appearing during treatment.
Action: Contact your prescriber immediately (or seek emergency care if the prescriber cannot be reached quickly) and expect an urgent white-blood-cell count. Agranulocytosis is a potentially life-threatening reaction to methimazole; do not wait or try to manage these symptoms yourself. (1)
Liver injury (hepatotoxicity), including rare acute liver failure — emergency
Symptoms: Loss of appetite, itching, pain in the upper right abdomen, dark urine, or yellowing of the skin or eyes (jaundice).
Action: Contact your prescriber immediately for liver-function testing. Methimazole has been associated with reports of hepatotoxicity, including acute liver failure; treatment is typically stopped promptly if significant liver-test abnormalities are confirmed by a clinician. Do not self-adjust or restart the dose. (1)
ANCA-positive vasculitis — urgent
Symptoms: New joint pain, rash, numbness or tingling, muscle pain, or other new systemic symptoms during treatment.
Action: Seek prompt medical assessment; cases of vasculitis, sometimes serious, have been reported with methimazole therapy. (1)
Pregnancy — fetal risk — high
Symptoms: Not applicable — this is a risk to the developing fetus, particularly with first-trimester use.
Action: Tell the prescriber immediately if pregnant or planning pregnancy. Rare congenital defects (including aplasia cutis and craniofacial or gastrointestinal malformations) have been reported in infants of mothers who took methimazole during pregnancy. Do not stop treatment for uncontrolled hyperthyroidism without medical guidance, since untreated hyperthyroidism in pregnancy also carries serious risk; this decision must be made with the prescriber. (1)
Interactions
| Medicine or test | Effect | Action |
|---|---|---|
| Warfarin and other oral anticoagulants | Methimazole may inhibit vitamin K activity, increasing anticoagulant effect and bleeding risk. | The prescriber should monitor prothrombin time/INR more closely, especially before surgical procedures. (1) |
| Beta-blockers, digoxin (digitalis glycosides), and theophylline | As hyperthyroidism resolves and the patient becomes euthyroid, clearance of these medicines can change, so a previously stable dose may become too high. | The prescriber may need to reduce the dose of these medicines as thyroid function normalizes; do not adjust them yourself. (1) |
Pregnancy, breastfeeding, kidney/liver function, age
Pregnancy
Methimazole crosses the placenta and can cause fetal harm, particularly with first-trimester use; rare congenital defects have been reported. The prescriber weighs the risks of treatment against the risks of untreated hyperthyroidism, and may use the lowest dose that controls the condition or consider an alternative agent, especially in the first trimester. (1)
Breastfeeding
Methimazole passes into breast milk. Some studies of mothers taking methimazole found no effect on nursing infants' clinical status, particularly when thyroid function was monitored frequently, but this requires specialist supervision rather than self-directed breastfeeding decisions. (1)
Children
Methimazole is the preferred antithyroid medicine for pediatric patients when one is needed, because of postmarketing reports of severe liver injury with propylthiouracil in children. Dosing is weight-based and must be set and monitored by the prescriber. (1)
Monitoring
Before starting
- Confirm the diagnosis of hyperthyroidism and the reason surgery or radioactive iodine is not being used first.
- Establish baseline blood counts and liver-function tests where clinically indicated.
- Review pregnancy status, breastfeeding plans, and all current medicines, especially anticoagulants, beta-blockers, digoxin, and theophylline.
During treatment
- Report fever, sore throat, mouth sores, unusual bruising/bleeding, jaundice, dark urine, or right-upper-abdominal pain immediately; these prompt urgent blood-count and liver-function testing.
- Periodic thyroid function tests (TSH, free T4) guide dose adjustment; a rising TSH after hyperthyroidism resolves usually means a lower maintenance dose is needed.
- Prothrombin time monitoring is advised, especially before surgery, because methimazole can affect blood clotting.
- Do not adjust or stop methimazole yourself based on how you feel — dose changes are guided by laboratory testing.
Storage
Store at controlled room temperature (20°C to 25°C / 68°F to 77°F), in a tightly closed, light-resistant container with a child-resistant closure, away from excess heat and moisture, and out of the reach of children. Dispose of unused medicine through a medicine take-back program rather than household trash or the toilet where possible.
Professional information
Clinician-facing context only. Use the current local label, dosing table, and monitoring protocol; this page does not provide impairment-specific dosing.
United States (FDA label)
Indication: Hyperthyroidism (Graves' disease, toxic multinodular goiter) or pre-treatment before thyroidectomy/RAI
Dose summary: Adults: initial 15 mg/day (mild), 30–40 mg/day (moderately severe), or 60 mg/day (severe) hyperthyroidism, given in 3 divided doses at ~8-hour intervals; maintenance 5–15 mg/day. Pediatric: initial 0.4 mg/kg/day in 3 divided doses at ~8-hour intervals; maintenance approximately half the initial dose.
Renal context: The current US label does not specify a distinct renal-impairment dosing schedule; use clinical judgement and monitoring. (1)
Hepatic guidance
Discontinue promptly for clinically significant liver-test abnormalities, including hepatic transaminases exceeding 3 times the upper limit of normal, or symptoms suggestive of hepatic dysfunction (anorexia, pruritus, right-upper-quadrant pain).
Overdose note
No established information on median lethal dose or toxic serum concentration; manage supportively per current poison-control guidance, considering possible multi-drug ingestion. (1)
Frequently asked questions
Does methimazole have a boxed warning?
The current FDA-approved methimazole label does not use a formal boxed-warning format; instead, agranulocytosis and hepatotoxicity (including rare acute liver failure) are described as prominent warnings within the WARNINGS section, with instructions to seek prompt evaluation for fever/sore throat or signs of liver dysfunction. These are treated as serious, potentially life-threatening risks regardless of the label format. (1)
Can I stop methimazole on my own if I feel better?
No. Stopping or changing the dose without medical guidance can allow hyperthyroidism to return or worsen. Dose changes should be based on your prescriber's assessment and laboratory tests. (1) (3)
Is carbimazole the same medicine as methimazole?
No. Carbimazole is a different, related medicine (a prodrug) that the body converts into thiamazole/methimazole. They are not automatically interchangeable products, and this page covers methimazole itself. (4)
What is the mechanism of action of methimazole?
Methimazole inhibits thyroid peroxidase-mediated synthesis of thyroid hormones. It does not inactivate thyroxine (T4) or triiodothyronine (T3) already stored in the thyroid or circulating, nor does it interfere with exogenously administered thyroid hormone. (1)
References
- METHIMAZOLE tablet — prescribing information (Chartwell RX, LLC)DailyMed, U.S. National Library of Medicine · United States · dated 2026-02 (Rev: 02/2026) · accessed 2026-07-23 · source 1
- METHIMAZOLE TABLETS, USP — original scanned label PDFFDA (accessdata.fda.gov, Par Pharmaceutical Companies) · United States · dated 01/2012 · accessed 2026-07-23 · source 2
- Methimazole: MedlinePlus Drug InformationMedlinePlus, U.S. National Library of Medicine · United States · accessed 2026-07-23 · source 3
- Carbimazole: medicine for treating overactive thyroid (hyperthyroidism)NHS · United Kingdom · accessed 2026-07-23 · source 4
- Carbimazole tablets — Summary of Product Characteristics (various strengths)electronic Medicines Compendium (UK) · United Kingdom · accessed 2026-07-23 · source 5
Brand names by country
Single-ingredient methimazole names are shown separately by country. Product availability, strengths, and supply rules can change; combination products are not included here.