methimazole
Brand names: Tapazole
Methimazole (Tapazole) is a medicine that lowers the amount of thyroid hormone your body makes. It helps treat hyperthyroidism (overactive thyroid).
Drug Pricing (NADAC)
Generic Price
$0.11/unit
Generic Available
Yes (8 manufacturers)
Pricing data from NADAC (CMS), effective December 18, 2024. Compare all drug costs →
What it does
This medicine treats hyperthyroidism, a condition where your thyroid gland makes too much thyroid hormone.
Common side effects
Nausea, Headache, Fatigue
Key warnings
Methimazole can cause serious side effects.
How It Works
Methimazole stops the thyroid gland from making thyroid hormone. It does this by blocking an enzyme needed to produce the hormones T3 and T4. This helps to bring thyroid hormone levels back to normal.
How to Take It
Your doctor will tell you how much methimazole to take. Follow their instructions carefully. You can take this medicine with or without food. Take it at the same time(s) every day.
Pregnancy & Breastfeeding
Talk to your doctor if you are pregnant or plan to become pregnant. Methimazole can harm your unborn baby. It is also important to discuss breastfeeding with your doctor while taking this medicine.
Missed Dose
If you miss a dose, take it as soon as you remember. If it is almost time for your next dose, skip the missed dose and continue with your regular schedule.
Storage
Store methimazole at room temperature, away from heat and moisture.
Side Effects (from patient reports)
Based on 3,746 FDA adverse event reports.
FDA Adverse Event Report Analysis
Detailed analysis of 7,411 reports from the FDA Adverse Event Reporting System (FAERS). Reports span 2004–2025.
Total Reports
7,411
Death-Related Reports
615
Hospitalization Reports
2,710
Top Indication
Product Used For Unknown Indication
Gender Distribution
Age Distribution
Most Reported Adverse Reactions (FAERS)
| # | Reaction | Reports |
|---|---|---|
| 1 | DRUG INEFFECTIVE | 607 |
| 2 | OFF LABEL USE | 463 |
| 3 | FATIGUE | 426 |
| 4 | HYPERTHYROIDISM | 384 |
| 5 | NAUSEA | 372 |
| 6 | DIARRHOEA | 325 |
| 7 | HEADACHE | 315 |
| 8 | PAIN | 296 |
| 9 | DIZZINESS | 292 |
| 10 | DYSPNOEA | 266 |
| 11 | AGRANULOCYTOSIS | 243 |
| 12 | ASTHENIA | 212 |
| 13 | ARTHRALGIA | 211 |
| 14 | RASH | 209 |
| 15 | ANXIETY | 208 |
Reactions in Death Reports
Reactions in Hospitalization Reports
Source: FDA FAERS (Adverse Event Reporting System) FDA FAERS (Adverse Event Reporting System) Reports are voluntary and do not establish causation
Serious Warnings
Methimazole can cause serious side effects. Talk to your doctor right away if you develop a fever, sore throat, or mouth sores, as these could be signs of a serious blood problem. This medicine may also harm your liver.
Known Drug Interactions
Drug Interactions Anticoagulants (oral) Due to potential inhibition of vitamin K activity by methimazole, the activity of oral anticoagulants (e.g., warfarin) may be increased; additional monitoring of PT/INR should be considered, especially before surgical procedures.
Mechanism: Methimazole can interfere with how your body uses Vitamin K, which makes the blood-thinning effect of warfarin even stronger.
What to do: Your doctor should check your blood clotting levels more often, especially before you have any type of surgery.
Theophylline Theophylline clearance may decrease when hyperthyroid patients on a stable theophylline regimen become euthyroid; a reduced dose of theophylline may be needed.
Mechanism: As methimazole treats your overactive thyroid, your body slows down how fast it removes theophylline from your system.
What to do: Your doctor may need to lower your dose of theophylline as your thyroid levels return to normal.
Common Questions
What should I do if I feel worse after starting methimazole?
Can I stop taking methimazole when I feel better?
Will methimazole cure my hyperthyroidism?
Can methimazole interact with other medicines I am taking?
How long does it take for methimazole to start working?
What happens if I take too much methimazole?
Can I drink alcohol while taking methimazole?
Does methimazole affect my ability to drive or operate machinery?
Will I need regular blood tests while taking methimazole?
Can methimazole cause weight gain?
What are the common side effects of methimazole?
Does methimazole interact with other medications?
What drug class is methimazole?
Is methimazole safe during pregnancy?
Related Medications in Anti-Thyroid Agent
Other drugs grouped near methimazole — same-class peers and common alternatives.
levothyroxine
Synthroid, Levoxyl
Levothyroxine is a medicine that replaces a hormone normally made by your thyroid gland.
Compare with methimazole →
liothyronine
Cytomel
Liothyronine (Cytomel) is a medicine that replaces a thyroid hormone called T3.
Compare with methimazole →
propylthiouracil
PTU
Propylthiouracil (PTU) is a medicine that treats an overactive thyroid.
Compare with methimazole →
thyroid (desiccated)
Armour Thyroid, Nature-Throid
Armour Thyroid is a natural thyroid hormone medicine.
Compare with methimazole →
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What the FDA Data Shows for methimazole
The FDA label for methimazole (sold under brand names such as Tapazole) classifies it as a prescription-only medication in the Anti-Thyroid Agent class. This medicine treats hyperthyroidism, a condition where your thyroid gland makes too much thyroid hormone. Official labeling lists 6 commonly reported side effects, including Nausea, Headache, Fatigue.
Post-market surveillance from the FDA Adverse Event Reporting System (FAERS) captures real-world experience. For this drug, FAERS contains 3,746 voluntary reports. The database also lists 2 documented drug interactions derived from FDA labeling, with the top-flagged interaction rated minor severity. NADAC pricing from CMS shows a generic unit cost of $0.11.
Report counts do not establish causation — a FAERS entry documents a temporal association, not proof that the drug produced the outcome. Widely prescribed medications naturally accumulate more reports than niche therapies, so raw totals must be interpreted alongside total exposure. Shortage status, recall history, and patent information further shape supply and switching decisions. This page summarizes public FDA data for educational purposes only and is not a substitute for professional medical advice — always consult a licensed healthcare provider before starting, stopping, or changing any medication.
Data Sources
Drug labeling: FDA Drug Labels (SPL/DailyMed). Adverse events: FDA Adverse Event Reporting System (FAERS). Pricing: CMS National Average Drug Acquisition Cost (NADAC).
FAERS reports are voluntary and do not establish causation. Drug interactions are derived from FDA labeling and clinical references. Always consult a healthcare professional before making medication decisions.
Last updated: September 10, 2025
Read our methodology — how this data is sourced, computed, and verified.
All federal data sources used on this page
- FDA Orange Book — approved drug products with therapeutic equivalence. accessdata.fda.gov/cder/ob
- FDA DailyMed — NIH-hosted drug labeling for FDA-approved meds. dailymed.nlm.nih.gov
- FDA Adverse Event Reporting System (FAERS) — post-marketing safety surveillance. fda.gov/drugs/faers
- NLM RxNorm — standardized clinical drug nomenclature. nlm.nih.gov/research/umls/rxnorm
- CMS Medicare Part B Drug Average Sales Price Files — federal drug pricing data. cms.gov/medicare/part-b-drugs/asp
- FDA Drug Shortages Database — current and resolved drug shortage tracking. accessdata.fda.gov/drugshortages