Methimazole: Generic, Uses, Side Effects, Dosages, Interactions, Warnings (2023)

• Uses
  • What Is Methimazole and How Does It Work?
• Dosage
  • What Are Dosages of Methimazole?
• Side Effects
  • What Are Side Effects Associated with Using Methimazole?
• Drug Interactions
  • What Other Drugs Interact with Methimazole?
• Warnings and Precautions
  • What Are Warnings and Precautions for Methimazole?

What Is Methimazole and How Does It Work?

Methimazole is used to treat overactive thyroid (hyperthyroidism). It works by stopping the thyroid gland from making too much thyroid hormone.

• Methimazole is available under the following different brand names: Northyx, and Tapazole.

What Are Dosages of Methimazole?

Dosages of Methimazole:

Adult and Pediatric Dosage Forms and Strengths

Tablet

• 5 mg
• 10 mg

Dosing Considerations – Should be Given as Follows:

Hyperthyroidism

Adult:

• Mild: 15 mg/day orally divided every 8 hours initially
• Moderate: 30-40 mg/day orally divided every 8 hours initially
• Severe: 60 mg/day orally divided every 8 hours initially
• Maintenance: 5-15 mg/day orally divided every 8 hours

Pediatric:

• Initial: 0.4-0.7 mg/kg/day orally divided every 8 hours
• Maintenance: 1/3-2/3 of initial orally dosage divided every 8 hours orally
• Not to exceed 30 mg/day

Graves Disease

• Adult: 10-20 mg/day orally; after euthyroidism is achieved, reduce dosage by 50% and administer for 12-18 months
• Pediatric: 0.2-0.5 mg/kg/day orally; after euthyroidism is achieved, reduce dosage by 50% and administered for 1-2 years

Thyrotoxic Crisis

• 20-30 mg every 6-12 hours for the short term, then reduce dosage to maintenance (5-15 mg/day) or reduce the frequency to every 12 hours or every 24 hours

What Are Side Effects Associated with Using Methimazole?

Common side effects of methimazole include:

• Agranulocytosis
• Aplastic anemia
• Dizziness
• Drowsiness
• Hair loss
• Headache
• Hepatotoxicity
• Hives
• Leukocytoclastic vasculitis
• Low blood platelets (thrombocytopenia)
• Muscle or joint pain
• Nausea
• Nerve pain/tingling/numbness
• Polyarthritis
• Salivary gland swelling
• Skin reactions (e.g., rash, itching, skin pigmentation)
• Spinning sensation (vertigo)
• Stomach upset
• Swelling
• Vomiting

Rare but serious side effects of methimazole include:

• Yellowing eyes/skin (jaundice)
• Dark urine
• Severe stomach or abdominal pain
• Persistent nausea or vomiting
• Changes in the amount of urine

This document does not contain all possible side effects and others may occur. Check with your physician for additional information about side effects.

What Other Drugs Interact with Methimazole?

If your doctor has directed you to use this medication, your doctor or pharmacist may already be aware of any possible drug interactions and may be monitoring you for them. Do not start, stop, or change the dosage of any medicine before checking with your doctor, health care provider, or pharmacist first.

• Severe interactions of methimazole include:
  • eliglustat
  • sodium iodide I-131
• Serious interactions of methimazole include:
  • carbamazepine
  • clozapine
  • propylthiouracil
• Methimazole has moderate interactions with at least 23 different drugs.
• Mild interactions of methimazole include:
  • cadexomer iodine
  • iodinated glycerol
  • iodine
  • potassium iodide
  • ruxolitinib

This information does not contain all possible interactions or adverse effects. Therefore, before using this product, tell your doctor or pharmacist of all the products you use. Keep a list of all your medications with you, and share this information with your doctor and pharmacist. Check with your health care professional or doctor for additional medical advice, or if you have health questions, concerns, or for more information about this medicine.

What Are Warnings and Precautions for Methimazole?

Warnings

This medication contains methimazole. Do not take Northyx or Tapazole if you are allergic to methimazole or any ingredients contained in this drug.

Keep out of reach of children. In case of overdose, get medical help or contact a Poison Control Center immediately

Contraindications

• Hypersensitivity
• Breastfeeding

Short-Term Effects

• See "What Are Side Effects Associated with Using Methimazole?"

Long-Term Effects

• See "What Are Side Effects Associated with Using Methimazole?"

Cautions

• Liver disease
• Hepatic reactions may occur
• Pregnancy
• Lupus-like syndrome reported
• Antineutrophil cytoplasmic autoantibody (ANCA)-positive vasculitis reported
• Dosage greater than 40 mg/day
• Patient age greater than 40 years
• Concurrent administration of other agranulocytosis-causing drugs
• Risk of rare but serious agranulocytosis, cholestatic jaundice, and rare congenital abnormalities (propylthiouracil [PTU] preferred in pregnancy)
• Prolonged treatment may cause hypothyroidism
• Discontinue if agranulocytosis, pancytopenia, hepatitis, fever, or exfoliative dermatitis occurs
• 10 times more potent than PTU, but action may be less consistent
• High rate of relapse (more likely in smokers)

Pregnancy and Lactation

• Use methimazole during pregnancy only in LIFE-THREATENING emergencies when no safer drug is available
• Methimazole crosses the placenta
• There is positive evidence of human fetal risk
• Methimazole is distributed in breast milk
• Its use is contraindicated by the manufacturer
• The American Academy of Pediatrics (AAP) Committee states that methimazole is compatible with nursing
• The American Academy of Family Physicians (AAFP) states that it is safe for nursing