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Methimazole Side Effects

For the Consumer

Applies to methimazole: oral tablet

Along with its needed effects, methimazole may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention.

Check with your doctor immediately if any of the following side effects occur while taking methimazole:


  • Backache
  • black, tarry stools
  • chest pain
  • chills
  • cough
  • fever
  • increase or decrease in urination
  • painful or difficult urination
  • shortness of breath
  • sore throat
  • sores, ulcers, or white spots on the lips or in the mouth
  • swelling of the feet or lower legs
  • swollen glands
  • unusual bleeding or bruising
  • unusual tiredness or weakness

Incidence Not Known

  • Abdominal or stomach pain
  • bleeding gums
  • blood in the urine or stool
  • burning, crawling, itching, numbness, prickling, "pins and needles", or tingling feelings
  • dark urine
  • difficulty with moving
  • general feeling of discomfort or weakness
  • headache
  • joint pain
  • light-colored stools
  • loss of appetite and weight
  • lower back or side pain
  • muscle aching or cramping
  • muscle pain or stiffness
  • nausea and vomiting
  • numbness or tingling of the hands, feet, or face
  • pinpoint red spots on the skin
  • sore muscles
  • swollen joints
  • swollen salivary glands
  • swollen, painful, or tender lymph glands in the neck, armpit, or groin
  • tightness in the chest
  • wheezing
  • yellow eyes or skin

Some side effects of methimazole may occur that usually do not need medical attention. These side effects may go away during treatment as your body adjusts to the medicine. Also, your health care professional may be able to tell you about ways to prevent or reduce some of these side effects. Check with your health care professional if any of the following side effects continue or are bothersome or if you have any questions about them:

Incidence Not Known

  • Abnormal loss of hair
  • dizziness or lightheadedness
  • feeling of constant movement of self or surroundings
  • heartburn
  • hives or welts
  • itching
  • loss of taste
  • pain or discomfort in the chest, upper stomach, or throat
  • red skin
  • skin rash
  • sleepiness
  • vomiting

For Healthcare Professionals

Applies to methimazole: compounding powder, oral tablet


Some of the more frequently reported adverse reactions have included skin rash, urticaria, nausea, loss of taste, and abnormal loss of hair.


The onset of aplastic anemia (pancytopenia) or agranulocytosis may occur in hours to days. Most cases of agranulocytosis occur within the first 90 days of treatment, but this complication can occur even a year or more after starting therapy.

Studies have suggested that the risk of agranulocytosis is greater in older patients and that they have a higher rate of death.[Ref]

Frequency not reported: Lymphadenopathy, agranulocytosis, granulocytopenia, thrombocytopenia, aplastic anemia, hypoprothrombinemia[Ref]


Frequency not reported: Lupus-like syndrome[Ref]


Frequency not reported: Jaundice, hepatitis[Ref]


Frequency not reported: Edema, periarteritis


A 5-year-old female experienced aplasia cutis congenita coincident with in utero exposure to methimazole. She presented with hairless scars on her scalp. At birth the lesions were friable and bled easily, however, over time they became scar-like and thicker. At the time of conception, her mother was receiving therapy for Graves' disease with methimazole 20 mg daily. Aplasia cutis congenita has been reported as a possible teratogenic effect of methimazole therapy in multiple case reports. However, the causal relationship between the therapy and the skin defect is controversial, and anomalies related to methimazole have low birth prevalence.[Ref]

Frequency not reported: Skin rash, urticaria, abnormal loss of hair, pruritus, skin pigmentation[Ref]


Frequency not reported: Arthralgia, myalgia[Ref]

Nervous system

Frequency not reported: Paresthesia, loss of taste, headache, drowsiness, neuritis[Ref]


Frequency not reported: Nausea, vomiting, epigastric distress, sialadenopathy[Ref]


Very rare (less than 0.01%): Nephritis[Ref]


Frequency not reported: Insulin autoimmune syndrome


Frequency not reported: Vertigo, drug fever[Ref]


1. "Product Information. Tapazole (methimazole)." King Pharmaceuticals Inc, Bristol, TN.

Further information

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Some side effects may not be reported. You may report them to the FDA.