Applies to methimazole: oral tablet.
Serious side effects of Methimazole
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:
Incidence not known
- Black, tarry stools
- bleeding gums
- bleeding under the skin
- bloody or cloudy urine
- burning, crawling, itching, numbness, prickling, "pins and needles", or tingling feelings
- chest pain
- chills
- cough
- dark urine
- difficulty in breathing
- dizziness or lightheadedness
- drowsiness
- feeling of constant movement of self or surroundings
- fever
- general feeling of discomfort, illness, or weakness
- headache
- hoarseness
- itching, skin rash
- light-colored stools
- loss of appetite and weight
- loss of consciousness
- lower back or side pain
- nausea
- painful or difficult urination
- pinpoint red spots on the skin
- redness, soreness, or itching skin
- sensation of spinning
- sore throat
- soreness of the muscles
- sores, ulcers, or white spots on the lips or in the mouth
- sores, welts, or blisters
- stomach pain
- swelling of the face, feet, or lower legs
- swollen or painful glands
- swollen salivary glands
- swollen, painful, or tender lymph glands in the neck, armpit, or groin
- tightness in the chest
- unusual bleeding or bruising
- unusual tiredness or weakness
- unusual weight gain
- upper right abdominal pain
- yellow eyes or skin
Other side effects of Methimazole
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
- changes in skin coloring
- difficulty in moving
- heartburn
- joint pain or swelling
- loss of taste
- muscle pain, cramps, or stiffness
- pain or discomfort in the chest, upper stomach, or throat
For Healthcare Professionals
Applies to methimazole: compounding powder, oral tablet.
General
Some of the more frequently reported adverse reactions have included skin rash, urticaria, nausea, loss of taste, and abnormal loss of hair.
Hematologic
Frequency not reported: Lymphadenopathy, agranulocytosis, granulocytopenia, thrombocytopenia, aplastic anemia, hypoprothrombinemia[Ref]
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]
Immunologic
Frequency not reported: Lupus-like syndrome[Ref]
Hepatic
Frequency not reported: Jaundice, hepatitis[Ref]
Cardiovascular
Frequency not reported: Edema, periarteritis
Dermatologic
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]
Musculoskeletal
Frequency not reported: Arthralgia, myalgia[Ref]
Nervous system
Frequency not reported: Paresthesia, loss of taste, headache, drowsiness, neuritis[Ref]
Gastrointestinal
Frequency not reported: Nausea, vomiting, epigastric distress, sialadenopathy
Postmarketing reports: Acute pancreatitis[Ref]
Case reports for recurrent acute pancreatitis with a decreased time to onset after re-exposure to carbimazole/thiamazole (methimazole) have been received. The mechanism is not completely understood, but the presentation suggests an immunological mechanism.[Ref]
Renal
Very rare (less than 0.01%): Nephritis[Ref]
Metabolic
Frequency not reported: Insulin autoimmune syndrome
Other
Frequency not reported: Vertigo, drug fever[Ref]
