Methimazole Side Effects
Not all side effects for methimazole may be reported. You should always consult a doctor or healthcare professional for medical advice. Side effects can be reported to the FDA here.
For the Consumer
Applies to methimazole: oral tablet
In addition to its needed effects, some unwanted effects may be caused by methimazole. In the event that any of these side effects do occur, they may require medical attention.
You should check with your doctor immediately if any of these side effects occur when taking methimazole:Rare
- black, tarry stools
- chest pain
- 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
- 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
- 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
- yellow eyes or skin
Some of the side effects that can occur with methimazole may not need medical attention. As your body adjusts to the medicine during treatment these side effects may go away. Your health care professional may also be able to tell you about ways to reduce or prevent some of these side effects. If any of the following side effects continue, are bothersome or if you have any questions about them, check with your health care professional:Incidence not known
- Abnormal loss of hair
- dizziness or lightheadedness
- feeling of constant movement of self or surroundings
- hives or welts
- loss of taste
- pain or discomfort in the chest, upper stomach, or throat
- red skin
- skin rash
For Healthcare Professionals
Applies to methimazole: compounding powder, oral tablet
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.
Hematologic side effects have included inhibition of myelopoiesis (agranulocytosis (0.35%), granulocytopenia, and thrombocytopenia), aplastic anemia, and hypoprothrombinemia. Approximately 10% of patients with untreated hyperthyroidism have experienced leukopenia, often with relative granulopenia.
Immunologic side effects have included insulin autoimmune syndrome and lupus-like syndrome.
Hepatic side effects have included hepatitis and jaundice (which may persist for several weeks following discontinuation of the drug).
Renal side effects have rarely included nephritis.
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.
Dermatologic side effects including urticaria (5%), macular rash (5%), rash, pruritus, abnormal loss of hair, and skin pigmentation have been reported. Aplasia cutis congenita has also been reported.
Musculoskeletal side effects have included arthralgia (5%) and myalgia.
Nervous system side effects have included paresthesia, headache, drowsiness, neuritis, and vertigo.
Gastrointestinal (GI) side effects have included GI upset (5%), nausea, vomiting, epigastric distress, and taste disturbances. Pancreatitis has been reported very rarely.
Ocular side effects including at least one case of carotid cavernous fistula have been reported.
A 67-year-old female with a history of Graves' disease and mild bilateral ophthalmopathy experienced carotid cavernous fistula coincident with methimazole therapy. She was treated with radioiodine following 10-years of therapy with methimazole. Evaluation of the patient with sensitive techniques lead to the diagnosis of carotid cavernous fistula. This is the first report of the simultaneous occurrence of Graves' ophthalmopathy and carotid cavernous fistula.
Cardiovascular side effects including vasculitis have been reported.
Metabolic side effects including rare reports of hypoglycemia have been reported.
Other side effects have included edema, periarteritis, sialadenopathy, lymphadenopathy, and fever. Abnormal sense of taste or smell has also been reported rarely.
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