Methimazole Side Effects
Some side effects of methimazole may not be reported. Always consult your doctor or healthcare specialist for medical advice. You may also report side effects to the FDA.
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: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 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
- 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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