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Northyx Side Effects

Generic Name: methimazole

Note: This page contains side effects data for the generic drug methimazole. It is possible that some of the dosage forms included below may not apply to the brand name Northyx.

For the Consumer

Applies to methimazole: oral tablet

As well as its needed effects, methimazole (the active ingredient contained in Northyx) may cause unwanted side effects that require medical attention.

Major Side Effects

If any of the following side effects occur while taking methimazole, check with your doctor immediately:

  • 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

Minor Side Effects

Some methimazole side effects may not need any medical attention. As your body gets used to the medicine these side effects may disappear. Your health care professional may be able to help you prevent or reduce these side effects, but do check with them if any of the following side effects continue, or if you are concerned 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


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]

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.[Ref]


Immunologic side effects have included insulin autoimmune syndrome and lupus-like syndrome.[Ref]


Hepatic side effects have included hepatitis and jaundice (which may persist for several weeks following discontinuation of the drug).[Ref]


Renal side effects have rarely included nephritis.[Ref]


A 5-year-old female experienced aplasia cutis congenita coincident with in utero exposure to methimazole (the active ingredient contained in Northyx) 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]

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.[Ref]


Musculoskeletal side effects have included arthralgia (5%) and myalgia.[Ref]

Nervous system

Nervous system side effects have included paresthesia, headache, drowsiness, neuritis, and vertigo.[Ref]


Gastrointestinal (GI) side effects have included GI upset (5%), nausea, vomiting, epigastric distress, and taste disturbances. Pancreatitis has been reported very rarely.[Ref]


A 67-year-old female with a history of Graves' disease and mild bilateral ophthalmopathy experienced carotid cavernous fistula coincident with methimazole (the active ingredient contained in Northyx) 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.[Ref]

Ocular side effects including at least one case of carotid cavernous fistula have been reported.[Ref]


Cardiovascular side effects including vasculitis have been reported.[Ref]


Metabolic side effects including rare reports of hypoglycemia have been reported.[Ref]


Other side effects have included edema, periarteritis, sialadenopathy, lymphadenopathy, and fever. Abnormal sense of taste or smell has also been reported rarely.[Ref]


1. "Product Information. Tapazole (methimazole)." Jones Medical-Western Research, St. Louis, MO.

2. Cooper DS "Antithyroid drugs." N Engl J Med 352 (2005): 905-17

3. Mikhail NE "Methimazole-induced cholestatic jaundice." South Med J 97 (2004): 178-82

4. Baid SK, Merke DP "Aplasia cutis congenita following in utero methimazole exposure." J Pediatr Endocrinol Metab 20 (2007): 585-6

5. Benson GD "Acetaminophen in chronic liver disease." Clin Pharmacol Ther 33 (1983): 95-101

It is possible that some side effects of Northyx may not have been reported. These can be reported to the FDA here. Always consult a healthcare professional for medical advice.

Disclaimer: Every effort has been made to ensure that the information provided is accurate, up-to-date and complete, but no guarantee is made to that effect. In addition, the drug information contained herein may be time sensitive and should not be utilized as a reference resource beyond the date hereof. This material does not endorse drugs, diagnose patients, or recommend therapy. This information is a reference resource designed as supplement to, and not a substitute for, the expertise, skill , knowledge, and judgement of healthcare practitioners in patient care. The absence of a warning for a given drug or combination thereof in no way should be construed to indicate safety, effectiveness, or appropriateness for any given patient. does not assume any responsibility for any aspect of healthcare administered with the aid of materials provided. The information contained herein is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. If you have questions about the substances you are taking, check with your doctor, nurse, or pharmacist.