What is methimazole (Tapazole)?
Methimazole prevents the thyroid gland from producing too much thyroid hormone. Methimazole is used to treat hyperthyroidism (overactive thyroid). It is also used before thyroid surgery or radioactive iodine treatment. Methimazole may also be used for other purposes not listed in this medication guide.
TAPAZOLE® (methimazole) Tablets, USP (1-methylimidazole-2-thiol) is a white, crystalline substance that is freely soluble in water. It differs chemically from the drugs of the thiouracil series primarily because it has a 5-instead of a 6-membered ring.
Each tablet contains 5 or 10 mg (43.8 or 87.6 μmol) methimazole, an orally administered antithyroid drug. Each tablet also contains lactose monohydrate, magnesium stearate, starch (corn), pregelatinized starch and talc.
The molecular weight is 114.16, and the molecular formula is C4H6N2S. The structural formula is as follows:
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TAPAZOLE is indicated:
- In patients with Graves’ disease with hyperthyroidism or toxic multinodular goiter for whom surgery or radioactive iodine therapy is not an appropriate treatment option.
- To ameliorate symptoms of hyperthyroidism in preparation forthyroidectomy or radioactive iodine therapy.
DOSAGE AND ADMINISTRATION
TAPAZOLE is administered orally. The total daily dosage is usually given in 3 divided doses at approximately 8-hour intervals.
Adult –The initial daily dosage is 15 mg for mild hyperthyroidism, 30 to 40 mg for moderately severe hyperthyroidism, and 60 mg for severe hyperthyroidism, divided into 3 doses at 8-hour intervals. The maintenance dosage is 5 to 15 mg daily.
Pediatric –Initially, the daily dosage is 0.4 mg/kg of body weight divided into 3 doses and given at 8-hour intervals. The maintenance dosage is approximately 1/2 of the initial dose.
HOW SUPPLIED
TAPAZOLE Tablets are available in:
The 5-mg tablets are round, white to off-white, scored on one side and the other side debossed with “J94”.
They are available as follows:
Bottles of 100 NDC 60793-104-01
The 10-mg tablets are round, white to off-white, scored on one side and the other side debossed with “J95”.
They are available as follows:
Bottles of 100 NDC 60793-105-01
Store at controlled room temperature, 15° to 30°C (59° to 86°F).
Distributed by: King Pharmaceuticals, Inc. Bristol, TN 37620. Manufactured by: AAI Pharma, 1726 North 23rd St., Wilmington, NC 28405. Revised February 2012
This monograph has been modified to include the generic and brand name in many instances.
What are the possible side effects of methimazole (Tapazole)?
Get emergency medical help if you have any of these signs of an allergic reaction: hives; difficulty breathing; swelling of your face, lips, tongue, or throat.
Stop using methimazole and call your doctor at once if you have a serious side effect such as:
- fever, chills, sore throat, body aches, flu symptoms;
- easy bruising or bleeding, unusual weakness;
- blood in your urine or stools;
- severe blistering, peeling, and red skin rash; or
- nausea, stomach pain, low fever, loss of appetite, dark urine, clay-colored stools, jaundice (yellowing of the skin or eyes).
Less serious side effects may include:
- headache, drowsiness, dizziness;
- mild nausea, vomiting, or stomach upset;
- itching, minor skin rash;
- muscle, joint, or nerve pain;
- swelling; or
- hair loss.
SIDE EFFECTS
Major adverse reactions (which occur with much less frequency than the minor adverse reactions) include inhibition of myelopoieses (agranulocytosis, granulocytopenia, thrombocytopenia, and aplastic anemia), drug fever, a lupus-like syndrome, insulin autoimmune syndrome (which can result inhypoglycemic coma), hepatitis (jaundice may persist for several weeks after discontinuation of the drug), periarteritis, and hypoprothrombinemia. Nephritis occurs very rarely.
Minor adverse reactions include skin rash, urticaria, nausea, vomiting, epigastric distress, arthralgia, paresthesia, loss of taste, abnormal loss of hair, myalgia, headache, pruritus, drowsiness, neuritis, edema, vertigo, skin pigmentation, jaundice, sialadenopathy, and lymphadenopathy.
DRUG INTERACTIONS
Anticoagulants (oral)
Due to potential inhibition of vitamin K activity by methimazole, the activity of oral anticoagulants (e.g., warfarin) may be increased; additional monitoring of PT/INR should be considered, especially before surgical procedures.
β-adrenergic blocking agents
Hyperthyroidism may cause an increased clearance of beta blockers with a high extraction ratio. A dose reduction of beta-adrenergic blockers may be needed when a hyperthyroid patient becomes euthyroid.
Digitalis glycosides
Serum digitalis levels may be increased when hyperthyroid patients on a stable digitalis glycoside regimen become euthyroid; a reduced dosage of digitalis glycosides may be needed.
Theophylline
Theophylline clearance may decrease when hyperthyroid patients on a stable theophylline regimen become euthyroid; a reduced dose of theophylline may be needed.
WARNINGS
Congenital Malformations
Methimazole readily crosses the placental membranes and can cause fetal harm, particularly when administered in the first trimester of pregnancy. Rare instances of congenital defects, including aplasia cutis, craniofacial malformations (facial Amalformations (esophageal atresia with or without tracheoesophageal fistula; umbilical abnormalities) have occurred in infants born to mothers who received TAPAZOLE during pregnancy. If TAPAZOLE is used during pregnancy or if the patient becomes pregnant while taking this drug, the patient should be warned of the potential hazard to the fetus.
Since the above congenital defects have been reported in offspring of patients treated with TAPAZOLE, it may be appropriate to use other agents in pregnant women requiring treatment for hyperthyroidism, particularly during organogenesis, in the first trimester of pregnancy. If TAPAZOLE is used, the lowest possible dose to control the maternal disease should be given.
Agranulocytosis
Agranulocytosis is potentially a life-threatening adverse reaction of TAPAZOLE therapy. Patients should be instructed to immediately report to their physicians any symptoms suggestive of agranulocytosis, such as fever or sore throat. Leukopenia, thrombocytopenia, and aplastic anemia(pancytopenia) may also occur. The drug should be discontinued in the presence of agranulocytosis, aplastic anemia (pancytopenia), ANCA-positive vasculitis, hepatitis, or exfoliative dermatitis and the patient’s bone marrow indices should be monitored.
Liver Toxicity
Although there have been reports of hepatotoxicity (including acute liver failure) associated with TAPAZOLE, the risk of hepatotoxicity appears to be less with methimazole than with propylthiouracil, especially in the pediatric population. Symptoms suggestive of hepatic dysfunction (anorexia, pruritis, right upper quadrant pain, etc.) should prompt evaluation of liver function (bilirubin, alkaline phosphatase) and hepatocellur integrity (ALT, AST). Drug treatment should be discontinued promptly in the event of clinically significant evidence of liver abnormality including hepatic transaminase values exceeding 3 times the upper limit of normal.
Hypothyroidism
TAPAZOLE can cause hypothyroidism necessitating routine monitoring of TSH and free T4 levels with adjustments in dosing to maintain a euthyroid state. Because the drug readily crosses placental membranes, methimazole can cause fetal goiter and cretinism when administered to a pregnant woman. For this reason, it is important that a sufficient, but not excessive, dose be given during pregnancy
OVERDOSE
Signs and Symptoms
Symptoms may include nausea, vomiting, epigastric distress, headache, fever, joint pain, pruritus, and edema. Aplastic anemia (pancytopenia) oragranulocytosis may be manifested in hours to days. Less frequent events are hepatitis, nephrotic syndrome, exfoliative dermatitis, neuropathies, and CNS stimulation or depression. No information is available on the median lethal dose of the drug or the concentration of methimazole in biologic fluids associated with toxicity and/or death.
Treatment
To obtain up-to-date information about the treatment of overdose, a good resource is your certified Regional Poison Control Center. In managing overdosage, consider the possibility of multiple drug overdoses, interaction among drugs, and unusual drug kinetics in the patient.
CONTRAINDICATIONS
TAPAZOLE is contraindicated in the presence of hypersensitivity to the drug or any of the other product components.
This monograph has been modified to include the generic and brand name in many instances.
CLINICAL PHARMACOLOGY
Methimazole inhibits the synthesis of thyroid hormones and thus is effective in the treatment of hyperthyroidism. The drug does not inactivate existingthyroxine and tri-iodothyronine that are stored in the thyroid or circulating in the blood nor does it interfere with the effectiveness of thyroid hormones given by mouth or by injection.
Methimazole is readily absorbed in the gastrointestinal tract, metabolized in the liver, and excreted in the urine.
How should I take methimazole (Tapazole)?
Take exactly as prescribed by your doctor. Do not take in larger or smaller amounts or for longer than recommended. Follow the directions on your prescription label.
Take methimazole with a full glass of water.
Methimazole can be taken with or without food, but you should take it the same way each time.
Methimazole can increase your risk of bleeding. If you need to have any type of surgery, tell the surgeon ahead of time that you are using this medication.
Methimazole can lower blood cells that help your body fight infections. This can make it easier for you to bleed from an injury or get sick from being around others who are ill. Your blood may need to be tested often. Visit your doctor regularly.
It is important to use methimazole regularly to get the most benefit. Get your prescription refilled before you run out of medicine completely.
Keep using this medication even if you feel fine or have no symptoms of hyperthyroidism. You may need to keep taking methimazole long term to control your condition. Stopping the medication could cause your symptoms to return.