Medically reviewed on September 3, 2018
Commonly used brand name(s)
In the U.S.
Available Dosage Forms:
Therapeutic Class: Antithyroid Agent
Chemical Class: Thionamide
Uses For methimazole
Methimazole is used to treat hyperthyroidism, a condition where the thyroid gland produces too much thyroid hormone. It is also used before thyroid surgery or radioactive iodine treatment. Methimazole is an antithyroid medicine. It works by making it harder for the body to make thyroid hormone.
Methimazole is available only with your doctor's prescription.
Before Using methimazole
In deciding to use a medicine, the risks of taking the medicine must be weighed against the good it will do. This is a decision you and your doctor will make. For methimazole, the following should be considered:
Tell your doctor if you have ever had any unusual or allergic reaction to methimazole or any other medicines. Also tell your health care professional if you have any other types of allergies, such as to foods, dyes, preservatives, or animals. For non-prescription products, read the label or package ingredients carefully.
Appropriate studies performed to date have not demonstrated pediatric-specific problems that would limit the usefulness of methimazole in children.
No information is available on the relationship of age to the effects of methimazole in geriatric patients.
|All Trimesters||D||Studies in pregnant women have demonstrated a risk to the fetus. However, the benefits of therapy in a life threatening situation or a serious disease, may outweigh the potential risk.|
There are no adequate studies in women for determining infant risk when using this medication during breastfeeding. Weigh the potential benefits against the potential risks before taking this medication while breastfeeding.
Interactions with Medicines
Although certain medicines should not be used together at all, in other cases two different medicines may be used together even if an interaction might occur. In these cases, your doctor may want to change the dose, or other precautions may be necessary. When you are taking methimazole, it is especially important that your healthcare professional know if you are taking any of the medicines listed below. The following interactions have been selected on the basis of their potential significance and are not necessarily all-inclusive.
Using methimazole with any of the following medicines is usually not recommended, but may be required in some cases. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
Using methimazole with any of the following medicines may cause an increased risk of certain side effects, but using both drugs may be the best treatment for you. If both medicines are prescribed together, your doctor may change the dose or how often you use one or both of the medicines.
Interactions with Food/Tobacco/Alcohol
Certain medicines should not be used at or around the time of eating food or eating certain types of food since interactions may occur. Using alcohol or tobacco with certain medicines may also cause interactions to occur. Discuss with your healthcare professional the use of your medicine with food, alcohol, or tobacco.
Other Medical Problems
The presence of other medical problems may affect the use of methimazole. Make sure you tell your doctor if you have any other medical problems, especially:
- Agranulocytosis (very low white blood cells) or
- Aplastic anemia (very low red blood cells) or
- Blood or bone marrow problems or
- Leukopenia (low white blood cells) or
- Thrombocytopenia (low platelets in the blood)—Use with caution. May make these conditions worse.
Proper Use of methimazole
Take methimazole only as directed by your doctor to benefit your condition as much as possible. Do not take more of it, do not take it more often, and do not take it for a longer time than your doctor ordered.
Methimazole works best when there is a constant amount in the blood. Do not miss any doses. If you are taking more than one dose per day, it is best to take the doses at evenly spaced times. If you need help planning when to take your medicine, check with your doctor or pharmacist.
The dose of methimazole will be different for different patients. Follow your doctor's orders or the directions on the label. The following information includes only the average doses of methimazole. If your dose is different, do not change it unless your doctor tells you to do so.
The amount of medicine that you take depends on the strength of the medicine. Also, the number of doses you take each day, the time allowed between doses, and the length of time you take the medicine depend on the medical problem for which you are using the medicine.
- For oral dosage form (tablets):
- For hyperthyroidism:
- Adults—At first, 15 to 60 milligrams (mg) per day, divided in 3 equal doses and given about every 8 hours. The maintenance dose is 5 to 15 mg per day.
- Children—Dose is based on body weight and must be determined by your doctor. The starting dose is 0.4 mg per kilogram (kg) of body weight per day, divided in 3 equal doses and given about every 8 hours. The maintenance dose is 0.2 mg per kg of body weight per day.
- For hyperthyroidism:
If you miss a dose of methimazole, take it as soon as possible. However, if it is almost time for your next dose, skip the missed dose and go back to your regular dosing schedule. Do not double doses.
Store the medicine in a closed container at room temperature, away from heat, moisture, and direct light. Keep from freezing.
Keep out of the reach of children.
Do not keep outdated medicine or medicine no longer needed.
Ask your healthcare professional how you should dispose of any medicine you do not use.
Precautions While Using methimazole
It is very important that your doctor check your progress at regular visits to make sure that the medicine is working properly. Blood tests may be needed to check for unwanted effects.
Using methimazole while you are pregnant can harm your unborn baby. Use an effective form of birth control to keep from getting pregnant. If you think you have become pregnant while using the medicine, tell your doctor right away.
In very rare situations, methimazole can temporarily lower the number of white blood cells in your blood, which increases the chance of getting an infection. It can also lower the number of platelets, which are necessary for proper blood clotting. If this occurs, there are certain precautions you can take, especially when your blood count is low, to reduce the risk of infection or bleeding:
- If you can, avoid people with infections. Check with your doctor immediately if you think you are getting an infection or if you get a fever or chills, cough or hoarseness, lower back or side pain, or painful or difficult urination.
- Check with your doctor immediately if you notice any unusual bleeding or bruising; black, tarry stools; blood in the urine or stools; or pinpoint red spots on your skin.
- Be careful when using a regular toothbrush, dental floss, or toothpick. Your medical doctor, dentist, or nurse may recommend other ways to clean your teeth and gums. Check with your medical doctor before having any dental work done.
- Do not touch your eyes or the inside of your nose unless you have just washed your hands and have not touched anything else in the meantime.
- Be careful not to cut yourself when you are using sharp objects such as a safety razor or fingernail or toenail cutters.
- Avoid contact sports or other situations where bruising or injury could occur.
Liver problems may occur while you are using methimazole. This is not a very common problem. Check with your doctor right away if you are having more than one of these symptoms: abdominal pain or tenderness; clay-colored stools; dark urine; decreased appetite; fever; headache; itching; loss of appetite; nausea and vomiting; skin rash; swelling of the feet or lower legs; unusual tiredness or weakness; or yellow eyes or skin.
Do not take other medicines unless they have been discussed with your doctor. This includes prescription or nonprescription (over-the-counter [OTC]) medicines and herbal or vitamin supplements.
Methimazole Side Effects
Along with its needed effects, a medicine 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:
- 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
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
- 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 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
Other side effects not listed may also occur in some patients. If you notice any other effects, check with your healthcare professional.
Call your doctor for medical advice about side effects. You may report side effects to the FDA at 1-800-FDA-1088.
See also: Side effects (in more detail)
Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.
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- Drug class: antithyroid agents
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