Methimazole (Oral)

Methimazole is an oral medicine that prevents the thyroid gland from over-producing thyroid hormone when it is used to treat patients with overactive thyroid gland (hyperthyroidism).

Overview

Hyperthyroidism occurs when too much thyroid hormone is produced by the thyroid gland. The medicine relieves the symptoms of hyperthyroidism by reducing the ability of the gland to make excess thyroid hormone.

It usually takes a few weeks before the medication takes full effect to relieve the symptoms of an overactive thyroid gland. Other medications may be prescribed to a patient to control the symptoms in the meantime.

There are health risks of using this drug. A patient's immune system may be weakened, making it easier for the patient to get infections. White blood cell levels may be lowered preventing blood from clotting as normal.

It is also not safe to use the medicine to treat pregnant women.

Methimazole, the generic version of the US brand Tapazole, is sold in the US by doctor's prescription only. It may have other uses not discussed here, such as the treatment of patients undergoing radioactive iodine therapy or thyroid surgery.

Condition treated:

  • Hyperthyroidism (overactive thyroid gland)

Type Of Medicine

  • Antithyroid agent

Side Effects

Unwanted symptoms may occur due to taking Methimazole. These are called adverse side effects. Other side effects that occur may be expected or required as the medicine works to treat the symptoms of the condition.

Side effects may range from mild to severe and some of them may be transient (go away after a short period of time). Severe or serious side effects usually require urgent medical care.

Call your doctor right away if you experience any of the following side effects:

Rarely occur

  • Shortness of breath
  • Chills
  • Sore throat
  • Fever
  • Stool that is black or tarry
  • Bruising or bleeding that is unusual
  • Pain in the chest
  • Cough
  • Feeling unusually weak or tired
  • Pain or difficulty urinating
  • Swollen glands
  • Backache
  • Urinating less or more than usual
  • White spots, sores or ulcers in the mouth or on the lips
  • Swelling of the lower legs or feet

Some side effects may occur but the frequency and severity with which they occur are unknown. Some of them are as follows. Call your doctor immediately if any of them occur.

Incidence unknown

  • Dark urine
  • Light-colored stools
  • A feeling of discomfort or weakness
  • Soreness of the muscles
  • Headache
  • Joint pain
  • Loss of appetite
  • Wheezing
  • Lower back or side pain
  • Difficulty moving the body
  • Tightness in the chest
  • Pain in the stomach or abdomen
  • Skin or eyes that appear yellow
  • Weight loss
  • Swollen joints
  • Aching or cramping of the muscle
  • Pain or stiffness in the muscles
  • Blood-stained urine or stool
  • Tingling or numbness of the face, hands or feet
  • Red spots on the skin that have a pinpoint
  • Bleeding gums
  • Painful, tender or swollen lymph glands in the groin, armpit or neck
  • Swollen salivary glands
  • Nausea or vomiting
  • Crawling, prickling, numbness, itching, tingling, burning, or sensation of "pins and needles" about the body

There are side effects that may occur that you can manage without having to seek medical attention. Some of them tend to go away on their own as the body adjusts to the medicine.

The following is a list of such side effects. If any of them occur but gets worse, bothersome or does not go away, tell your doctor.

Incidence unknown

  • Loss of taste
  • Feeling lightheaded or dizzy
  • Pain or discomfort in the chest, throat or upper stomach
  • Vomiting
  • Skin rash
  • Itching
  • Feeling as if you or your surroundings are constantly moving
  • Unusual loss of hair
  • Red skin
  • Hives or welts
  • Sleepiness
  • Heartburn

Some patients may experience other side effects not listed in this guide. If you notice any unusual symptom that bothers you, let your doctor or healthcare professional know.

You may ask them about ways to manage, reduce and prevent side effects.

You may report side effects by calling the U.S. Food and Drug Administration (FDA) at 1-800-FDA-1088.

Dosage

Methimazole is supplied as tablets of 5 mg and 10 mg strengths and is taken by mouth. Each dose is usually taken three times a day, every 8 hours. It can be taken with or without food, as directed by your doctor.

Your dose, how often you take it, and the time between each dose will vary from other patients. It depends on the condition you are being treated for and how long your treatment will last.

Follow all directions given to you by your doctor, pharmacist and the prescription label found on the packet of your medicine. You should also read the patient information leaflet inserted into the medicine packet.

If you have any questions about using the medicine, ask your doctor or pharmacist.

The following is only a recommended dosage guide. Stick to the dose your doctor told you to take. To get the best out of your treatment, take your dose on time each day to keep the medicine at a constant level in your body.

Methimazole tablet for the treatment of hyperthyroidism:

Adults:

The starting dose is usually 3 equal doses of between 15 to 60 milligrams (mg) of the tablet, daily. It should be taken approximately every 8 hours.

The exact amount and strength of your dose will depend on whether your symptoms of hyperthyroidism are mild, moderate or severe. The maintenance dose ranges from 5 to 15 mg daily.

Children:

The dose will be determined by your doctor based on body weight. The initial dose is usually 0.4 mg of the tablet for each kilogram (kg) of your body weight, per day.

It is divided and taken as 3 equal doses, approximately every 8 hours. The maintenance dose is usually one half (0.2 mg) of the tablet for each kg of your body weight, per day.

Missed dose

If you missed a dose of this medicine, take it as soon as possible. However, if it is time to take the next dose, skip the missed dose and take the next dose as scheduled. Continue with your regular dosing schedule. Do not take an extra dose to make up for the missed dose.

Overdose

Call your local poison control center at 1-800-222-1222 if you overdose on this drug. If the patient collapses or is not breathing, call 911.

Interactions

Food, alcohol, and tobacco

Sometimes food, alcohol or tobacco may affect the way a medicine works and should not be taken during treatment. Your doctor will let you know if you should avoid any of them while taking Methimazole.

Your doctor will need to know about all medications you take. Also tell your doctor about all medical problems you have or have a history of. This will help your doctor determine if to treat you and what precautions should be taken.

Other substances such as those found in street drugs may also interact and affect the medicine

Other medicines

The following medicines may cause significant adverse interaction when used during treatment with Methimazole. It does not always mean that you must stop taking any of them in order to take Methimazole.

Your doctor may decide to treat you but change your dose or how often you take one or more of your medicines.

It is usually not recommended to use Methimazole with the following other medications. If due to your health it is best to continue using any of them during your treatment for overactive thyroid, your doctor may change the dose or how often you use any of your medicines.

  • Tegafur
  • Bupropion

The risk of certain side effects may increase if any of the following other medicines is used together with Methimazole. Your doctor may decide it is best to allow you to continue all your medicines but may change the dose or how often you use any of them.

  • Phenprocoumon
  • Warfarin
  • Anisindione
  • Dicumarol
  • Acenocoumarol
  • Phenindione

Other drugs not listed here may also interact with this medicine. Ask your doctor for more information.

Other medical problems

Medical conditions present in patients taking Methimazole may affect or be affected by the medicine, especially patients with the following conditions. To avoid making the conditions worse, the medicine should be used with caution.

  • Blood or bone marrow problems
  • Low platelets in the blood (Thrombocytopenia)
  • Very low red blood cells (Aplastic Anemia)
  • Low white blood cells (Leukopenia)
  • Very low white blood cells (Agranulocytosis)

Warnings

  • Use Methimazole only if your doctor prescribed it to treat you for an overactive thyroid gland.
  • If you have a medical condition that may be treated by this medicine, other than hyperthyroidism, speak with your doctor first before taking this drug.
  • Do not use it if you are allergic to it or to any of its inactive ingredients.
  • Let your doctor know if you are allergic to other medicines, certain foods, dyes, preservatives or animals.
  • This medication can harm an unborn baby. Tell your doctor if you are pregnant. You should not be treated with this medicine unless your doctor determines that the potential benefits outweigh the risks involved.

Your doctor may advise you to use birth control during treatment. Tell your doctor right away if you become pregnant while being treated with this medicine.

  • Tell your doctor if you are breastfeeding. Your doctor should determine if it is safe to treat you with the medicine while you are breastfeeding. There is a potential risk of harm to a breastfeeding infant.
  • There is no limitation on the use of the medicine in children or elderly adults. Your doctor will determine use and dose based on your age and/or weight.
  • Tell your doctor about all medications you take or plan to take. These include prescription and non-prescription drugs. Also disclose your use of vitamin supplements or herbal products.
  • Certain medical problems may affect or be affected by the medicine. Ensure you tell your doctor about all medical problems you have or have a history of.
  • You may need to do regular blood tests or other lab work, so your doctor can check your progress during treatment. Ensure you keep all your follow-up appointments so your doctor can check you for unwanted side effects.
  • Methimazole may cause a decrease in the levels of white blood cells in your body. This increases the risk of getting infections. Call your doctor right away if you experience symptoms of an infection such as headache, fever, sore throat, skin rash, or if you feel generally unwell.
  • This drug may reduce the body's ability to clot blood and increase the risk of bleeding. To reduce the risk bruising or bleeding your doctor may advise you to clean your teeth and gums in ways other than by using a toothbrush, toothpick, or dental floss.
  • Use sharp devices, such as razors or finger or toenail cutters, with care to avoid cutting yourself.
  • Avoid any activity, such as contact sport, which increases your risk of bruising or injury.
  • Check with your doctor before doing any dental procedure.
  • In rare cases, this drug may cause liver problems in some patients. Call your doctor right away if you notice any of these symptoms: headache, nausea, vomiting, yellow skin or eyes or clay-colored stools.

Other symptoms include loss of appetite, pain or tenderness in the abdomen, swelling of the lower legs or feet, dark color urine, fever, itching, skin rash or unusual tiredness or weakness.

  • If your doctor instructs you to monitor and report any change in your weight, do so as soon as possible if your weight changes.
  • Do not stop taking the medicine if your symptoms are not getting better right away. The medicine may take a few weeks before taking full effect in relieving your symptoms.

Storage

Store the medicine, in a closed container, at room temperature. Keep away from moisture, heat, and direct light. Keep from freezing.

Keep out of the reach of children.

Using a safe method of disposal, throw away the medicine once it expires or is no longer needed.

Summary

Methimazole has been effectively used in children and adults to manage the symptoms of hyperthyroidism (overactive thyroid gland).

But it is not recommended to be used in pregnant women due to the risk of harm to an unborn baby. Women treated with Methimazole may be required to take birth control to prevent pregnancy during treatment.

This drug may reduce the body's ability to clot blood and increase the risk of bleeding. It also weakens the immune system making patients vulnerable to infections.

The medication may take a few weeks to take full effect in relieving the symptoms of the overactive thyroid gland. Patients will need to continue taking it, and any other medication that may have been prescribed to control the symptoms in the meantime.

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