Propylthiouracil belongs to the class of medications called antithyroid medications. It is used to treat an overactive thyroid gland that is producing too much thyroid hormone (sometimes referred to as hyperthyroidism). It works by decreasing the production of thyroid hormone.

Signs of improvement are usually seen within 3 weeks of the start of treatment. Propylthiouracil usually needs to be taken for a period lasting between 6 months and 3 years.

This medication may be available under multiple brand names and/or in several different forms. Any specific brand name of this medication may not be available in all of the forms or approved for all of the conditions discussed here. As well, some forms of this medication may not be used for all of the conditions discussed here.

Your doctor may have suggested this medication for conditions other than those listed in these drug information articles. If you have not discussed this with your doctor or are not sure why you are taking this medication, speak to your doctor. Do not stop taking this medication without consulting your doctor.

Do not give this medication to anyone else, even if they have the same symptoms as you do. It can be harmful for people to take this medication if their doctor has not prescribed it.

The usual adult starting dose of propylthiouracil is 50 mg to 100 mg every 8 hours, with increases as necessary up to a maximum of 500 mg daily or as directed by your doctor. When doses larger than 300 mg per day are needed, the medication should be taken every 4 to 6 hours. Children's doses are based on their body weight.

Propylthiouracil may be taken with or without food. Do not chew, split, or crush the tablets. Avoid taking any tablets that are cracked or broken.

Many things can affect the dose of medication that a person needs, such as body weight, other medical conditions, and other medications. If your doctor has recommended a dose or schedule different from the ones listed here, do not change the way that you are taking the medication without consulting your doctor.

It is important to take this medication exactly as prescribed by your doctor. This medication should not be stopped suddenly but should be withdrawn over a period of 1 to 2 months under the direction of your doctor.

If you miss a dose, take it as soon as possible and continue with your regular schedule. If it is almost time for your next dose, skip the missed dose and continue with your regular dosing schedule. Do not take a double dose to make up for a missed one. If you are not sure what to do after missing a dose, contact your doctor or pharmacist for advice.

Store this medication at room temperature, protect it from light and moisture, and keep it out of the reach of children.

Each white, circular, biconvex tablet, approximately 6.5 mm by 3 mm, contains 50 mg of propylthiouracil. Nonmedicinal ingredients: lactose monohydrate, acacia spray-dried, croscarmellose sodium, sodium lauryl sulfate, and magnesium stearate.

Do not take propylthiouracil if you:

• are allergic to propylthiouracil or any ingredient of the medication
• are breast-feeding

Many medications can cause side effects. A side effect is an unwanted response to a medication when it is taken in normal doses. Side effects can be mild or severe, temporary or permanent.

The side effects listed below are not experienced by everyone who takes this medication. If you are concerned about side effects, discuss the risks and benefits of this medication with your doctor.

The following side effects have been reported by at least 1% of people taking this medication. Many of these side effects can be managed, and some may go away on their own over time.

Contact your doctor if you experience these side effects and they are severe or bothersome. Your pharmacist may be able to advise you on managing side effects.

• drowsiness
• headache
• hearing changes
• loss of sense of taste
• muscle aches
• nausea
• stomach pain
• vomiting

Although most of the side effects listed below don't happen very often, they could lead to serious problems if you do not seek medical attention.

Check with your doctor as soon as possible if any of the following side effects occur:

• backache
• change of hair colour
• hair loss
• lupus-like symptoms (e.g., fever, general feeling of being unwell, joint pain, confusion, muscle aches, skin rash)
• numbness or tingling of fingers, toes, or face
• pain, swelling, or redness in joints
• signs of anemia (low red blood cells; e.g., dizziness, pale skin, unusual tiredness or weakness, shortness of breath)
• signs of clotting problems (e.g., unusual nosebleeds, bruising, blood in urine, coughing blood, bleeding gums, cuts that don't stop bleeding)
• signs of infection (symptoms may include fever or chills, severe diarrhea, shortness of breath, prolonged dizziness, headache, stiff neck, weight loss, or listlessness)
• signs of kidney problems (e.g., increased urination at night, decreased urine production, blood in the urine, change of urine colour)
• skin rash
• symptoms of hypothyroidism (e.g., dry skin, constipation, weight gain, fatigue, aches, pains and stiffness, intolerance to cold, depression, memory problems)
• symptoms of liver problems (such as yellow eyes or skin, abdominal pain, pale stools, dark urine, itching, nausea, or fatigue)

Stop taking the medication and seek immediate medical attention if any of the following occur:

• signs of a serious allergic reaction (e.g., abdominal cramps, difficulty breathing, nausea and vomiting, or swelling of the face and throat)
• signs of a severe skin reaction such as blistering, peeling, a rash covering a large area of the body, a rash that spreads quickly, or a rash combined with fever or discomfort
• symptoms of encephalopathy (disease affecting the function or structure of the brain; memory loss, personality changes, inability to concentrate, muscle twitches, sleep problems, decreased coordination, and slurred speech)

Some people may experience side effects other than those listed. Check with your doctor if you notice any symptom that worries you while you are taking this medication.

Before you begin using a medication, be sure to inform your doctor of any medical conditions or allergies you may have, any medications you are taking, whether you are pregnant or breast-feeding, and any other significant facts about your health. These factors may affect how you should use this medication.

Blood counts: This medication can decrease the number of neutrophils (a type of white blood cell that helps fight infection), red blood cells (which carry oxygen), and platelets (which help your blood to clot). If you notice any signs of infection (e.g., fever, chills, or sore throat) or unusual bleeding or bruising, contact your doctor immediately.

Hypothyroidism: Your dose of propylthiouracil will be reduced or temporarily discontinued by your doctor if signs of hypothyroidism (such as weight gain, clumsiness, and intolerance of cold temperatures) occur during treatment.

Inflamed blood vessels: Rarely, propylthiouracil can cause blood vessels to become inflamed. This inflammation can cause changes to the blood vessels, reducing blood flow and possibly causing scarring, weakening, or narrowing of the blood vessels. This can result in organ or tissue damage. If you experience any unexpected side effects, such as difficulty breathing, rash, blood in the urine, or coughing up blood, contact your doctor as soon as possible.

Kidney problems: If you have kidney problems, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed.

Lactose: Some formulations of this medication contain lactose. If you have hereditary problems of galactose intolerance, lactase deficiency, or glucose-galactose malabsorption, talk to your doctor about whether this medication is appropriate for you.

Liver function: Propylthiouracil may reduce liver function and can cause liver failure. If you have liver problems, discuss with your doctor how this medication may affect your medical condition, how your medical condition may affect the dosing and effectiveness of this medication, and whether any special monitoring is needed. Your doctor may want to test your liver function regularly with blood tests while you are taking this medication.

If you experience symptoms of liver problems such as fatigue, feeling unwell, loss of appetite, nausea, yellowing of the skin or whites of the eyes, dark urine, pale stools, abdominal pain or swelling, and itchy skin, contact your doctor immediately.

Size of thyroid gland: Tell your doctor if the size of your thyroid gland increases during treatment with propylthiouracil. An enlarged thyroid gland is a sign that you may need a lower dose of the medication.

Pregnancy: Propylthiouracil may be taken during pregnancy only if the benefits outweigh the risks. This medication crosses the placenta and may harm the baby’s liver if the dose is too high. If you become pregnant while taking this medication, contact your doctor immediately. Those who may become pregnant should use effective birth control while taking propylthiouracil.

Breast-feeding: This medication passes into breast milk. If you are breast-feeding and taking propylthiouracil, it may affect your baby. Talk to your doctor about whether you should continue breast-feeding.

There may be an interaction between propylthiouracil and any of the following:

• bacillus Calmette-Guérin (BCG)
• cladribine
• clozapine
• deferiprone
• digoxin
• medications that contain iodine
• mesalamine
• olsalazine
• sulfasalazine
• theophyllines (e.g., aminophylline, theophylline)
• warfarin

If you are taking any of these medications, speak with your doctor or pharmacist. Depending on your specific circumstances, your doctor may want you to:

• stop taking one of the medications,
• change one of the medications to another,
• change how you are taking one or both of the medications, or
• leave everything as is.

An interaction between two medications does not always mean that you must stop taking one of them. Speak to your doctor about how any drug interactions are being managed or should be managed.

Medications other than those listed above may interact with this medication. Tell your doctor or prescriber about all prescription, over-the-counter (non-prescription), and herbal medications you are taking. Also tell them about any supplements you take. Since caffeine, alcohol, the nicotine from cigarettes, or street drugs can affect the action of many medications, you should let your prescriber know if you use them.