Allopurinol is an inhibitor of xanthine oxidase, a substance in the body that is responsible for producing uric acid. It is used to treat gout and kidney problems due to high uric acid levels, as well as to prevent kidney stones for people with high uric acid levels. It is also used to reduce the levels of uric acid in the blood that occur because of blood disorders associated with certain treatments for cancers such as leukemias and lymphomas.

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 recommended dose ranges from 100 mg to 800 mg daily in 1 to 3 divided doses, depending on the condition being treated. Take allopurinol after eating, to help minimize upset stomach.

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 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. 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 at room temperature in a dry place and keep out of the reach of children.

Do not take this medication if you:

• are allergic to allopurinol or any ingredients of this medication
• have previously developed a severe reaction to allopurinol or any ingredients of this medication
• are a child (except to treat high levels of uric acid caused by cancer or Lesch-Nyhan syndrome)
• are breast-feeding (except to treat high levels of uric acid caused by cancer)

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.

• change in normal bowel habits
• changed sense of taste
• decreased fertility (men)
• decreased sexual ability
• diarrhea
• dizziness
• drowsiness
• hair colour changes
• hair loss
• headache
• indigestion
• itchy, flaky skin
• mouth sores
• nausea
• pale, bulky, or foul-smelling stool
• 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:

• boils (red, swollen, painful bumps under the skin)
• chest pain
• enlarged breasts (men)
• numbness, tingling, pain, or weakness in hands or feet
• signs of clotting problems (e.g., unusual nosebleeds, bruising, blood in urine, coughing blood, bleeding gums, cuts that don't stop bleeding)
• signs of depression (e.g., poor concentration, changes in weight, changes in sleep, decreased interest in activities, thoughts of suicide)
• signs of kidney problems (e.g., increased urination at night, decreased urine production, blood in the urine, change of urine colour, rapid weight gain)
• signs of liver problems (e.g., nausea, vomiting, diarrhea, loss of appetite, weight loss, yellowing of the skin or whites of the eyes, dark urine, pale stools)
• swollen glands in the armpit or groin
• symptoms of increased blood pressure (e.g., chest pain, blurred vision, dizziness, excessive tiredness, headache, fast or racing heartbeat)
• symptoms of high blood sugar (e.g., frequent urination, increased thirst, excessive eating, unexplained weight loss, poor wound healing, infections, fruity breath odour)
• symptoms of a slow heartbeat (e.g., dizziness, light-headedness, or fainting)
• unsteadiness when walking
• vision changes

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

• chills, fever, muscle aches or pains, nausea, or vomiting (especially if this happens with or shortly after a skin rash)
• signs of a serious allergic reaction (e.g., abdominal cramps, difficulty breathing, nausea and vomiting, or swelling of the face and throat)
• signs of bleeding in the stomach (e.g., bloody, black, or tarry stools; spitting up of blood; vomiting blood or material that looks like coffee grounds)
• 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

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 taking 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 take this medication.

Drowsiness/reduced alertness: This medication may cause drowsiness, affecting your ability to drive or operate machinery. Avoid driving, operating machinery, or performing other potentially hazardous tasks until you have determined how you are affected by this medication.

Use appropriate caution if you plan to do activities requiring alertness.

Gout: Do not start allopurinol treatment until an acute attack of gout has completely subsided, as further attacks may be caused by this medication. Acute gout attacks may occur at the start of treatment with allopurinol for people who have not taken the medication before. If an acute gout attack develops after starting this medication, call your doctor.

Hypersensitivity syndrome: A severe allergic reaction called hypersensitivity syndrome has occurred for some people with the use of allopurinol. This reaction often starts as a rash and can progress to involve a number of organs in the body. This type of reaction may be fatal if not treated quickly. Stop taking the medication and get immediate medical attention if you have symptoms of a severe allergic reaction, including fever, swollen glands, yellowing of the skin or eyes, or flu-like symptoms with skin rash or blistering.

People with long term kidney problems and taking diuretics (water pills) may be more likely to experience severe skin or hypersensitivity reactions.

Kidney function: Kidney disease or reduced kidney function may cause this medication to build up in the body, causing side effects. If you have reduced kidney function or kidney disease, 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.

Liver function: Allopurinol 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.

Skin rash: If you develop a skin rash, stop taking this medication and contact your doctor. The skin rash may be the first sign of a serious allergic reaction to the medication.

Thyroid function: People taking allopurinol may have changes in thyroid function that produce abnormal thyroid test results. If you have a history of thyroid disease, 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.

Pregnancy: Allopurinol is not recommended for people who are or may become pregnant unless the potential benefits outweigh the possible risks. If you are or may be pregnant, talk to your doctor about the risks and benefits of this medication.

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

Children: Children should not take allopurinol, except in cases where increased uric acid levels in the blood are associated with cancer or with Lesch-Nyhan syndrome. The safety and effectiveness of allopurinol for this age group have not been established for treating other conditions.

Seniors: Kidney function often decreases with age, affecting how allopurinol is removed from the body. Seniors may require lower doses of this medication.

For a full list of interactions, use the Drug Interaction Checker available on the Drugs.com website.

If you are taking other 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.

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.