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Allopurinol: 7 things you should know

Medically reviewed by Carmen Fookes, BPharm. Last updated on July 13, 2022.

1. How it works

  • Allopurinol may be used to decrease levels of uric acid in people with gout, kidney stones, or who are receiving certain types of chemotherapy.
  • Allopurinol reduces the production of uric acid (a compound produced by the body associated with gout and kidney stones), by inhibiting an enzyme called xanthine oxidase. Allopurinol reduces levels of uric acid in the blood and urine.
  • Allopurinol belongs to the class of medicines called xanthine oxidase inhibitors.

2. Upsides

  • May be used to prevent acute attacks of gout and to reduce ongoing complications of gout such as tophi (deposits of crystalline uric acid that form painful nodules), joint destruction, uric acid lithiasis (the formation of kidney or bladder stones), and kidney disease. Note that allopurinol does not treat acute attacks of gout, although once started, it should be continued throughout an acute attack.
  • May be used temporarily to reduce elevated uric acid levels caused by certain cancer treatments in people with leukemia, lymphoma, and other malignancies.
  • May also be used to treat recurrent kidney stones (calcium oxalate calculi) in males with a urinary uric acid level of more than 800 mg/day, or in females with levels greater than 750 mg/day. The need for continued treatment should be reassessed periodically.
  • Available as an oral tablet and as an injection.
  • Can be taken once a day.
  • Generic allopurinol is available.

3. Downsides

If you are between the ages of 18 and 60, take no other medication or have no other medical conditions, side effects you are more likely to experience include:

  • May cause a skin rash which may be a sign of an allergic reaction to allopurinol. This is considered a serious event and allopurinol should be discontinued immediately and urgent medical advice sought. People with kidney disease who are receiving thiazide diuretics in addition to allopurinol are considered at higher risk.
  • May precipitate gout attacks during the early stages of administration, even when uric acid levels are normal. Coadministration with colchicine or an anti-inflammatory agent is recommended initially, in addition to gradually increasing the dosage of allopurinol.
  • May also cause diarrhea, nausea, and laboratory test changes.
  • Do not restart in people who have previously had a severe reaction to allopurinol.
  • Should not be used to treat asymptomatic hyperuricemia. When used to treat high uric acid levels caused by treatments for leukemia, lymphoma, and other malignancies, it should be discontinued when the potential for overproduction of uric acid is no longer present.
  • Occasionally may cause drowsiness. Affected people should not drive or operate machinery.
  • May interact with several different drugs including mercaptopurine and azathioprine (the dosage of these two drugs needs to be reduced to approximately one-third to one-fourth of their normal dosage).
  • Rarely, may cause reversible liver damage or elevations in liver enzymes. Monitoring is necessary for people with pre-existing liver disease and an investigation into the functioning of the liver should be conducted if symptoms such as weight loss or itchy skin develop.
  • Rarely, may cause bone marrow depression. May occur anywhere from 6 weeks to years after administration of allopurinol and the risk is greatest in those receiving other drugs that also have the potential to cause bone marrow depression.
  • The dosage of allopurinol needs reducing in people with kidney disease and their renal function monitored at least during the early stages of therapy. People administered allopurinol for high uric acid levels occurring as a result of cancer are at higher risk of developing kidney damage from allopurinol.
  • There is a lack of data regarding the use of allopurinol during pregnancy or breastfeeding. Consider the risks versus benefits before starting.

Note: In general, seniors or children, people with certain medical conditions (such as liver or kidney problems, heart disease, diabetes, seizures) or people who take other medications are more at risk of developing a wider range of side effects. View complete list of side effects

4. Bottom Line

Allopurinol is used in the treatment of high uric acid levels associated with gout, kidney stones, and cancer treatment, it should not be used for asymptomatic high uric acid levels. If a rash occurs while taking allopurinol, immediate discontinuation of allopurinol is required and urgent medical advice should be sought.

5. Tips

  • Allopurinol may be better tolerated if taken with food.
  • If you forget a dose of allopurinol, you do not need to double up the next dose. Just continue taking allopurinol as scheduled.
  • Discontinue allopurinol at the first appearance of a skin rash, painful urination, blood in the urine, eye irritation, or facial swelling, and seek urgent medical advice.
  • Allopurinol is a maintenance treatment for gout or high uric acid levels so it should be taken regularly even once a gout attack has resolved. Other treatments (such as NSAIDs or colchicine) are usually prescribed to be taken during an acute gout attack.
  • Seek immediate medical advice if unexplained weight loss or itchiness develops while you are taking allopurinol.
  • Starting allopurinol may cause an acute attack of gout. Your doctor should have advised you on how to manage this, if not, seek further advice from them.
  • Do not drive or operate machinery if allopurinol makes you drowsy or impairs your judgment.
  • Ensure you maintain a good intake of fluid while taking allopurinol. A fluid intake of 2 liters per day is recommended to reduce the risk of kidney stones developing.
  • Tell your doctor if you are pregnant or intending to become pregnant before taking allopurinol because it may not be suitable for you.

6. Response and effectiveness

  • Peak levels of allopurinol are reached within 1.5 hours and 4.5 hours for its active metabolite. Allopurinol may be taken once daily.
  • Normal serum uric acid levels are normally achieved within one to three weeks.
  • It may take two to six weeks before allopurinol effectively reduces gout attacks. It is not unusual to experience a gout attack in the first few weeks of allopurinol initiation.
  • In people with severe kidney disease, a dosage of 100 mg/day or 300mg twice a week may be sufficient to reduce high uric acid levels.

7. Interactions

Medicines that interact with allopurinol may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with allopurinol. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed.

Common medications that may interact with allopurinol include:

  • ACE inhibitors, such as benazepril, captopril, or fosinopril
  • antacids, such as those containing aluminum
  • antibiotics, such as amoxicillin
  • anticoagulants, such as warfarin
  • anticonvulsants, such as phenytoin
  • antiviral medications, such as didanosine or entecavir
  • azathioprine
  • chemotherapy treatments, such as cyclophosphamide or mercaptopurine
  • cyclosporine
  • diuretics, such as hydrochlorothiazide
  • pegloticase
  • probenecid
  • sucralfate.

The dosage of allopurinol should be reduced by one-third to one-fourth in people also receiving mercaptopurine (Purinethol) or azathioprine (Imuran).

Note that this list is not all-inclusive and includes only common medications that may interact with allopurinol. You should refer to the prescribing information for allopurinol for a complete list of interactions.


Further information

Remember, keep this and all other medicines out of the reach of children, never share your medicines with others, and use allopurinol only for the indication prescribed.

Always consult your healthcare provider to ensure the information displayed on this page applies to your personal circumstances.

Copyright 1996-2023 Revision date: July 13, 2022.