What is Allopurinol? Uses, warnings & side effects

Updated May 14, 2025  •  Published Dec 30, 2020

Allopurinol is a generic prescription drug used to lower uric acid levels in the blood. Uric acid is a normal product of the body’s metabolism, but uric acid can cause problems like gout or kidney stones when levels get too high. When doctors prescribe allopurinol, most people take allopurinol tablets. However, people with high uric acid levels due to cancer  chemotherapy may be given intravenous injections.

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Allopurinol overview

BRAND NAMES:

  • Zyloprim, Aloprim

Generic Names:

  • Allopurinol

Uses:

  • Treats gout, kidney stones, and high uric acid levels due to cancer chemotherapy

Therapeutic Classes:

  • Urinary stone agent, antigout agent, xanthine oxidase inhibitor

Forms:

  • Tablet, injection

How it’s taken:

  • By mouth, intravenous injection

What is Allopurinol used for?

Healthcare professionals use allopurinol, a drug that reduces uric acid levels, to treat gout, calcium kidney stones, or high uric acid levels due to cancer chemotherapy, a condition called tumor lysis syndrome. Healthcare professionals may sometimes use it off-label for other conditions caused by high uric acid levels. However, they usually avoid allopurinol treatment when high uric acid levels aren’t associated with symptoms or other problems.

What form(s) does Allopurinol come in?

  • Tablet 

    • 100 mg

    • 200 mg

    • 300 mg

  • Injection

    • 20 mg/mL (after dilution)

What are common Allopurinol dosages?

  • For gout: 200–600 mg tablets taken once per day or in divided doses

  • For recurrent calcium oxalate stones in people with high uric acid levels: 200–300 mg tablets taken once per day

  • For tumor lysis syndrome (starting one to two days before chemotherapy treatment and ending two to three days after chemotherapy):

    • Intravenous dosage: 200–400 mg/day administered in two to four divided doses

    • Tablet: 300–800 mg/day taken once daily or in divided doses

  • Individual doses cannot exceed 300 mg. Daily doses that are larger than 300 mg must be taken in divided doses

Find more detailed information about allopurinol dosage, forms, and strengths here.

Allopurinol images - What does Allopurinol look like?

  • Allopurinol
    100mg tablet

  • Allopurinol
    200mg tablet

  • Allopurinol
    300mg tablet

How to take Allopurinol

  • Take this medicine as prescribed. Doses will change at the start of treatment. 

  • Do not stop taking allopurinol doses until talking to the prescriber or other healthcare professional. Keep taking doses during gout attacks or if symptoms go away.

  • Take the tablets at about the same time every day.

  • Take allopurinol tablets after a meal to avoid stomach upset, nausea, and similar problems.

  • Swallow allopurinol tablets whole with a glass of water.

  • Drink at least two liters of fluids daily when taking allopurinol.

  • Store allopurinol tablets at room temperature in a light-resistant container. Protect from moisture.

What to do if you miss a dose of Allopurinol

Allopurinol contraindications - Who shouldn't take Allopurinol?

  • This medicine is not right for everyone. 

  • Do not use it if you have had an allergic reaction to allopurinol.

Allopurinol warnings

  • The prescriber will require blood tests before prescribing allopurinol. After you start taking the drug, the prescriber will need regular blood tests and office visits to monitor treatment and monitor for adverse effects. Keep all appointments.

  • When starting allopurinol, make sure the prescriber knows if you are pregnant, breastfeeding, or have:

    • Kidney problems

    • Liver problems

    • Diabetes

    • High blood pressure

  • Allopurinol can cause serious and potentially fatal skin reactions. The risk of a serious drug reaction is higher in people with a gene type called HLA-B*5801. If you know that you have this gene type, tell the prescriber.

  • Stop taking allopurinol doses and immediately inform the prescriber at the first sign of a skin rash or allergic reaction.

  • This medicine may cause the following:

    • Liver problems

    • Kidney problems

    • Serious and potentially life-threatening skin reactions

  • Women should not breastfeed while taking allopurinol or for one week after the last dose has been taken.

  • When starting allopurinol, you may have more gout flares but do not stop taking doses. During this initial stage of treatment, the prescriber can prescribe medications to treat or prevent gout flares. The attacks should become shorter and less severe after a few months.

Allopurinol interactions & what to avoid when taking Allopurinol

  • To avoid possible drug interactions, tell the prescriber about all prescription drugs, over-the-counter medications, and supplements being taken, particularly:

    • Drugs that make you urinate are called thiazide diuretics

    • The immune suppressants cyclosporine, azathioprine, and mercaptopurine

    • The gout drug Krystexxa (pegloticase)

    • The blood thinners warfarin and dicumarol

    • The antibiotics amoxicillin or ampicillin

    • Chemotherapy drugs, particularly capecitabine

Find more information about allopurinol interactions to avoid here.

Allopurinol side effects - How does Allopurinol make you feel?

Serious Side Effects

Allopurinol side effects are relatively minimal and not commonly experienced. Serious side effects are possible but rare. Talk to a doctor if you notice any signs or symptoms of a possible serious side effect, including:

  • Trouble breathing or slow breathing

  • Change in how much or how often you urinate, pain when you urinate, cloudy or bloody urine, pain in the flanks of the abdomen

  • Dark urine or pale stools, loss of appetite, stomach pain, nausea, vomiting, yellow skin or eyes

  • Fever, fatigue, heart rate abnormalities, skin color changes, and other signs of infection

  • Blistering, peeling, red skin rash

  • Allergic reaction: Itchy skin, hives, swelling in your face, mouth, lips, or throat, chest tightness, trouble breathing

Less serious side effects

Allopurinol’s most common side effects have an incidence rate of 1% or less. The most common include: 

  • Nausea

  • Vomiting

  • Diarrhea

Find more information about allopurinol side effects here.

Sources:

Allopurinol News & Research

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