Allopurinol is a medication that lowers uric acid levels in the blood. It’s primarily used to prevent gout flares and joint damage, but it also treats certain types of kidney stones and protects the kidneys during cancer treatment. It works by blocking the enzyme that produces uric acid in the first place, making it one of the most widely prescribed urate-lowering therapies in the world.
How Allopurinol Works
Your body naturally breaks down substances called purines (found in foods like red meat and beer, and also produced by your own cells). The final step of that breakdown is handled by an enzyme called xanthine oxidase, which converts purines into uric acid. Allopurinol blocks this enzyme.
What’s interesting is that allopurinol itself isn’t the main player. Once you take it, your body converts it into an active form called oxipurinol, which binds tightly to the enzyme and shuts it down. This reduces the amount of uric acid your body produces, lowering levels in both the blood and urine. Less uric acid means fewer crystals forming in your joints and kidneys.
Gout Prevention
Gout is the most common reason people take allopurinol. When uric acid builds up in the blood, it can form sharp, needle-like crystals in joints, causing intense pain, swelling, and redness. These are gout flares, and they tend to recur and worsen over time. Left untreated, uric acid crystals can form visible lumps called tophi and permanently damage joints.
Allopurinol is a long-term preventive treatment, not a painkiller for active flares. The goal is to bring your serum uric acid level below 6 mg/dL and keep it there. For people who already have tophi, guidelines from the American College of Rheumatology suggest aiming even lower, below 5 mg/dL, to help dissolve those deposits over time. The typical starting dose is 100 mg per day, increased by 100 mg every two to five weeks until you hit your target level.
One important detail that surprises many people: you may actually experience more gout flares when you first start taking allopurinol. Shifting uric acid levels can destabilize existing crystal deposits and trigger attacks. This is normal and doesn’t mean the medication isn’t working. Flares can continue for up to 12 to 18 months after starting treatment, though they become less frequent over time and eventually stop if you maintain your uric acid target. For this reason, doctors typically prescribe a separate anti-inflammatory medication alongside allopurinol during the early months to prevent or manage these flares. Older guidance recommended waiting until a flare resolved before starting allopurinol, but clinical trials have shown that starting it during an active flare (with appropriate pain treatment) does not prolong the attack.
Kidney Stone Prevention
Allopurinol is approved for people with recurrent calcium oxalate kidney stones who excrete high amounts of uric acid in their urine, specifically above 800 mg per day in men or 750 mg per day in women. Excess uric acid in the urine can act as a seed for calcium oxalate stones to form. By reducing uric acid production, allopurinol lowers the concentration of uric acid in the urine, which helps prevent new stones from developing.
Protection During Cancer Treatment
When chemotherapy kills large numbers of cancer cells quickly, those cells release their contents into the bloodstream, including massive amounts of purines. The body converts these purines into uric acid, which can overwhelm the kidneys and cause a dangerous condition called tumor lysis syndrome. This is especially common in blood cancers like leukemia and lymphoma, where treatment can destroy a huge volume of cells at once.
Allopurinol is given before and during chemotherapy to block uric acid production and protect the kidneys. Treatment typically starts one to two days before chemotherapy begins and continues for three to seven days afterward, depending on the ongoing risk. One limitation is that allopurinol only prevents new uric acid from forming. It can’t break down uric acid that’s already in the blood, so in patients who start with very high levels (above 7.5 mg/dL), a different medication that actively degrades existing uric acid is preferred.
Genetic Screening Before Starting
A small percentage of people carry a gene variant called HLA-B*58:01 that dramatically increases the risk of a severe allergic reaction to allopurinol. This reaction can affect the skin and internal organs and, in rare cases, is life-threatening. The 2020 American College of Rheumatology guidelines recommend genetic testing before starting allopurinol for people of Southeast Asian descent (including Han Chinese, Korean, and Thai populations) and African American patients, since the variant is more common in these groups. A simple blood test can identify carriers, allowing doctors to choose an alternative medication instead.
Important Drug Interactions
Allopurinol has a potentially life-threatening interaction with two immunosuppressant medications: azathioprine and mercaptopurine. Both of these drugs are normally broken down by the same enzyme that allopurinol blocks. When allopurinol shuts down that enzyme, these medications build up to dangerously high levels in the body, which can cause severe bone marrow suppression. This means your body stops producing enough blood cells, leading to serious infections, uncontrolled bleeding, or both.
If you’re taking either of these medications (commonly prescribed for organ transplant patients, autoimmune conditions, or certain cancers), combining them with allopurinol requires reducing the immunosuppressant dose to about 25 percent of the usual amount and closely monitoring blood counts. In most cases, the combination is avoided entirely.
What Allopurinol Is Not For
The FDA label carries a notable warning: allopurinol is not recommended for people who have elevated uric acid levels but no symptoms. High uric acid on a blood test alone, without gout, kidney stones, or cancer treatment needs, is not a reason to start taking it. The medication carries real risks, and the benefits only outweigh those risks when there’s an actual condition to manage. It also does nothing for acute gout pain on its own. If you’re in the middle of a flare and not already on allopurinol, anti-inflammatory medications are the first step for immediate relief.