Allopurinol lowers uric acid by blocking the enzyme that produces it. It’s the most commonly prescribed long-term medication for gout and works by interrupting uric acid production at its source, rather than helping your body eliminate uric acid faster. The goal is to bring your blood uric acid level below 6 mg/dL and keep it there, which over time dissolves the uric acid crystals responsible for gout attacks.
The Enzyme It Blocks
Your body constantly breaks down compounds called purines, which come from the food you eat and from normal cell turnover. The final step in that breakdown process is handled by an enzyme called xanthine oxidase, which converts precursor molecules into uric acid. Allopurinol and its active metabolite, oxypurinol, both inhibit xanthine oxidase. With that enzyme blocked, less uric acid gets produced, and the precursors that would have become uric acid are excreted through your kidneys instead.
After you take allopurinol, your liver converts it into oxypurinol, which is actually the form that does most of the work. Oxypurinol sticks around in your body much longer than allopurinol itself, which is why the medication works with once-daily dosing. Both forms target the same enzyme, but oxypurinol provides the sustained uric acid suppression throughout the day.
What It’s Prescribed For
The primary use of allopurinol is preventing gout flares in people with chronically elevated uric acid. It’s not a painkiller and won’t help during an active gout attack. Instead, it’s a long-term strategy: by keeping uric acid levels consistently low, the crystals already deposited in your joints gradually dissolve, and new ones stop forming. This is why doctors prescribe it as a daily medication you take indefinitely, not something you reach for when a flare starts.
Allopurinol is also used to prevent uric acid kidney stones and to manage high uric acid levels that develop during cancer treatment, when large numbers of cells break down rapidly and flood the body with purines.
Dosing and How It’s Adjusted
Treatment typically starts at 100 mg per day, with the dose increased by 100 mg each week until your uric acid drops below 6 mg/dL. This slow titration matters. Starting low and increasing gradually reduces the risk of both gout flares and serious adverse reactions. The effective range for most people falls between 100 and 200 mg daily, though the maximum dose can go up to 800 mg daily for those who need it.
If you have reduced kidney function, the starting dose is lower (50 mg daily), and increases happen more slowly, typically every two to four weeks in 50 mg increments. Because allopurinol and oxypurinol are primarily eliminated through the kidneys, impaired kidney function causes the drug to accumulate. For people with severely reduced kidney function, the daily dose may need to stay at 100 mg or less, and the time between doses may need to be extended.
How Long It Takes to Work
You won’t feel a difference right away. Uric acid levels drop gradually over weeks as the dose is titrated upward. Most treatment plans involve checking your levels every two to five weeks and adjusting accordingly. Even once your uric acid reaches the target, it takes months for the crystals already sitting in your joints to fully dissolve. This is a slow process, and the full benefit of treatment often isn’t apparent for six months or longer.
Why It Can Trigger Flares at First
One of the most frustrating aspects of starting allopurinol is that it can actually provoke gout attacks in the first weeks or months. This seems counterintuitive, but it happens because lowering uric acid levels causes existing crystal deposits in your joints to shift and partially dissolve. That remodeling process irritates the joint lining and triggers inflammation. The greater the initial drop in uric acid, the more likely a flare becomes.
To prevent this, doctors typically prescribe a low-dose anti-inflammatory medication alongside allopurinol during the early months. Current guidelines recommend continuing this preventive treatment for at least three months after reaching your target uric acid level if you don’t have visible uric acid deposits (tophi), or six months if tophi were present but have resolved. The flares don’t mean the medication isn’t working. They’re actually a sign that crystal deposits are being disrupted, which is part of the process.
A Rare but Serious Reaction
Allopurinol causes severe skin reactions in a small number of people, estimated at 0.1 to 0.4% of those who take it. These reactions, which include conditions called Stevens-Johnson syndrome and toxic epidermal necrolysis, carry a mortality rate of up to 25%. The risk is strongly linked to a genetic variant called HLA-B*58:01. In a landmark study of Han-Chinese patients, 100% of those who developed these severe reactions carried this gene variant, compared to only 15% of those who tolerated the drug without problems.
You only need one copy of the HLA-B*58:01 variant to be at increased risk. The risk climbs substantially in people who also have reduced kidney function, ranging from roughly 2 to 13% with mildly impaired kidneys and up to 100% with severely impaired kidneys. Genetic testing for HLA-B*58:01 is now recommended before starting allopurinol, particularly for people of Southeast Asian, African American, or Korean descent, where the variant is more common. A negative test doesn’t eliminate all risk, since other factors also contribute, but it significantly narrows it.
What to Expect on Long-Term Treatment
Allopurinol is a lifelong medication for most people with gout. Stopping it allows uric acid to climb back up, crystals to reform, and flares to return. The treat-to-target approach, endorsed by the American College of Rheumatology, means your dose gets adjusted based on periodic blood tests rather than set at a fixed amount. The target is a uric acid level below 6 mg/dL, and your dose stays at whatever level maintains that threshold.
Once you’ve been on a stable dose long enough for existing crystal deposits to dissolve and your uric acid stays consistently below target, gout flares become rare or stop entirely. For many people, this represents a dramatic improvement in quality of life compared to repeated acute attacks. The medication works best when taken daily without interruption, since even brief gaps can cause uric acid to spike and potentially trigger a flare.