Does Allopurinol Cause Diarrhea? What to Do About It

Diarrhea is one of the recognized early side effects of allopurinol, the xanthine oxidase inhibitor most commonly prescribed to lower uric acid in people with gout or hyperuricemia. It is not the most frequent side effect (skin rashes hold that distinction), but it appears consistently enough in prescribing information and clinical reports that gastrointestinal upset is a genuine concern for people starting the drug. The good news is that for most people, the problem is manageable and does not necessarily mean stopping treatment.

How Common Is the Problem

Allopurinol-related diarrhea falls into the category of common early side effects alongside skin rashes and nausea.¹ Exact prevalence figures vary across studies, partly because many trials group all gastrointestinal complaints together rather than breaking out diarrhea on its own. In one genotyping-guided prescribing cohort of 97 patients, about 2% experienced gastrointestinal symptoms described as nausea and diarrhea.² That is a relatively small number, but it represents a carefully monitored group on standard doses. In broader clinical practice, where doses and patient populations are more varied, GI complaints are reported more frequently. The pattern most doctors see is that diarrhea tends to show up in the first weeks of therapy and often settles down as the body adjusts.

It is worth noting that the overall rate of adverse events on allopurinol at standard doses (200–300 mg daily) is not dramatically different from that of other urate-lowering drugs. A systematic review and meta-analysis comparing allopurinol to febuxostat across five randomized controlled trials found the combined risk of any adverse event was essentially the same between the two drugs.³ So while diarrhea is real, allopurinol is not an unusually problematic drug in the grand scheme of gout treatment.

Why Allopurinol Can Upset Your Gut

Allopurinol works by blocking xanthine oxidase, the enzyme responsible for converting hypoxanthine into uric acid. That is how it lowers urate levels. But xanthine oxidase is not just sitting around in the liver waiting to make uric acid. It plays roles elsewhere in the body, including in the lining of the gut. When you inhibit this enzyme system-wide, you also interfere with purine metabolism in the intestinal tissue itself.

Research in animal models has shown that allopurinol does more than simply block the uric acid pathway. At doses matching what humans typically take, it shifts adenylate and creatine metabolism in the gut, disrupts a key energy-sensing pathway called AMPK, and interferes with the ability of intestinal cells to proliferate and heal.⁴ In plainer terms, the drug can make it harder for the gut lining to repair itself and maintain its normal barrier function. This is especially relevant in people who already have some degree of intestinal inflammation, even if they do not know it. The effect was demonstrated in a murine colitis model, where a therapeutic dose of allopurinol worsened tissue damage. While mouse studies do not translate directly to every human gut, they help explain why some people feel gastrointestinal effects that go beyond simple stomach upset.

The mechanism also explains why the diarrhea from allopurinol is not typically violent or bloody the way it might be with, say, an antibiotic that wipes out gut flora wholesale. It is more of a low-grade disruption: looser stools, more frequent bowel movements, occasional cramping. The gut lining is slightly compromised, not destroyed.

Allopurinol and the Gut Microbiome

Beyond its direct effects on intestinal cells, allopurinol also reshapes the community of bacteria living in the gut. A study using gene sequencing to track bacterial populations in rats with hyperuricemia found that allopurinol treatment increased the abundance of certain bacterial groups, including Bifidobacterium and Collinsella, while decreasing others like Adlercreutzia and Anaerostipes. Allopurinol also specifically reduced three additional genera: Bilophila, Morganella, and Desulfovibrio.⁵

Some of those shifts sound beneficial on the surface. Bifidobacterium, for instance, is a genus often associated with gut health and is the basis of many probiotic products. But the overall picture is more complicated. Anaerostipes bacteria produce butyrate, a short-chain fatty acid that nourishes the cells lining the colon, so a drop in that population could reduce the gut lining’s resilience. Desulfovibrio species, while sometimes linked to inflammation, also play a role in sulfur metabolism in the gut. The point is not that allopurinol destroys healthy gut flora but that it rearranges the ecosystem in ways that could make some people more susceptible to loose stools or digestive discomfort, particularly in the early weeks before the microbial community finds a new equilibrium.

This research is still in its early stages and was conducted in rats, so it would be premature to say “take a probiotic and the diarrhea will stop.” But it does offer a plausible biological reason why some people experience gut symptoms that are not fully explained by the drug’s direct pharmacological action on xanthine oxidase alone.

Practical Steps to Manage Diarrhea on Allopurinol

If you are experiencing diarrhea after starting allopurinol, several practical approaches are worth trying before concluding the drug is not for you.

• Take it with food: Allopurinol is well absorbed regardless of meals, but taking it with food buffers the stomach and can reduce nausea and loose stools. Many people find that taking it right after a meal makes a noticeable difference.
• Stay hydrated: This sounds obvious, but diarrhea plus the increased urination that sometimes accompanies changes in urate metabolism can leave you mildly dehydrated without realizing it. Dehydration itself worsens GI motility problems.
• Watch for dietary triggers: Gout patients are often advised to adjust their diets at the same time they start allopurinol, and some of those dietary changes (more vegetables, more water, less alcohol) can independently affect bowel habits. It is easy to blame the pill when the real culprit is the sudden increase in fiber or the loss of a binding food you used to eat regularly.
• Give it time: For many people, GI side effects are most pronounced in the first two to four weeks and then taper off. If the diarrhea is mild and tolerable, waiting it out is a reasonable strategy, though you should keep your doctor informed.

The gradual dose-escalation approach that most guidelines now recommend for allopurinol also helps. Starting at a low dose and working up slowly gives your gut time to adapt. In an up-titration study, researchers found that gradually increasing the dose allowed the vast majority of patients to reach their target uric acid level at a median dose of 300 mg daily, with minimal side effects along the way.⁶ Starting high and staying high is more likely to provoke GI complaints than inching up over several weeks.

When Diarrhea Is Not Just a Nuisance

Most allopurinol-related diarrhea is mild and self-limiting. But there are situations where GI symptoms warrant prompt medical attention. If diarrhea is severe, bloody, or accompanied by fever, these could signal something beyond a simple drug side effect. Allopurinol can, in rare cases, trigger a hypersensitivity syndrome that involves multiple organ systems, and GI symptoms can be part of that picture. The hallmark of allopurinol hypersensitivity is usually a serious skin reaction, but the early phase can include nausea, diarrhea, and general malaise before the rash appears.¹

People with impaired kidney function are at higher risk for drug accumulation and more intense side effects, because allopurinol’s active metabolite (oxipurinol) is cleared through the kidneys. If your kidney function is reduced, even a standard dose can behave like a higher dose in your body, potentially amplifying GI and other side effects. This is one reason dose adjustments are especially important for people with chronic kidney disease.

As a general rule, if you have been on allopurinol for more than a month and new diarrhea appears (rather than persisting from the start), that is also worth discussing with your doctor. Late-onset GI symptoms are less likely to be simple drug intolerance and more likely to have another cause that happens to be coinciding with the medication.

Could It Be the Colchicine Instead

This is a surprisingly common source of confusion. When people start allopurinol, they are frequently also prescribed colchicine as a short-term measure to prevent gout flares during the early weeks of urate-lowering therapy. Colchicine is notorious for causing diarrhea. In fact, diarrhea is one of its most predictable dose-limiting side effects and occurs at a much higher rate than it does with allopurinol alone.

If you started both drugs around the same time and are now dealing with loose stools, colchicine is a strong suspect. The way to tell is straightforward: if the diarrhea began at the same time as (or shortly after) starting colchicine and improves when the colchicine course ends, that was your answer. Many people attribute their GI problems to the allopurinol because it is the “new” long-term medication, while the colchicine, which was always meant to be temporary, flies under the radar. Your prescribing doctor can help sort this out.

Switching Medications if Diarrhea Persists

If you have given allopurinol a fair trial with gradual dose escalation, taken it with food, ruled out colchicine as the culprit, and the diarrhea is still intolerable after several weeks, switching to a different urate-lowering drug is a reasonable conversation to have with your doctor. The most commonly discussed alternative is febuxostat, which works by the same mechanism (blocking xanthine oxidase) but has a different chemical structure.

The evidence on whether febuxostat is easier on the gut is mixed. Meta-analyses comparing the two drugs at standard doses show no significant difference in the overall rate of adverse reactions.⁷ At higher febuxostat doses (240 mg), diarrhea was actually more frequent with febuxostat than with allopurinol in one large trial.⁸ So switching does not guarantee relief from diarrhea specifically, but individual responses vary enough that some people do find febuxostat more tolerable. The drugs are chemically different enough that an intolerance to one does not predict an intolerance to the other.

Other alternatives exist as well, including uricosuric agents like probenecid and lesinurad, which lower uric acid by a completely different route (increasing its excretion through the kidneys rather than blocking its production). These carry their own side-effect profiles and are not right for everyone, particularly people with kidney stones or significantly impaired kidney function. But they represent an option for the relatively small group of patients who simply cannot tolerate xanthine oxidase inhibitors at any dose.

Side Effects and the Problem of Sticking With Treatment

Gout is a disease that rewards consistency. Uric acid needs to stay below a certain threshold for months to years before existing urate crystal deposits dissolve and flares stop coming. That makes medication adherence critical, and side effects like diarrhea are one of the most commonly cited barriers to sticking with urate-lowering therapy. In qualitative research with gout patients, concerns about side effects ranked alongside cost, forgetfulness, and difficulty swallowing pills as major reasons people skip doses or stop treatment entirely.⁹

The flip side of that same research is revealing: patients who experienced few or no side effects listed that lack of problems as one of the main reasons they were able to stay on their medication. They also cited understanding why they needed to take the drug every day (to prevent flares, keep pain from becoming chronic, and avoid emergency room visits for acute attacks) as a powerful motivator. The takeaway here is that if diarrhea is undermining your willingness to keep taking allopurinol, addressing it actively rather than just enduring it is important for your long-term gout outcomes. Talk to your prescriber about dose adjustments or alternatives rather than quietly abandoning a drug that could be protecting your joints.

People With Inflammatory Bowel Conditions

One group that deserves special mention is people who already have inflammatory bowel disease or other conditions affecting the gut lining. The animal research showing that allopurinol worsens experimental colitis by disrupting purine salvage and impairing intestinal wound healing raises a real concern for anyone with Crohn’s disease, ulcerative colitis, or even less severe but chronic inflammatory gut conditions.⁴ The compromised gut lining in these conditions is already struggling to repair itself, and adding a drug that further hampers that repair process could theoretically worsen symptoms or complicate disease management.

This is not to say people with IBD cannot take allopurinol. Many do, and there are situations where the benefits for joint health clearly outweigh the GI risks. But it does mean that if you have a pre-existing bowel condition and you are starting allopurinol, both your rheumatologist and your gastroenterologist should be in the loop. More careful dose titration, closer monitoring of bowel symptoms, and a lower threshold for switching to an alternative drug are all reasonable precautions in this scenario.

Interestingly, allopurinol is sometimes used in combination with other drugs in the treatment of IBD itself (specifically, it is added to thiopurine therapy to shift drug metabolism in a favorable direction). That is a different clinical context with different doses and goals, and people sometimes confuse the two uses. If your gastroenterologist has prescribed low-dose allopurinol alongside azathioprine or mercaptopurine for IBD, the rationale and risk profile are quite different from taking allopurinol for gout. The doses used in the IBD context are typically lower, and the drug is being used to modify another drug’s metabolism rather than to lower uric acid directly.