Febuxostat Side Effects: Heart Risks, Liver, and Flares

Febuxostat lowers uric acid effectively, but its side-effect profile has drawn more scrutiny than most gout medications, including a boxed warning from the FDA over cardiovascular risk. The most common complaints are mild and predictable: abnormal liver enzyme readings, diarrhea, headache, and joint pain. The serious concerns, though, are what keep physicians and patients cautious. Two large clinical trials reached opposite conclusions about whether febuxostat raises the risk of heart-related death compared to allopurinol, and that unresolved tension still shapes prescribing decisions today.

Common, Everyday Side Effects

Most people who take febuxostat tolerate it well enough. The side effects that show up frequently in clinical trials are the kind that might prompt a phone call to your doctor but rarely lead to stopping the drug. These include abnormal liver function test results, diarrhea, headache, joint pain, and general musculoskeletal symptoms.¹ Nausea and dizziness also appear on prescribing labels, though they tend to be less frequent. None of these side effects are unique to febuxostat; allopurinol, the older and more commonly used xanthine oxidase inhibitor, causes a similar set of minor complaints. For most people, these issues are manageable or fade after the first few weeks of treatment.

The Cardiovascular Controversy

The biggest cloud hanging over febuxostat is its relationship to cardiovascular events, and the story is genuinely complicated. In 2018, the results of the CARES trial, a large FDA-mandated study of over 6,000 gout patients who already had cardiovascular disease, were published. The trial found that patients on febuxostat had a higher rate of cardiovascular death and all-cause death compared to those on allopurinol. The hazard ratio for cardiovascular death was about 1.34, and for death from any cause about 1.22.² Based largely on those results, the FDA added a boxed warning to febuxostat’s label, advising that it be reserved for patients who cannot tolerate or have not responded to allopurinol.³

Then came the FAST trial, a European study of similar scale that followed gout patients in the UK, Denmark, and Sweden. Its conclusion was strikingly different: febuxostat was non-inferior to allopurinol for the primary cardiovascular endpoint, and its long-term use was not linked to a higher risk of death or serious adverse events. The rate of cardiovascular events was actually numerically lower in the febuxostat group, at roughly 1.72 events per 100 patient-years compared to 2.05 for allopurinol.⁴

Why the contradictory results? The CARES trial had a very high dropout rate, with many patients stopping treatment but still being tracked for outcomes, which some researchers argue may have distorted the mortality signal. The FAST trial, by contrast, specifically analyzed outcomes among patients who stayed on their medication, which arguably better reflects what happens in real-world treatment. The CARES participants also had more established cardiovascular disease at baseline, making them a higher-risk group from the start. A population-based cohort study from Taiwan using insurance data from over 44,000 patients per group found that febuxostat users had a higher risk of heart failure hospitalization, atrial fibrillation hospitalization, and cardiovascular death compared to allopurinol users, though it found no difference in heart attacks, strokes, or overall mortality.⁵

In practical terms, this means the cardiovascular signal is real enough to warrant caution, particularly if you already have heart disease. The FDA boxed warning remains in place. But the evidence is not unanimous, and many rheumatologists feel the CARES trial findings have been over-interpreted, especially for patients without significant heart problems.

Liver Injury

Mild elevations in liver enzymes are among the most common side effects reported in trials, and for most patients they are just numbers on a lab report that normalize with continued use or dose adjustment. However, febuxostat can, in rare cases, cause clinically significant liver damage. A case study published in Hepatology documented a patient who developed jaundice while on febuxostat, likely because the drug was continued after liver injury had already begun. The injury presented with a cholestatic pattern, meaning bile flow was disrupted rather than liver cells being directly destroyed. It resolved quickly once febuxostat was stopped.⁶

A more recent cohort study comparing febuxostat to benzbromarone, another urate-lowering drug, found that about 37% of patients had some degree of liver enzyme abnormality. The incidence of hepatotoxicity was roughly 40 per 1,000 patient-years for febuxostat users compared to about 17 per 1,000 patient-years for benzbromarone users, and the risk was especially elevated in patients who already had raised liver enzymes before starting the drug.⁷ The takeaway is straightforward: your doctor will want to check your liver function periodically, and if you start feeling unusually fatigued, develop dark urine, or notice yellowing of the skin, those are reasons to call immediately rather than wait for the next scheduled blood draw.

Skin Reactions and Cross-Reactivity With Allopurinol

Skin reactions to febuxostat are uncommon but worth understanding, particularly if you were switched to febuxostat after having a rash from allopurinol. A nationwide study found that the rate of cutaneous adverse reactions with febuxostat was substantially lower than with allopurinol, at roughly 3.5 per 1,000 patient-years compared to about 15.4 for allopurinol.⁸ Since febuxostat has a completely different chemical structure from allopurinol (it is not a purine-based molecule), cross-reactivity was originally expected to be negligible.⁹

But the reality is more nuanced. Among patients who previously developed a rash from allopurinol and then switched to febuxostat, about 15% also developed a rash with febuxostat. One patient who had experienced Stevens-Johnson syndrome from allopurinol also developed it with febuxostat.¹⁰ A separate case report documented a severe cross-reaction between the two drugs despite their different chemical structures.¹¹ Researchers have concluded that the risk of skin reaction with febuxostat is moderately increased in people with a history of allopurinol rash, though the mechanism does not appear to be traditional cross-reactivity.¹²

In exceptionally rare cases, febuxostat has been linked to DRESS syndrome, a serious drug reaction involving rash, fever, swollen lymph nodes, and damage to internal organs. It requires immediate discontinuation of the drug.¹³

Gout Flares When Starting Treatment

One of the most frustrating side effects of febuxostat has nothing to do with the drug’s toxicity and everything to do with its effectiveness. When uric acid levels drop quickly, crystals already deposited in the joints begin to dissolve, and the resulting immune response can trigger new gout attacks. This mobilization flare is common with any urate-lowering therapy, not just febuxostat, but it catches many patients off guard because they feel worse right when they expect to feel better.

A study of patients starting febuxostat for chronic gout found that prophylactic treatment made a substantial difference. Patients given low-dose colchicine or low-dose steroids during the first six months experienced far fewer flares and reported less severe pain compared to patients who received no prophylaxis. Colchicine performed better than steroids at preventing flares, and both were well tolerated.¹⁴ This is now standard practice: most prescribers will co-prescribe colchicine or an anti-inflammatory for at least the first several months of treatment. If your doctor starts you on febuxostat without mentioning flare prevention, it is worth asking about it.

A related question patients sometimes have is whether it is safe to start febuxostat during an active gout attack. A randomized trial tested this directly, starting febuxostat during an acute flare alongside diclofenac (an anti-inflammatory), and found that beginning the drug early did not prolong the current attack.¹⁵ This challenges the older conventional wisdom that urate-lowering therapy should always be delayed until after a flare has fully resolved.

Drug Interactions to Watch

Febuxostat has a relatively clean interaction profile compared to many chronic medications, but there is one interaction that can be dangerous. Azathioprine and mercaptopurine, two immunosuppressant drugs commonly used in organ transplant recipients and people with autoimmune conditions, are broken down by the same enzyme that febuxostat blocks. When febuxostat slows that breakdown, the immunosuppressant accumulates to toxic levels and can suppress the bone marrow severely. A review of FDA adverse event reports found 19 cases of bone marrow suppression in patients taking febuxostat alongside one of these drugs. Most cases required hospitalization and blood transfusions, and the typical onset was about a month and a half after combining the medications.¹⁶ This is a well-known interaction that carries a formal contraindication, but real-world errors still happen, especially when patients see multiple specialists who may not review each other’s medication lists.¹⁷

Blood Cell Count Changes

One uncommon but potentially serious side effect is a drop in white blood cell counts, specifically the neutrophils that your body relies on to fight bacterial infections. Case reports have documented agranulocytosis, a severe depletion of these cells, occurring anywhere from three days to about three months after starting febuxostat.¹⁸ Early reports linked this reaction to patients with chronic kidney disease in particular.¹⁹ The risk is low enough that routine blood count monitoring is not universally recommended for all patients, but if you develop unexplained fevers, mouth sores, or signs of infection shortly after starting febuxostat, a blood count check is warranted.

Febuxostat and Kidney Disease

Gout and kidney disease frequently coexist, and since the kidneys are the main route for eliminating uric acid, impaired kidney function raises uric acid levels and makes gout harder to control. Febuxostat has an advantage here because it is primarily broken down by the liver rather than excreted by the kidneys, which means it can be used in patients with reduced kidney function without the dose-ceiling problems that limit allopurinol in that population.

A retrospective study found that patients with severely reduced kidney function (stage 4 or 5 chronic kidney disease) needed lower doses of febuxostat to reach their uric acid targets compared to patients with normal kidneys or milder impairment.²⁰ A randomized controlled study comparing low-dose febuxostat to allopurinol in chronic kidney disease patients found that febuxostat was better at slowing the decline in kidney function, particularly at stage 3 disease, with no difference in drug-related side effects between the two groups.²¹ This kidney-protective signal has generated considerable interest and is one of the reasons febuxostat remains a valued option despite the cardiovascular concerns.

Older Adults

Since gout becomes more common with age, many febuxostat users are elderly. The limited trial data available suggests that febuxostat does not cause dramatically different or more frequent side effects in older adults compared to younger patients. A study specifically examining elderly women found no significant difference in adverse events compared to younger participants.²² A pooled analysis of older trial data in patients aged 65 and above also showed comparable adverse event rates across treatment groups.²³

However, an analysis of the FDA’s own adverse event reporting database painted a more cautious picture, finding that elderly gout patients on febuxostat had higher reported rates of heart failure, ischemic heart disease, hypertension, and cardiomyopathy.²⁴ Adverse event reporting databases are tricky to interpret because they capture reports without confirming cause and effect, and they are subject to reporting bias, especially after a safety warning draws attention to a drug. Still, the signal fits with the broader cardiovascular concern and suggests older adults with heart disease deserve especially careful monitoring.

Sticking With the Treatment

Side effects do not just matter for their direct health impact; they also affect whether people keep taking the drug. A study following 220 male gout patients on febuxostat in routine practice found that adherence dropped steadily over time. About 72% were still adherent at one year, roughly 66% at two years, and around 58% at three years.²⁵ These numbers are not terrible by chronic-medication standards, but they mean that by three years, nearly half of patients had either stopped or become inconsistent. Some of that dropout is driven by side effects, but much of it reflects the broader challenge of taking daily medication for a condition that only flares intermittently. Stopping urate-lowering therapy typically causes uric acid levels to rebound, which eventually triggers new flares and continued joint damage.

Use in Tumor Lysis Syndrome

Outside gout, febuxostat is sometimes used to prevent tumor lysis syndrome, a dangerous buildup of uric acid that can occur when cancer cells are rapidly destroyed by chemotherapy. A phase III trial comparing febuxostat to allopurinol for this purpose found no difference in safety outcomes between the two drugs.²⁶ A meta-analysis of multiple studies confirmed that liver enzyme elevation was the most frequently reported adverse effect in this setting, but the rate was similar between febuxostat and allopurinol.²⁷ In cancer patients, the short duration of use likely limits the accumulation of risk seen with chronic therapy for gout.

Cost and the Side-Effect Calculation

Febuxostat is more expensive than allopurinol, and the cost gap influences how doctors and patients think about tolerating side effects. In the general gout population, economic analyses have found that febuxostat’s higher drug price is partially offset by its greater efficacy at reaching uric acid targets, which translates into fewer flares and fewer downstream complications. One analysis estimated it cost roughly $1,773 per flare avoided over five years. For patients with moderate-to-severe chronic kidney disease, febuxostat actually dominated allopurinol economically, meaning it delivered better outcomes at a lower total cost when kidney-related complications were factored in.²⁸ A systematic review of cost-effectiveness studies concluded that febuxostat is highly cost-effective as a second-line treatment for patients who fail or cannot tolerate allopurinol.²⁹ This framing matters because the FDA’s boxed warning can make febuxostat feel like a last resort, when in practice it may be the better economic and clinical choice for certain patients, particularly those with kidney complications or a documented inability to tolerate allopurinol.