Allopurinol

Allopurinol works by inhibiting xanthine oxidase, the enzyme responsible for the final two steps of purine catabolism in the human body ^[1][2].

Normally, purines (from dietary sources and cellular turnover) are broken down through a series of steps: hypoxanthine is oxidized to xanthine, and xanthine is further oxidized to uric acid. Xanthine oxidase catalyzes both of these reactions ^[2]. In patients with gout, excess uric acid (hyperuricemia) leads to the deposition of monosodium urate (MSU) crystals in joints and soft tissues, triggering the intensely painful inflammatory response characteristic of gout ^[3].

Allopurinol and its active metabolite oxypurinol bind to and inhibit xanthine oxidase, blocking both conversion steps ^[1]. This reduces the production of uric acid and lowers serum urate levels. The accumulated hypoxanthine and xanthine are more soluble than uric acid and are readily excreted by the kidneys without forming crystals ^[2].

Over time (typically 3-6 months), sustained reduction of serum urate below the saturation point of 6.8 mg/dL allows existing urate crystal deposits in joints to gradually dissolve, reducing the frequency and severity of gout flares ^[3][4]. The ACR target for most patients is a serum urate level below 6 mg/dL ^[4].

Starting allopurinol requires a "start low, go slow" approach. Your doctor will likely begin with 100 mg daily (or even 50 mg if you have kidney problems) and increase the dose by 100 mg every 2-4 weeks until your uric acid level reaches the target of <6 mg/dL ^[4].

Critically, starting or changing the dose of allopurinol can paradoxically trigger gout flares during the first 3-6 months. This happens because changing urate levels destabilizes existing crystal deposits. Your doctor will prescribe a flare prophylaxis medication (usually low-dose colchicine or an NSAID) for at least 3-6 months during initiation ^[4].

Do not stop allopurinol during a flare. If a gout attack occurs, treat the flare while continuing allopurinol at the current dose ^[4].

Expect regular uric acid blood tests every 2-4 weeks during dose titration and every 6 months once stable. The goal is to get your uric acid below 6 mg/dL and keep it there ^[4].

Be alert for skin rash — particularly during the first few months. While most rashes are benign, report any rash immediately as it could be an early sign of a severe hypersensitivity reaction (DRESS syndrome) ^[5].

Food: Allopurinol can be taken with or without food. Taking after meals may reduce GI upset in sensitive patients ^[1]. Food does not significantly affect absorption.

Alcohol: Alcohol should be limited during allopurinol therapy ^[4]. Beer is particularly problematic for gout patients because it contains purines (from yeast) and alcohol itself impairs renal urate excretion. Wine in moderation appears to have less effect on uric acid levels. Spirits have an intermediate effect.

Purine-Rich Foods: While allopurinol allows more dietary flexibility than diet alone, patients should still moderate intake of high-purine foods (organ meats, shellfish, red meat, anchovies, sardines) ^[4]. Allopurinol is not a substitute for dietary management but rather a complement to it.

Hydration: Adequate fluid intake (at least 2 liters daily) is important to promote uric acid excretion and reduce the risk of urate kidney stones during the early treatment period ^[1].

Vitamin C: Vitamin C (500 mg/day) may modestly lower uric acid by 0.5 mg/dL and can be used adjunctively ^[4].

Gout (standard approach): Start at 100 mg daily (50 mg daily if CrCl <30 mL/min). Increase by 100 mg every 2-4 weeks until target serum urate <6 mg/dL is achieved ^[4]. Many patients require 300-400 mg/day; some require up to 800 mg/day.

Maintenance: Once target urate is achieved, continue at that dose indefinitely. Allopurinol is a lifelong therapy — stopping it allows uric acid to rise and gout flares to recur ^[4].

Flare Prophylaxis: Continue colchicine 0.6 mg daily or twice daily (or low-dose NSAID) for at least 3-6 months after achieving target urate, to prevent initiation flares ^[4].

Urate Kidney Stone Prevention: 200-300 mg/day for patients with uric acid stones and hyperuricosuria ^[1].

Renal Impairment: Traditional guidelines (Hande's) recommended fixed-dose caps based on CrCl. Current ACR guidelines allow titration to target urate regardless of renal function, with monitoring ^[4]. Start lower (50 mg/day) and titrate more slowly.

Maximum Dose: 800 mg/day (FDA-approved). Most patients respond to 300-600 mg/day.

**HLA-B*5801 Testing: Recommended before starting in patients of Southeast Asian descent (prevalence 6-8%), African American descent (3-4%), and Korean** descent (12%) ^[4].

Generic allopurinol is one of the most affordable medications available. A 30-day supply of 300 mg tablets costs approximately $4-10 at most pharmacies ^[6].

Cost-saving strategies: - Generic is universal: Brand-name Zyloprim is rarely stocked. Generic allopurinol has been available for decades and is essentially a commodity drug - $4 generic lists: Allopurinol 100 mg and 300 mg are included on virtually all pharmacy discount programs - Once-daily dosing: For doses up to 300 mg, once-daily dosing is standard and convenient. For higher doses, splitting into twice-daily administration is recommended but increases pill count - GoodRx coupons: Can reduce costs to $3-6 for a 30-day supply - Avoid febuxostat unless necessary: Febuxostat (Uloric) is an alternative XOI that costs significantly more ($300+ brand, $30-60 generic). It should be reserved for allopurinol-intolerant patients - Blood test costs: Factor in the cost of periodic uric acid and CBC monitoring during dose titration

Pregnancy (Category C): There is limited data on allopurinol use in human pregnancy ^[1]. Animal studies at high doses have shown fetal abnormalities. Allopurinol is generally not recommended during pregnancy unless the benefits clearly outweigh the risks. Gout is uncommon in premenopausal women, so this rarely arises.

Breastfeeding: Allopurinol and oxypurinol are excreted in breast milk. The American Academy of Pediatrics considers it compatible with breastfeeding, as the amounts excreted are small ^[7]. However, some experts advise monitoring the infant for rash or other adverse effects.

Fertility: No significant effects on fertility have been documented in humans at therapeutic doses ^[1].

Generic allopurinol has been available for decades and is manufactured by numerous companies. Brand-name Zyloprim is rarely prescribed or stocked at retail pharmacies. All generic versions are FDA "AB" rated as therapeutically equivalent.

Allopurinol tablets are available in 100 mg and 300 mg strengths. The 300 mg tablet is the most commonly dispensed.

There is no clinical reason to seek brand-name Zyloprim. The IV formulation (Aloprim) is available for patients who cannot take oral medication, typically in hospital settings.