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Medical Disclaimer: This information is for educational purposes only and is not a substitute for professional medical advice.
Xanthine Oxidase Inhibitor [EPC]
Allopurinol is a potent xanthine oxidase inhibitor used primarily to manage chronic gout, prevent calcium oxalate kidney stones, and treat hyperuricemia associated with cancer therapy. It works by lowering the production of uric acid in the body.
Name
Allopurinol
Raw Name
ALLOPURINOL
Category
Xanthine Oxidase Inhibitor [EPC]
Drug Count
3
Variant Count
159
Last Verified
February 17, 2026
RxCUI
197319, 197320, 245422, 252931, 261265
UNII
63CZ7GJN5I, 428673RC2Z
About Allopurinol
Allopurinol is a potent xanthine oxidase inhibitor used primarily to manage chronic gout, prevent calcium oxalate kidney stones, and treat hyperuricemia associated with cancer therapy. It works by lowering the production of uric acid in the body.
Detailed information about Allopurinol
This page is for informational purposes only and does not replace medical advice. Consult a qualified healthcare professional before using any medication containing Allopurinol.
Allopurinol is a pharmacological agent belonging to the class of medications known as xanthine oxidase inhibitors [EPC]. It is primarily utilized to manage conditions associated with elevated levels of uric acid in the body (hyperuricemia). Uric acid is a waste product formed during the breakdown of purines, which are natural substances found in the body and in certain foods. When uric acid levels become too high, it can lead to the formation of sharp, needle-like crystals in the joints (gout) or the kidneys (renal calculi or kidney stones). Allopurinol was first approved by the U.S. Food and Drug Administration (FDA) in 1966 and has since become a cornerstone in the long-term management of chronic gout and other hyperuricemic states.
Unlike medications used to treat acute gout attacks, such as colchicine or nonsteroidal anti-inflammatory drugs (NSAIDs), Allopurinol is a preventive medication. It does not provide immediate pain relief during an active flare-up. In fact, starting Allopurinol during an acute attack can sometimes worsen the symptoms. Instead, it is prescribed for long-term use to lower the overall 'urate burden' in the body, eventually reducing the frequency and severity of gout flares over months and years. Your healthcare provider will typically wait until an acute attack has fully resolved before initiating this therapy.
To understand how Allopurinol works, it is necessary to look at the purine metabolic pathway. Purines (adenine and guanine) are broken down into hypoxanthine, then into xanthine, and finally into uric acid. The enzyme responsible for the last two steps of this process is called xanthine oxidase. Allopurinol is a structural analog (a molecule with a similar shape) of hypoxanthine. Because of this similarity, it acts as a 'decoy' for the xanthine oxidase enzyme.
At the molecular level, Allopurinol binds to and inhibits xanthine oxidase. Furthermore, Allopurinol is metabolized by the same enzyme into its active metabolite, oxypurinol (also known as alloxanthine). Oxypurinol is an even more potent inhibitor of xanthine oxidase and remains in the body much longer than the parent drug. By blocking this enzyme, Allopurinol effectively reduces the production of uric acid. This leads to a decrease in serum (blood) urate levels and a decrease in the amount of uric acid excreted in the urine. Over time, the lower levels of uric acid in the blood allow existing urate crystals in the joints and tissues to slowly dissolve, preventing future inflammatory responses.
Understanding how the body processes Allopurinol is essential for optimizing its therapeutic effect and minimizing toxicity.
Allopurinol is FDA-approved for several specific clinical indications:
Off-label uses, which are not FDA-approved but may be considered by specialists, include the management of hyperuricemia in certain types of kidney disease or as an adjunct therapy in specific inflammatory bowel disease cases (to modulate the metabolism of other drugs like azathioprine).
Allopurinol is most commonly available in the following forms:
> Important: Only your healthcare provider can determine if Allopurinol is right for your specific condition. The choice of dose and form depends on your uric acid levels, kidney function, and the specific condition being treated.
The dosage of Allopurinol is highly individualized. Healthcare providers typically start with a low dose and gradually increase it (titration) until the target serum uric acid level (usually below 6 mg/dL) is achieved. This 'start low, go slow' approach helps reduce the risk of triggering a gout flare when treatment begins.
Allopurinol is primarily used in children for the management of hyperuricemia secondary to cancer or certain rare genetic enzyme deficiencies (such as Lesch-Nyhan syndrome).
Pediatric use for gout is extremely rare, as gout is primarily an adult-onset condition. All pediatric dosing must be strictly supervised by a specialist.
Because the active metabolite oxypurinol is cleared by the kidneys, patients with kidney disease are at a significantly higher risk of drug toxicity. Healthcare providers use the Creatinine Clearance (CrCl) rate to adjust the dose:
Patients with liver disease should be monitored closely. While specific dose adjustment formulas for liver disease are less standardized than for renal disease, periodic liver function tests (LFTs) are mandatory, as Allopurinol can rarely cause hepatotoxicity (liver damage).
Older adults often have naturally declining kidney function. Therefore, healthcare providers usually start elderly patients at the lowest possible dose (e.g., 50 mg to 100 mg) and monitor renal function frequently.
If you miss a dose, take it as soon as you remember. However, if it is almost time for your next scheduled dose, skip the missed dose and return to your regular schedule. Do not 'double up' or take two doses at once to make up for a missed one, as this increases the risk of side effects.
Signs of an Allopurinol overdose may include severe nausea, vomiting, diarrhea, or dizziness. In extreme cases, it could lead to acute kidney injury. If an overdose is suspected, contact a poison control center or seek emergency medical attention immediately. Treatment is generally supportive, focusing on hydration and monitoring kidney function.
> Important: Follow your healthcare provider's dosing instructions exactly. Do not adjust your dose or stop taking the medication without medical guidance, even if you feel better, as stopping can trigger a severe gout flare.
The most frequently reported side effect when starting Allopurinol is an increase in gout flares. This occurs because as the blood uric acid level drops, urate crystals stored in the joints begin to dissolve and move, which can trigger an inflammatory response. To prevent this, doctors often prescribe a 'prophylactic' (preventative) dose of colchicine or an NSAID for the first 3 to 6 months of Allopurinol therapy.
Other common side effects include:
> Warning: Stop taking Allopurinol and call your doctor immediately or seek emergency care if you experience any of the following:
With prolonged use, Allopurinol is generally well-tolerated if the dose is correct. However, long-term monitoring is required for:
There are currently no FDA black box warnings for Allopurinol. However, the risk of severe hypersensitivity reactions is considered a 'major warning' in the official prescribing information and is treated with the same clinical gravity as a black box warning.
Report any unusual symptoms or changes in your health to your healthcare provider promptly. Early detection of side effects is key to safe management.
Allopurinol is a powerful metabolic modifier. The most critical safety point is the risk of severe cutaneous adverse reactions (SCARs). These reactions can be fatal. If you develop any skin rash, regardless of how minor it looks, you must stop the medication and contact your healthcare provider immediately. Treatment should not be restarted in patients who have had a severe reaction.
No FDA black box warnings for Allopurinol. However, the FDA-approved labeling contains prominent warnings regarding hypersensitivity and the need for immediate discontinuation at the first sign of a rash.
Patients taking Allopurinol require regular laboratory monitoring, especially during the first year of therapy:
Allopurinol may cause somnolence (drowsiness) or dizziness in some patients. You should determine how you react to the medication before driving, operating heavy machinery, or engaging in tasks that require full mental alertness.
Alcohol consumption, particularly beer and hard liquor, increases the production of uric acid and interferes with its excretion. Drinking alcohol can trigger gout flares and counteract the benefits of Allopurinol. It is generally recommended to limit or avoid alcohol while managing gout.
Allopurinol is typically a lifelong medication for chronic gout. If you stop taking it, your uric acid levels will likely return to their previous high levels within a few days or weeks, leading to a recurrence of gout flares and the potential for joint damage. There is no 'withdrawal syndrome,' but the underlying condition will worsen. Always consult your doctor before stopping the medication.
> Important: Discuss all your medical conditions, especially kidney or liver disease, with your healthcare provider before starting Allopurinol.
Allopurinol is intended to change lab results (specifically lowering serum uric acid). However, it does not typically interfere with the chemical processes of other common lab tests, though it may occasionally cause a false elevation in certain liver or kidney markers if toxicity occurs.
> Important: Tell your doctor about ALL medications, supplements, and herbal products you are taking. A complete medication review is the best way to prevent dangerous interactions.
Allopurinol must NEVER be used in the following circumstances:
In these cases, the benefits must be carefully weighed against the risks:
There is no significant evidence of cross-sensitivity between Allopurinol and other common drug classes like sulfonamides or penicillins. However, patients who are sensitive to oxypurinol (the metabolite) will naturally be sensitive to Allopurinol. Additionally, patients who have had skin reactions to other medications should be monitored with extra caution.
> Important: Your healthcare provider will evaluate your complete medical history, including any previous drug allergies, before prescribing Allopurinol.
Allopurinol is classified as FDA Pregnancy Category C. This means that animal studies have shown some potential for harm to the fetus, but there are no adequate, well-controlled studies in humans. It should be used during pregnancy only if the potential benefit justifies the potential risk to the fetus. Most experts recommend avoiding Allopurinol during the first trimester unless it is absolutely essential (e.g., for managing hyperuricemia during cancer treatment). It is not typically used in fertility treatments.
Allopurinol and its active metabolite, oxypurinol, are excreted in human breast milk. While there are limited reports of adverse effects on nursing infants, the potential for hypersensitivity reactions in the baby exists. A decision should be made whether to discontinue nursing or discontinue the drug, taking into account the importance of the drug to the mother. If breastfeeding while taking Allopurinol, monitor the infant for any signs of a rash or unusual symptoms.
Allopurinol is approved for use in children primarily for hyperuricemia caused by cancer or rare genetic metabolic disorders (like Lesch-Nyhan syndrome). It is NOT typically approved or recommended for the treatment of gout in children, as gout is extremely rare in this population. Long-term effects on growth and development have not been extensively studied, though it has been used safely for decades in pediatric oncology.
Elderly patients (65 and older) are at an increased risk of side effects, primarily because kidney function naturally declines with age.
This is the most critical special population for Allopurinol. Patients with a Creatinine Clearance (CrCl) of less than 20 mL/min require significant dose reductions (often 100 mg/day or less). In patients on chronic hemodialysis, Allopurinol or its metabolites are removed during the session. Dosing may be timed to occur after dialysis, or supplemental doses may be required, depending on the patient's urate levels.
There are no specific 'Child-Pugh' based dose adjustments for Allopurinol, but the drug can be hepatotoxic. Patients with existing liver cirrhosis or hepatitis should have their liver enzymes (ALT, AST) monitored every few weeks during the initial phase of treatment. If liver function worsens, the drug should be discontinued.
> Important: Special populations require individualized medical assessment and more frequent laboratory monitoring.
Allopurinol is a potent inhibitor of xanthine oxidase, the enzyme responsible for the oxidation of hypoxanthine to xanthine and xanthine to uric acid. Allopurinol is a purine analog; it is a structural isomer of hypoxanthine (the positions of the carbon and nitrogen atoms at 7 and 8 are interchanged).
Upon administration, Allopurinol is rapidly converted by xanthine oxidase to its primary metabolite, oxypurinol. While Allopurinol itself is a competitive inhibitor of the enzyme, oxypurinol acts as a non-competitive 'suicide' inhibitor. It binds tightly to the reduced molybdenum atom in the active site of the enzyme, effectively 'locking' it and preventing further catalytic activity. This dual action results in a significant decrease in the production of uric acid, leading to a rise in the concentrations of the more soluble precursors, hypoxanthine and xanthine, which are then easily excreted by the kidneys.
| Parameter | Value |
|---|---|
| Bioavailability | 67% - 90% |
| Protein Binding | < 1% (Negligible) |
| Half-life (Parent) | 1 - 2 hours |
| Half-life (Metabolite) | 15 - 30 hours (up to 125h in renal failure) |
| Tmax | 1.5 - 2 hours |
| Metabolism | Hepatic/Enzymatic (to Oxypurinol) |
| Excretion | Renal 80%, Fecal 20% |
Allopurinol is the prototypical Xanthine Oxidase Inhibitor. Other drugs in this class include Febuxostat (Uloric). While Febuxostat is a newer, non-purine inhibitor, Allopurinol remains the first-line therapy for most patients due to its long track record of safety and cost-effectiveness.
Medications containing this ingredient
Common questions about Allopurinol
Allopurinol is primarily used to prevent gout attacks by lowering high levels of uric acid in the blood. It is also prescribed to prevent certain types of kidney stones, specifically calcium oxalate stones, in patients with high urinary uric acid. Additionally, it is used in cancer patients to prevent a dangerous buildup of uric acid that can occur during chemotherapy. It is important to note that Allopurinol is a preventive medication and is not used to treat the pain of an active gout flare. Your doctor will determine the appropriate use based on your specific medical history and lab results.
The most common side effect is an initial increase in gout flares when first starting the medication, which occurs as the body adjusts to lower uric acid levels. Other frequent side effects include stomach upset, nausea, and diarrhea, which can often be managed by taking the tablet after a meal. Drowsiness and headaches are also reported by some patients during the first few weeks of therapy. However, the most critical side effect to watch for is a skin rash, which can be a sign of a severe allergic reaction. Always report any new skin changes or persistent symptoms to your healthcare provider immediately.
While there is no direct chemical interaction between alcohol and Allopurinol, drinking alcohol is generally discouraged for patients with gout. Alcohol, especially beer and spirits, increases the production of uric acid and can trigger painful gout flares, essentially working against the benefits of the medication. Furthermore, heavy alcohol use can put additional stress on the liver, which is also a potential site of side effects for Allopurinol. Most healthcare providers recommend limiting alcohol intake significantly or avoiding it entirely to achieve the best results in managing your condition. Always discuss your lifestyle and alcohol consumption habits with your doctor.
Allopurinol is classified as Pregnancy Category C, meaning its safety in human pregnancy has not been fully established. Animal studies have suggested potential risks, but there is insufficient data from well-controlled human trials to confirm its safety for a developing fetus. Consequently, it is usually only prescribed during pregnancy if the benefits to the mother clearly outweigh the potential risks to the baby. If you are planning to become pregnant or find out you are pregnant while taking Allopurinol, you should contact your doctor immediately. They will help you weigh the risks of your condition against the risks of the medication.
Allopurinol begins to lower blood uric acid levels within 24 to 48 hours of the first dose, but it takes much longer to see the full clinical benefits. It may take 1 to 3 weeks of consistent use for your uric acid levels to reach their lowest point and stabilize. Furthermore, it can take several months—sometimes up to six months or longer—of continuous treatment before you notice a significant reduction in the frequency of gout attacks. This is because it takes time for the existing uric acid crystals in your joints to slowly dissolve. Patience and consistency are key to the success of this treatment.
You should not stop taking Allopurinol suddenly without consulting your healthcare provider, even if you feel better or haven't had a gout flare in a long time. Allopurinol is typically a long-term, often lifelong, medication used to manage a chronic metabolic condition. If you stop taking it, your uric acid levels will likely rise back to their original levels within a few days, putting you at high risk for a severe return of gout attacks or kidney stones. If you are experiencing side effects that make you want to stop, talk to your doctor about adjusting the dose or switching to an alternative rather than quitting abruptly.
If you miss a dose of Allopurinol, take it as soon as you remember, provided it is not almost time for your next scheduled dose. If your next dose is only a few hours away, skip the missed dose and continue with your regular schedule. Never take two doses at the same time to 'catch up,' as this can increase the risk of side effects like stomach upset or more serious reactions. To help remember your medication, try taking it at the same time every day, such as right after breakfast. If you frequently miss doses, consider using a pill organizer or a reminder app on your phone.
Weight gain is not a commonly reported side effect of Allopurinol in clinical trials or post-marketing surveillance. If you experience rapid or unusual weight gain while taking this medication, it is more likely related to other factors, such as changes in diet, physical activity, or other underlying health conditions. In some cases, swelling (edema) can occur as part of a rare allergic reaction, which might feel like weight gain, but this would usually be accompanied by a rash or other symptoms. If you are concerned about your weight, discuss it with your healthcare provider to identify the true cause and develop an appropriate management plan.
Allopurinol can interact with several other medications, some of which are very serious. For example, it can dangerously increase the levels of azathioprine and 6-mercaptopurine, requiring a significant dose reduction of those drugs. It may also interact with certain blood pressure medications (ACE inhibitors), water pills (diuretics), and blood thinners like warfarin. Because of these potential interactions, it is vital that you provide your doctor and pharmacist with a complete list of all prescription drugs, over-the-counter medicines, vitamins, and herbal supplements you are taking. They will monitor you for interactions and adjust your treatment plan as necessary to ensure your safety.
Yes, Allopurinol is widely available as a generic medication and is generally very affordable. Generic versions are required by the FDA to have the same active ingredient, strength, dosage form, and route of administration as the brand-name version (formerly known as Zyloprim). Because it has been off-patent for many decades, it is one of the most cost-effective treatments for chronic gout management. Most insurance plans cover generic Allopurinol with a low co-pay. If you have concerns about the cost of your medication, ask your pharmacist about the generic options available to you.