Zyloric

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Zyloric on Prescriptsy

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Zyloric is a brand of allopurinol, a xanthine oxidase inhibitor manufactured by Aspen.

It is prescribed for the long-term management of gout and hyperuricaemia, reducing the production of uric acid (urate) to prevent recurrent gout flares, tophaceous deposits, joint damage, and urate nephropathy.

Zyloric is available as 100 mg and 300 mg tablets and is the most widely prescribed urate-lowering therapy in the UK.

It is a prescription-only medicine (POM) in the United Kingdom.

Gout is the most common inflammatory arthritis in the UK, affecting approximately 2.5% of adults (around 1.5 million people), with a rising prevalence linked to ageing, obesity, alcohol consumption, and increasing use of medications that raise serum urate (such as thiazide diuretics and low-dose aspirin).

Gout is caused by the deposition of monosodium urate crystals in joints and soft tissues when serum urate concentrations exceed the saturation point (approximately 360 micromol/L at body temperature).

Despite being eminently treatable and preventable with effective urate-lowering therapy, gout remains poorly managed in UK primary care, with studies showing that fewer than 50% of patients on allopurinol achieve target serum urate levels.

This page provides a comprehensive clinical guide to Zyloric, covering its mechanism of action, dosage, side effects, safety warnings, and how to obtain it in the United Kingdom.

Important safety information about Zyloric

Before reading further, note these essential safety points about Zyloric.

• Do not start Zyloric during an acute gout flare. Wait until the attack has fully resolved.
• Start at 100 mg daily and increase gradually. Never start at a high dose.
• Colchicine or NSAID prophylaxis must be co-prescribed for at least 6 months to prevent mobilisation flares.
• Stop Zyloric immediately if any skin rash develops and seek medical advice.
• Dose reduction is mandatory in renal impairment to reduce the risk of serious hypersensitivity reactions.

Understanding gout and hyperuricaemia

Gout results from the crystallisation of monosodium urate (MSU) in joints and periarticular tissues when serum urate levels exceed the physiological saturation threshold (approximately 360 micromol/L or 6 mg/dL at 37 degrees Celsius).

Uric acid is the end product of purine metabolism in humans.

Purines are derived from dietary intake (particularly from red meat, offal, game, shellfish, and beer) and from endogenous turnover of nucleic acids (DNA and RNA).

Most mammals possess the enzyme uricase, which converts uric acid to the more soluble allantoin, but humans lost functional uricase through evolutionary mutations approximately 15 million years ago, leaving them susceptible to hyperuricaemia and gout.

Approximately two-thirds of daily uric acid excretion occurs via the kidneys (through a complex process of filtration, reabsorption, and secretion in the proximal tubule) and one-third via the gastrointestinal tract.

Hyperuricaemia develops when uric acid production exceeds excretion. Under-excretion (renal inefficiency) accounts for approximately 90% of primary hyperuricaemia, while overproduction accounts for approximately 10%.

Risk factors for gout include male sex (gout is 3 to 4 times more common in men, with women protected by the uricosuric effect of oestrogen until menopause), advancing age, genetic factors (particularly polymorphisms in urate transporter genes such as SLC2A9, ABCG2, and SLC22A12), obesity, metabolic syndrome, alcohol consumption (particularly beer, which is rich in guanosine, a purine nucleoside), high-purine diet, renal impairment, and certain medications (thiazide and loop diuretics, low-dose aspirin, ciclosporin, pyrazinamide).

Acute gouty arthritis is triggered when MSU crystals activate the NLRP3 inflammasome in tissue macrophages, leading to production of interleukin-1 beta and a powerful inflammatory cascade.

The typical presentation is sudden onset of severe joint pain, swelling, redness, and tenderness, most commonly affecting the first metatarsophalangeal joint (podagra, occurring in approximately 60% of first attacks).

Untreated gout progresses through 4 clinical stages: asymptomatic hyperuricaemia, acute intermittent gout (recurrent flares with symptom-free intervals), intercritical gout (clinically silent crystal deposition between flares), and chronic tophaceous gout (large crystal aggregates called tophi in joints, subcutaneous tissue, and other sites, causing chronic pain, joint destruction, and disability).

How Zyloric works

Allopurinol is a structural isomer of hypoxanthine (a naturally occurring purine base).

It inhibits xanthine oxidase, the enzyme catalysing the final 2 steps of uric acid biosynthesis: the oxidation of hypoxanthine to xanthine, and the oxidation of xanthine to uric acid.

By blocking this enzyme, allopurinol diverts purine catabolism away from uric acid production.

The accumulated substrates (hypoxanthine and xanthine) are more water-soluble than uric acid and are more readily excreted by the kidneys without crystallising.

Allopurinol itself has a short plasma half-life of 1 to 2 hours, but its primary active metabolite, oxypurinol (alloxanthine), has a half-life of 14 to 30 hours and is the principal mediator of sustained xanthine oxidase inhibition.

Oxypurinol binds tightly to the reduced form of xanthine oxidase and is released only slowly.

At therapeutic doses, allopurinol reduces serum urate by 25 to 50%, with the degree of reduction proportional to the dose.

The target serum urate (below 360 micromol/L for most patients, below 300 micromol/L for those with tophi) guides dose titration.

Allopurinol is well absorbed orally (approximately 80% bioavailability) and can be taken with food. It is metabolised primarily by xanthine oxidase and aldehyde oxidase in the liver.

Oxypurinol is excreted predominantly by the kidneys, which is why dose reduction is required in renal impairment (to avoid oxypurinol accumulation and the associated risk of hypersensitivity).

Clinical evidence for Zyloric in gout management

Allopurinol has been the first-line urate-lowering therapy for gout for over 50 years.

The British Society for Rheumatology (BSR) guideline on gout management (2017) recommends allopurinol as first-line urate-lowering therapy for all patients with a diagnosis of gout and an indication for treatment (2 or more flares per year, tophi, joint damage, renal stones, or renal impairment).

The treat-to-target approach, titrating the allopurinol dose to achieve and maintain serum urate below 360 micromol/L (or below 300 micromol/L with tophi), is strongly recommended.

The ALL HEART trial (2022, published in The Lancet), a large UK-based randomised controlled trial of allopurinol versus placebo in patients with ischaemic heart disease (but without gout), did not demonstrate cardiovascular benefit from allopurinol in this population.

However, this does not affect the established role of allopurinol for gout and symptomatic hyperuricaemia.

NICE guideline NG219 on gout (2022) reinforces the treat-to-target strategy and recommends discussing lifestyle modifications alongside urate-lowering therapy.

Dosage and administration

Start at 100 mg once daily (50 mg daily in severe renal impairment).

Increase by 100 mg every 2 to 4 weeks, guided by serum urate levels checked 2 to 4 weeks after each dose change.

Target: serum urate below 360 micromol/L (or below 300 micromol/L for tophaceous gout). Usual maintenance: 200 to 600 mg daily.

Maximum: 900 mg daily (doses above 300 mg in divided doses). Co-prescribe colchicine 0.5 mg once or twice daily for at least 6 months as flare prophylaxis.

Do not start during an acute flare. Take after food. Drink at least 2 litres of fluid daily.

Side effects of Zyloric

Common side effects

Skin rash (2 to 5%), nausea, and altered liver function tests. Mobilisation flares during dose titration are expected and managed with prophylaxis.

Rare but serious side effects

Allopurinol hypersensitivity syndrome (0.1 to 0.4% incidence, 20 to 25% mortality): severe skin reactions (SJS/TEN), fever, eosinophilia, hepatitis, renal failure.

Risk factors include renal impairment, high starting dose, and HLA-B*5801 positivity (screen in high-risk ethnic groups). Stop allopurinol immediately at the first sign of rash.

Very rare: bone marrow suppression, vasculitis, peripheral neuropathy. Report suspected adverse reactions to the MHRA at yellowcard.mhra.gov.uk .

Warnings and precautions

Do not start during an acute flare. Mandatory dose reduction in renal impairment. HLA-B*5801 testing recommended for Han Chinese, Thai, Korean, and African American patients.

Stop immediately if rash develops. Critical drug interactions: azathioprine and mercaptopurine (reduce dose to one-quarter), warfarin (monitor INR), amoxicillin/ampicillin (increased rash risk).

Co-prescribe flare prophylaxis for at least 6 months. Pregnancy: avoid. Breastfeeding: not recommended.

Monitoring during allopurinol therapy

Regular monitoring is essential for effective and safe allopurinol therapy.

Serum urate should be checked 2 to 4 weeks after each dose change during the titration phase, then every 6 to 12 months once the target level is achieved and the dose is stable.

The target serum urate is below 360 micromol/L (6 mg/dL) for most patients, or below 300 micromol/L (5 mg/dL) for patients with tophi, chronic arthropathy, or frequent flares.

Achieving and maintaining target urate levels dissolves existing crystal deposits over 12 to 24 months (a process called debulking) and prevents new crystal formation.

Renal function (eGFR, creatinine) and liver function tests should be checked before starting treatment and periodically during treatment (at least annually, more frequently in patients with renal impairment or those on high doses).

A full blood count should be checked if symptoms suggest bone marrow toxicity (unexplained bruising, sore throat, fever).

Alternative urate-lowering therapies

Febuxostat (Adenuric) is a non-purine selective xanthine oxidase inhibitor that may be used in patients who are intolerant of allopurinol or in whom allopurinol is contraindicated (for example, HLA-B*5801-positive patients).

Febuxostat does not require dose adjustment in mild to moderate renal impairment. NICE TA164 recommends febuxostat for patients in whom allopurinol is not appropriate.

Uricosuric agents such as sulfinpyrazone increase renal uric acid excretion and may be used as second-line alternatives or as add-on therapy if allopurinol alone does not achieve target urate levels.

Lesinurad (in combination with a xanthine oxidase inhibitor) was previously available but has been withdrawn from the UK market.

Pegloticase (a recombinant uricase enzyme that converts uric acid to allantoin) is available for refractory gout through specialist centres but is administered intravenously and is reserved for patients who have failed or cannot tolerate other urate-lowering therapies.

How to get Zyloric in the UK

Zyloric is available on NHS prescription from your GP or an authorised online prescriber registered with the General Pharmaceutical Council (GPhC).

Most prescriptions are dispensed as generic allopurinol, which is bioequivalent and widely available.

Serum urate monitoring is essential for dose titration, typically performed via blood tests at your GP surgery every 2 to 4 weeks during titration and then annually once the target is achieved.

The standard NHS prescription charge in England is 9.90 pounds per item; prescriptions are free in Scotland, Wales, and Northern Ireland.

Sources

• Zyloric 300 mg, Summary of Product Characteristics (EMC)
• Allopurinol, British National Formulary (BNF)
• NICE NG219: Gout: diagnosis and management
• BSR Guideline for the Management of Gout
• Gout, NICE CKS
• Allopurinol, NHS
• MHRA Yellow Card Scheme