What is Allopurinol and How Does it Work? 

Allopurinol is the most commonly prescribed medication in the UK for the long term prevention of gout. While other drugs are used to treat the immediate pain of an attack, allopurinol addresses the root cause of the condition. It is a preventative treatment, meaning it does not provide instant relief during a flare, but instead works slowly over time to stop flares from happening in the first place. For the majority of patients, allopurinol is a lifelong medication that is essential for protecting the joints from permanent damage and dissolving the crystal deposits that cause agony. 

What We Will Discuss in This Article 

• The biological mechanism of allopurinol as a xanthine oxidase inhibitor 

• Why this medication is used for prevention rather than acute pain relief 

• The treat to target strategy used by UK clinicians 

• Why it is vital to continue taking allopurinol during a flare 

• Common side effects and safety considerations 

• When a sudden joint flare requires an urgent clinical review via 999 

How Allopurinol Works: The Biological Mechanism 

Allopurinol belongs to a class of drugs known as xanthine oxidase inhibitors. To understand how it works, it is helpful to look at how your body produces waste. Uric acid is a byproduct created when your body breaks down substances called purines, which are found naturally in your cells and in certain foods. 

The final step of this uric acid production is managed by an enzyme called xanthine oxidase. Allopurinol works by “blocking” this enzyme. By slowing down the activity of xanthine oxidase, allopurinol reduces the total amount of uric acid your body produces.⁴ According to NICE clinical guidelines, once the production is slowed, the level of uric acid in your blood begins to drop, eventually falling below the level where it can form painful crystals in your joints. 

• Enzyme Blocking: Stops the final stage of uric acid production. 

• Lowering Concentration: Reduces the saturation of urate in the bloodstream. 

• Dissolving Deposits: When blood levels are low, existing crystals in the joints can slowly dissolve back into the blood to be filtered out by the kidneys. 

The “Treat to Target” Strategy 

In the UK, allopurinol is not prescribed at a “one size fits all” dose. Instead, GPs use a treat to target approach. This means they will start you on a low dose (usually 100mg once a day) and check your blood uric acid levels every few weeks. 

If your uric acid is still high, they will gradually increase the dose, sometimes up to 800mg or 900mg per day, until your blood level is below a specific target. This target is typically 360 µmol/L or 300 µmol/L for more severe cases. This strategy ensures that the medication is working at the exact strength needed for your specific metabolism to keep your joints crystal free. 

Why You Must Continue During a Flare 

A common point of confusion for patients is what to do if they experience a gout attack while taking allopurinol. It is a vital rule in the UK that you should not stop taking allopurinol during a flare. 

If you stop the medication, your uric acid levels will suddenly bounce back up, which can actually prolong the inflammation or make the flare more severe. If you have an attack, you should continue your daily allopurinol dose and use a separate acute treatment, such as naproxen or colchicine, to manage the pain. Allopurinol is the “long term shield,” while acute meds are the “short term fire extinguishers.” 

Side Effects and Safety 

Like any medication, allopurinol can cause side effects, although most people tolerate it very well. When you first start the drug, it can actually trigger a temporary “initiation flare” as your uric acid levels shift.¹¹ This is a sign the medication is working, and your GP will often prescribe a low dose of colchicine for the first few months to help prevent this. 

Other potential side effects include: 

1. Skin Rash: This can occasionally be serious. If you develop a rash after starting allopurinol, you must contact your GP immediately. 

2. Indigestion: Often helped by taking the tablet after a meal. 

3. Drowsiness: Some people feel slightly sleepy when they first start the medication.  

Before starting allopurinol, your doctor will perform a blood test to check your kidney function. Because the kidneys help clear the medication from your body, the dose may need to be adjusted if your kidneys are not working at full capacity. 

Summary of Allopurinol Facts 

Feature	Allopurinol Detail
Drug Class	Xanthine Oxidase Inhibitor.
Primary Goal	To prevent gout flares by lowering uric acid.
Dosage Style	Start low and slow, then “treat to target.”
Duration	Usually a lifelong commitment.
Flare Advice	Never stop taking it during an active attack.

Conclusion 

Allopurinol is the cornerstone of gout prevention in the UK, working by blocking the enzyme responsible for producing uric acid. By lowering the concentration of urate in your blood, it allows existing crystals to dissolve and prevents new ones from forming. While it requires regular blood tests and a long term commitment, it is the most effective way to ensure a future free from the agony of gout flares. Understanding its role as a preventative shield rather than a quick painkiller is the key to successful management. 

If you experience severe, sudden joint pain accompanied by a high temperature, chills, or feeling generally unwell, call 999 or go to A&E immediately, as this may be a sign of a serious infection. 

Authority Snapshot (E-E-A-T Block) 

This article was written by Dr. Stefan Petrov, a UK trained physician with an MBBS and experience in general medicine, surgery, and emergency care. Dr. Petrov has worked in various NHS settings, helping patients manage chronic conditions according to NICE and NHS standards. This guide provides an authoritative overview of the role and mechanism of allopurinol in the UK.