A xanthine oxidase inhibitor (gout-prevention medicine)
Allopurinol
The usual first-choice medicine to prevent gout attacks by lowering uric acid over the long term — a preventer, not a treatment for a flare that is already happening.
What is Allopurinol?
Allopurinol is the standard first-line medicine to prevent gout by lowering the uric acid in the blood, which stops the crystals that cause attacks from forming. It is a long-term preventer, not a painkiller for an attack in progress. It is started at a low amount and increased to a target uric-acid level, and can briefly trigger a flare when first started — so it is often begun with cover from an anti-inflammatory or colchicine.
Education and reference only. This is a plain-language guide to Allopurinol — it deliberately contains no doses. Doses depend on the person, the brand and the reason for treatment, and belong with your prescriber. Always check the BNF, the product labelling (SmPC) and follow medical advice.
What it is
Allopurinol is the most commonly used medicine for preventing gout, a form of arthritis caused by uric-acid crystals building up in the joints. By lowering the level of uric acid in the blood over time, it gradually dissolves these crystals and prevents new attacks, and it can also help prevent certain kinds of kidney stones and manage high uric acid in some other settings. Importantly, it is a preventer taken every day for the long term — it does not relieve an attack that is already happening. It is taken as a daily tablet.
How it works
Allopurinol blocks an enzyme called xanthine oxidase, which the body uses to make uric acid. With less uric acid being produced, the level in the blood falls below the point at which crystals form, so over months the existing crystals dissolve and new attacks stop. Because lowering uric acid can temporarily stir up the crystals already present, treatment is started at a low amount and increased slowly to a target blood level, often with anti-inflammatory or colchicine "cover" for the first few weeks.
What it treats
Conditions Allopurinol is used for
Practical use
How to take Allopurinol
General, dose-free guidance — always follow your prescriber's and the leaflet's specific instructions.
- Take it every day, long-term, to keep uric acid down — including during an attack; do not stop it when a flare happens.
- Take it after food, with plenty of water, to reduce stomach upset and help protect the kidneys.
- Expect that starting it may trigger a flare; keep taking it and use the "cover" medicine your doctor provides for the first weeks.
- Stop and seek urgent advice if you develop a new widespread skin rash, especially with fever, blistering or the mouth/eyes involved.
- Attend blood tests so the amount can be adjusted to your target uric-acid level.
Weighing it up
Advantages & disadvantages of Allopurinol
Advantages
- Effective, well-established first-line prevention that can make gout attacks stop altogether once uric acid is controlled.
- Once-daily, low cost, and also useful for some kidney stones and high uric acid in other settings.
- The amount can be tailored to a measured target level.
Disadvantages
- Can trigger a flare when first started, and does nothing for an attack already in progress.
- Rarely causes a serious skin reaction, and interacts dangerously with azathioprine and mercaptopurine.
- Needs the amount reduced in significant kidney impairment.
Practical use
Good to know
Two points matter most. First, allopurinol prevents attacks but does not treat one in progress — you keep taking it through a flare (adding a separate painkiller or anti-inflammatory), and you do not stop it just because an attack happens. Second, starting it can itself trigger a flare as the crystals shift, which is normal and does not mean it is not working; this is why it is introduced gradually, often with cover. The amount is adjusted to reach a target uric-acid level on blood tests. Rarely, allopurinol causes a serious skin reaction — stop it and seek urgent advice if you develop a widespread rash, especially with fever or blistering, particularly in the first weeks.
Who should not take it / use with caution
- People having an acute gout attack should not START it during the flare (but should continue it if already established) — it is begun once the attack has settled.
- People taking azathioprine or mercaptopurine, unless under specialist supervision with major dose adjustment, as the combination can be dangerous.
- Used at reduced amounts and with care in significant kidney impairment; certain ethnic backgrounds may be offered a gene test before starting.
Monitoring
- Blood uric-acid level (to reach and hold a target)
- Kidney function
- For skin rash, especially in the first weeks
Side effects
- A gout flare in the early weeks as uric acid falls — expected, not a sign of failure.
- Skin rash (common and usually mild) and digestive upset; take it after food.
- Rarely, a serious skin or hypersensitivity reaction (widespread rash, fever, blistering, feeling very unwell) — stop and seek urgent help; this is most likely in the first weeks.
Key interactions
- Azathioprine and mercaptopurine levels are greatly increased by allopurinol — a potentially serious combination that must be specially managed.
- It can increase the effect of warfarin, and care is needed with some other medicines.
- Certain water tablets (diuretics) can raise uric acid and the risk of hypersensitivity; ACE inhibitors add a small extra reaction risk.
Available as: Tablets; can be crushed or a liquid used for people who cannot swallow tablets.
Answers
Allopurinol: frequently asked questions
Can I take allopurinol during a gout attack?
You should keep taking it if you are already established on it — stopping during a flare makes control worse. But you do not START allopurinol in the middle of an attack; it is begun once the attack has settled, because starting it can briefly worsen a flare. For the pain of an attack you need a separate treatment such as an anti-inflammatory or colchicine.
Why did my gout get worse after starting allopurinol?
Lowering uric acid can temporarily stir up the crystals already in your joints, triggering a flare in the first weeks. This is expected and does not mean the medicine is failing. It is why allopurinol is started at a low amount and increased slowly, often with "cover" from an anti-inflammatory or colchicine for the first weeks or months.
How long do I need to take allopurinol for?
Usually long-term, often for life. It works by keeping uric acid low so crystals cannot form; if you stop, the level rises again and attacks return. Once the target level is reached and held, many people find their attacks stop completely.
What rash should I worry about?
A mild rash can occur and is usually managed easily, but a widespread rash — especially with fever, blistering, or involvement of the mouth or eyes — can signal a rare but serious reaction. Stop the allopurinol and seek urgent medical advice; this is most likely in the first few weeks.
Authoritative sources
- BNF: Allopurinol.
- electronic Medicines Compendium (SmPC): Allopurinol (Zyloric).
- NICE NG219: Gout – diagnosis and management.
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