What is allopurinol?
Allopurinol is used for the long-term treatment and prevention of gout. Taken regularly, it can stop attacks of gout and help prevent damage to the joints.
The body naturally produces a substance called urate, which is normally dissolved in the blood until it's passed out of the body in the urine. When too much urate is produced, or if the body cannot get rid of it properly, the blood can't dissolve all the urate and solid crystals can form in and around joints causing inflammation and pain.
Allopurinol blocks an enzyme called xanthine oxidase which is involved in producing urate. Over a few weeks allopurinol can lower urate levels in the blood and stop new crystals forming. Once urate levels are low enough existing crystals begin to dissolve. This can take some months and you may have more attacks of gout during this time. This is more likely if your urate levels are very high or you've had gout for a long time. It doesn't mean the drug isn't working. Attacks of gout usually stop within a year as long as your urate level has lowered sufficiently.
Usually, you'll be offered allopurinol for gout if blood tests show that your urate level is high and one or more of the following applies:
- you're having repeated attacks of gout
- your joints or kidneys have been damaged by attacks of gout
- you have deposits of urate crystals affecting your skin (gouty tophi).
Your doctor may be cautious about prescribing allopurinol if your kidney or liver function is significantly impaired.
How is it taken?
Allopurinol is taken in tablet form once daily, preferably after food. The tablet should be swallowed with water.
Your doctor will advise you about the correct dose. Usually you'll start with between 100 mg and 300 mg daily but your doctor will take regular blood tests and may increase the dose (up to 900 mg daily) if your urate level isn't coming down far enough for the urate crystals to dissolve.
If you have kidney or liver disease then you may need a lower dose.
Your doctor may advise you not to start taking allopurinol until after an acute attack of gout has completely settled. If this isn't possible treatment may be started while the attack is in a mild phase.
Once your urate levels are low and steady, you'll have further checks every few months to make sure the urate isn't starting to build up again.
Allopurinol isn't a treatment for acute attacks of gout and is generally needed for life. It's important to keep taking allopurinol (unless you have severe side-effects):
- even if it doesn't seem to be working at first
- even if you have more attacks of gout when you first start allopurinol
- even when you stop having attacks of gout – otherwise urate levels may start to build up again and your symptoms may come back.
How long does it take to work?
Allopurinol lowers the level of urate in the blood within a few weeks. However, urate crystals in the tissues dissolve very slowly and so acute attacks may still occur. Acute attacks of gout will usually stop during the first year of treatment with allopurinol.
Side-effects and risks
Most people on allopurinol don’t experience side-effects. However, some possible side-effects include:
- skin rashes
- drowsiness, dizziness or headaches
- feeling sick (nausea) or vomiting
- taste disturbance.
If you develop a rash you should stop the allopurinol and see your doctor straight away. If you become dizzy or drowsy while taking allopurinol, don’t drive or operate machinery, and see your doctor as soon as possible. You should also speak to your doctor if you develop any other new symptoms that concern you.
Even if side-effects occur, it may be possible to restart allopurinol using a special ‘desensitisation’ course, which involves beginning allopurinol at a very low dose and increasing the dosage gradually.
Sometimes when you start allopurinol it can actually trigger an attack of gout, as the crystals begin to dissolve. You'll probably be given an additional medicine for the first few months to reduce this risk. This will be a small dose of either a non-steroidal anti-inflammatory drug (NSAID), colchicine or steroids.
If you do have an acute attack of gout while on allopurinol, don’t stop taking it. Treat the acute attack as you would normally.
If you're unable to continue taking allopurinol because of side-effects, you may be able to take another drug instead called febuxostat, which works in a similar way to allopurinol. Otherwise your doctor may suggest a drug such as benzbromarone or sulfinpyrazone which reduce urate levels by increasing the amount of urate passed in the urine.
Effects on other treatments
You'll probably need a non-steroidal anti-inflammatory drug (NSAID), colchicine or steroids to deal with attacks of gout – especially in the early stages of allopurinol treatment. However, some drugs may interact with allopurinol, so you should discuss any new medication with your doctor before starting it, and you should always tell anyone else treating you that you're taking allopurinol.
- Allopurinol interacts with the immunosuppressant drugs azathioprine and mercaptopurine and these drugs should not generally be used with allopurinol.
- Allopurinol may increase the risk of developing a rash with the antibiotics ampicillin and amoxicillin. Allopurinol may increase the effect of warfarin and other drugs that thin the blood (anticoagulants), so you may need to have your blood clotting tested more frequently.
You can have vaccinations while on allopurinol.
There's no known interaction between allopurinol and alcohol.
However, alcohol increases the level of urate in the blood. As part of the general treatment of gout, try to keep within the recommended limits for alcohol (14 units per week for adults). It's advisable to have alcohol-free days – but without saving up units to drink in one go.
Fertility, pregnancy and breastfeeding
It's rare for women to get gout before the menopause, but allopurinol is unlikely to affect fertility.
We don't yet know what effect allopurinol might have on an unborn baby, so you shouldn't take it if you're pregnant. If you're planning a family or become pregnant while taking allopurinol you should discuss this with your doctor.
Some allopurinol will pass into the breast milk and the effects on the baby aren't yet known. Therefore it's recommended that you shouldn't breastfeed if you're taking allopurinol.