Allopurinol is a medicine used to lower levels of uric acid in your blood. If you produce too much uric acid or your kidneys do not filter enough out, it can build up and cause tiny, sharp crystals to form in and around your joints.

Allopurinol is used to treat gout and kidney stones.

It may also be prescribed if you're having some types of cancer treatment. Some treatments can cause a build-up of uric acid.

Allopurinol comes as 100mg and 300mg tablets and is only available on prescription.

Allopurinol is also known by the brand names Zyloric and Uricto.

• Allopurinol reduces the amount of uric acid made by your body's cells. This reduces symptoms such as swollen and painful joints (gout).
• It may take several months before you feel the full benefit of allopurinol.
• During the first few months of treatment, as allopurinol starts to work, you may get more gout attacks. However, your doctor will prescribe non-steroidal anti-inflammatory drugs (NSAIDs) or colchicine to help with this.
• When allopurinol is taken regularly, it can lower the number of gout attacks and help prevent damage to the joints.
• Usually you will start allopurinol after an acute attack of gout has completely settled.

Allopurinol can be taken by adults and sometimes children.

Allopurinol is not suitable for certain people.

Talk to a doctor or pharmacist if you:

• have ever had an allergic reaction to allopurinol or any other medicine
• are of Han Chinese, Thai or Korean origin
• have problems with your liver or kidneys
• currently have an attack of gout
• have thyroid problems

The usual dose of allopurinol is 100mg to 300mg a day. Follow your doctor's advice on how many tablets to take, and how many times a day.

You'll have regular blood tests to monitor your uric acid levels. If your uric acid level does not come down far enough, your doctor may increase your dose (up to 900mg daily in severe cases).

If you have kidney or liver disease, your doctor may prescribe a lower dose and will monitor you more closely.

How to take it

Swallow the allopurinol tablets with water, ideally after food. You'll usually take it once a day, but if you're on a high dose, your doctor may advise you to split the dose and take it twice a day.

If your doctor has recommended you take allopurinol with lots of fluid, try to drink 2 to 3 litres of fluids every day.

You can take allopurinol at any time of the day, however, try to take your doses at the same time of day each day.

Will my dose go up or down?

Your doctor will usually start you on a low dose of allopurinol.

You'll have regular blood tests to monitor your uric acid levels.

Your doctor may increase or decrease your dose depending on the results of your blood tests.

What if I forget to take it?

If you take allopurinol:

• once a day – take the missed dose as soon as you remember. If you do not remember until the following day, skip the missed dose.

• twice or more a day - if you do not remember until your next dose is due, skip the missed dose and take the dose that is due.

Never take a double dose to make up for a missed dose.

If you often forget to take your medicines, it may help to set an alarm to remind you. You could also ask a pharmacist for advice on ways to help you remember to take your medicines.

What if I take too much?

If you need to go to A&E, do not drive yourself. Get someone else to drive you or call for an ambulance.

Take the allopurinol packet or leaflet inside it, plus any remaining medicine, with you.

Like all medicines, allopurinol can cause side effects, although not everyone gets them.

Common side effects

The most common side effects are feeling or being sick. These can be reduced if you eat little and often and drink lots of fluids such as water or squash. If you are being sick, take small, frequent sips of water to avoid dehydration.

Serious side effects

Skin rashes

If you notice a skin rash or redness, tell a doctor straight away, as this can develop into a life-threatening skin condition called Stevens-Johnson syndrome.

Stevens-Johnson syndrome is a rare side effect of allopurinol. It causes flu-like symptoms, followed by a red or purple rash that spreads and forms blisters. The affected skin eventually dies and peels off.

It's more likely to happen in the first 8 weeks of taking allopurinol, or when the dose is increased too quickly. It can also happen if allopurinol is stopped suddenly for a few days and then restarted at the same dose as before. It's better to reduce the dose and then increase it slowly.

It's also best to not start taking allopurinol within 2 weeks of a viral infection, vaccination, or rash caused by something else.

Other serious side effects

It's unusual to have serious side effects after taking allopurinol. Tell a doctor straight away if you:

• get yellow skin or the whites of your eyes go yellow – these can be signs of a liver problem
• get a high temperature, sore throat and swollen glands or feel generally unwell – this could mean there are problems with your white blood cells
• have bruising for no obvious reason or bleeding gums (which takes a long time to stop) when brushing your teeth
• are unusually thirsty, going to the toilet to pee a lot, unusually tired, losing weight without trying, blurred vision – these could be signs of diabetes

Serious allergic reaction

In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to allopurinol.

These are not all the side effects of allopurinol. For a full list, see the leaflet inside your medicines packet.

Allopurinol is not usually recommended during pregnancy. There is not enough evidence to say that it's safe.

Talk to a doctor if you're thinking about trying for a baby. There may be other medicines that are safer for you.

Find out more about how allopurinol can affect you and your baby during pregnancy from Best Use of Medicines in Pregnancy (BUMPS).

Allopurinol and breastfeeding

If your doctor or health visitor says your baby is healthy, allopurinol can be taken while you're breastfeeding.

Allopurinol passes into breast milk in small amounts and has been linked with side effects in very few breastfed babies.

If you notice that your baby is not feeding as well as usual, or seems unusually sleepy, or if you have any other concerns about your baby, then talk to your health visitor or doctor.

Some medicines and allopurinol can interfere with each other and increase the chances of you having side effects.

Tell a doctor or pharmacist if you're taking any of these medicines before you start taking allopurinol:

• aspirin or medicines used to thin your blood (anticoagulants), such as warfarin
• any antibiotics
• medicines used to reduce your immune response (for arthritis or after you've had an organ transplant)
• tablets that make you pee more (diuretics) such as furosemide or ACE inhibitors to treat high blood pressure (hypertension) such as enalapril and ramipril

If you take aluminium hydroxide (found in some antacids such as Maalox and Mucogel), leave a 3 hour gap between the aluminium hydroxide and your allopurinol dose.

Taking allopurinol with painkillers

You can take allopurinol with paracetamol and anti-inflammatory medicines such as ibuprofen, naproxen, or indomethacin.

Your doctor may prescribe a NSAID (such as diclofenac or naproxen or a medicine called colchicine to help prevent or to deal with attacks of gout – especially in the early stages of allopurinol treatment.

Mixing allopurinol with herbal remedies and supplements

There's very little information about taking herbal medicines and supplements with allopurinol.