Like all medicines, allopurinol can cause side effects, but many people have no side effects or only minor ones.
Common side effects
There are things you can do to cope with these common side effects of allopurinol:
Feeling or being sick (nausea or vomiting)
If you're feeling sick, eat little and often and drink lots of fluids such as water or squash. Also try taking your medicine with, or just after, a meal or snack. Stick to simple meals and do not eat rich or spicy food.
If you're being sick, take small, frequent sips of water to avoid dehydration. Signs of dehydration include peeing less than usual or having dark, strong-smelling pee.
If you take the combined contraceptive pill or progestogen-only pill and you're being sick, your contraception may not protect you from pregnancy. Check the pill packet for advice.
Serious side effects
It's unusual to have serious side effects after taking allopurinol. Tell a doctor or call 111 now if:
- the whites of your eyes or your skin turn yellow, although this is less obvious on black or brown skin – these can be signs of a liver problem
- you get a high temperature, sore throat and swollen glands or feel generally unwell – this could mean there are problems with your white blood cells
- you have bruising for no obvious reason or bleeding gums (which takes a long time to stop) when brushing your teeth
- you are unusually thirsty, going to the toilet to pee a lot, unusually tired, losing weight without trying, or have blurred vision – these could be signs of diabetes
Skin rashes
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.
Stevens-Johnson syndrome is more common in:
- children
- people who developed a rash with an epilepsy medicine in the past
- people who are allergic to an antibiotic called trimethoprim
- people also taking a medicine called sodium valproate
To help prevent the chance of you getting a rash that could be confused with Stevens-Johnson syndrome, it's best not to try new medicines or food during the first 3 months of treatment with allopurinol.
It's also best to not start taking allopurinol within 2 weeks of a viral infection, vaccination, or rash caused by something else.
Immediate action required: Go to A&E now if:
- you get a skin rash with flushing, blisters or ulcers – these can be signs of Stevens-Johnson syndrome
Serious allergic reaction
In rare cases, it's possible to have a serious allergic reaction (anaphylaxis) to allopurinol.
Immediate action required: Call 999 now if:
- your lips, mouth, throat or tongue suddenly become swollen
- you're breathing very fast or struggling to breathe (you may become very wheezy or feel like you're choking or gasping for air)
- your throat feels tight or you're struggling to swallow
- your skin, tongue or lips turn blue, grey or pale (if you have black or brown skin, this may be easier to see on the palms of your hands or soles of your feet)
- you suddenly become very confused, drowsy or dizzy
- someone faints and cannot be woken up
- a child is limp, floppy or not responding like they normally do (their head may fall to the side, backwards or forwards, or they may find it difficult to lift their head or focus on your face)
You or the person who's unwell may also have a rash that's swollen, raised, itchy, blistered or peeling.
These can be signs of a serious allergic reaction and may need immediate treatment in hospital.
Other side effects
These are not all the side effects of allopurinol. For a full list, see the leaflet inside your medicines packet.
You can report any suspected side effect using the Yellow Card safety scheme.