Allopurinol: what it treats, how to take it, side effects
Last updated September 3, 2026.
Allopurinol is a xanthine oxidase inhibitor - it reduces the amount of uric acid your body produces. You may know it as Zyloprim. It is the standard long-term gout drug, with one twist that surprises everyone: it can trigger more gout attacks in its first months before it starts preventing them.
What does it treat?
Gout - the sudden attacks of redness, swelling, pain, and heat in a joint caused by uric acid buildup; high uric acid in people whose tumors break down during certain cancer treatments; and recurring kidney stones in people with high uric acid in their urine. It prevents gout attacks. It does not treat an attack once one starts.
How do you take it?
Once or twice a day, preferably after a meal, at the same times every day. The label starts at 100 mg daily and increases by 100 mg at weekly intervals until uric acid is controlled - the maximum recommended dose is 800 mg per day. Here is the twist: allopurinol may increase the number of gout attacks during the first few months, so your doctor may prescribe colchicine alongside it for that stretch. This is expected, not a failure - keep taking it. The full benefit takes several months or longer. Never stop without your doctor. Missed dose: take it when you remember unless the next one is close; never double up.
Side effects to know
- Common: nausea, diarrhea, and drowsiness - no driving until you know how it affects you.
- Serious - stop taking allopurinol and call your doctor immediately: rash, itching, or hives; peeling, blistering, or shedding skin; red or purple spots on the skin; painful urination or blood in the urine; eye irritation; swelling of the lips or mouth; fever or flu-like symptoms; swollen glands; and yellowing of the skin or eyes, pain in the upper right stomach, vomiting, itching, or extreme tiredness. A rash on allopurinol is never a wait-and-see.
Who should not take it?
Flag high blood pressure, diabetes, kidney or liver disease, and heart failure - kidney function in particular sets the dose, and the label lowers it for reduced kidney function. Alcohol is an ask-your-doctor item. If you become pregnant, call your doctor. And if you ever had a rash reaction to allopurinol in the past, that history is the first thing your prescriber needs to hear.
When is it an emergency?
A rash with fever, peeling or blistering skin, or swelling of the lips or mouth means stopping the drug and getting emergency care - this is the severe-reaction picture MedlinePlus tells you to act on immediately. Trouble breathing is a 911 call. For an overdose, call Poison Control at 1-800-222-1222. Pymander's escalation routing is built and tested specifically for this class of decision; see the safety architecture working paper.
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Common questions
Why did my gout get worse after starting allopurinol?
It is expected, and it passes. MedlinePlus says allopurinol may increase the number of gout attacks during the first few months even as uric acid comes down - shifting uric acid out of tissue deposits stirs things up before it settles. That is why doctors often prescribe colchicine alongside it for the first months. The mistake is quitting during this stretch; the attacks fade as the uric acid pool empties.
Why can't I take allopurinol during a gout attack and expect relief?
Because it is a prevention drug, not a pain drug. MedlinePlus is explicit: allopurinol is used to prevent gout attacks, not to treat them once they occur. It lowers uric acid over months; it does nothing for the joint that is on fire tonight. Attacks are treated with colchicine, NSAIDs, or steroids - your doctor sets that plan so you have it before you need it.
Why is a rash on allopurinol such a big deal?
Because it can be the first sign of a severe reaction. MedlinePlus says to stop taking allopurinol and call your doctor immediately for rash, itching, hives, peeling or blistering skin, fever or flu-like symptoms, or swollen glands - this combination can escalate fast. Do not wait to see whether the rash fades. Stop, call, same day.
Can I drink alcohol while taking allopurinol?
Ask your doctor - MedlinePlus lists alcohol as an ask-about item for this drug. Beyond the interaction question, alcohol raises uric acid, which works directly against what you are taking allopurinol to do. Get the personal rule at your next visit.
How long does allopurinol take to work?
Several months or longer for the full benefit, per MedlinePlus. The label starts at 100 mg daily and climbs by 100 mg a week until your uric acid target is hit - your blood tests, not your joints, are the scoreboard in the early months. The payoff is steady: fewer attacks, then none.
What happens if I miss a dose of allopurinol?
Take it as soon as you remember, unless it is almost time for the next one - then skip it and never double up. One missed dose barely moves your uric acid. The pattern matters: uric acid control is a months-long accumulation, and it is the daily habit that keeps it there.