Allopurinol: what it does and what to watch
Also known as: Xanthine oxidase inhibitor · Xanthine oxidase inhibitor for gout and high uric acid · Updated 2026-08-19 · Educational — not medical advice
In plain language: Uric acid builds up in the body as a byproduct of breaking down purines, molecules found in many foods and in the body's own cells. An enzyme called xanthine oxidase is a key step in producing uric acid, and this medicine blocks that enzyme, which reduces how much new uric acid is made. Because it works on production rather than clearing out crystals already deposited in joints, it's used for long-term prevention rather than for treating a gout attack that's already underway.
What it's used for
- Long-term prevention of gout attacks by lowering uric acid
- High uric acid levels linked to certain cancer treatments
- Prevention of uric acid kidney stones
Common side effects
- Skin rash
- Nausea or stomach upset
- Headache
- Diarrhea
- Drowsiness
Get medical help if…
A severe skin reaction with blistering, peeling, or mucous membrane involvement, which can rarely be life-threatening and needs urgent care
Fever, rash, and yellowing of the skin or eyes together, which can indicate a rare hypersensitivity syndrome affecting multiple organs
Signs of a worsening gout flare shortly after starting, which is a recognized early effect rather than a treatment failure
Unusual bruising, bleeding, or signs of infection, which can point to a rare effect on blood cell counts
Lab values it can move
Few guides tell you this part: medicines show up in blood work. If you take Allopurinol and a value below looks off on your report, this may be part of the story — never stop a medicine without your doctor.
Uric acid levels fall gradually over weeks as production is reduced, with the target range guided by follow-up blood tests.
Uric AcidLiver enzymes are checked periodically, since mild elevations can occur, particularly early in treatment.
SGPT (ALT)Kidney function influences dosing, since reduced clearance can raise blood levels of the medicine and, rarely, contribute to the severe hypersensitivity syndrome.
eGFR (Estimated Glomerular Filtration Rate)Good to know
- Starting this medicine can trigger a gout flare in the first weeks, which is a known effect and not a sign the medicine isn't working
- Any new, widespread rash should be reported and evaluated promptly rather than waited out, given the rare risk of a severe skin reaction
- Kidney function is usually checked before starting, since dosing depends on it
- Staying well hydrated supports the kidneys in clearing uric acid and the medicine itself
- People of certain genetic backgrounds may be screened for a specific gene variant before starting, since it raises the risk of severe reactions
Questions people ask
Why might gout get worse right after starting this medicine?
As uric acid levels begin to fall, crystals already deposited in and around joints can start to dissolve and shift, which can paradoxically trigger a gout flare during the early weeks of treatment. This is a well-recognized, temporary effect rather than a sign the medicine has failed, and many prescribers pair it with a short course of anti-inflammatory treatment to reduce flares while the uric acid level comes down.
Why is a severe rash on this medicine taken so seriously?
A rare but serious hypersensitivity reaction has been linked to this medicine, sometimes appearing as a widespread rash, fever, and involvement of internal organs like the liver, and it can be life-threatening if not caught early. Certain genetic backgrounds carry a higher risk, which is why some people are tested beforehand, and why any new or spreading rash on this medicine is treated as urgent rather than routine.
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