7 Allopurinol Side Effects to Know About

Allopurinol (Zyloprim, Lopurin) is a medication that’s prescribed to treat gout, a type of arthritis (joint inflammation). Allopurinol is also used to prevent certain types of kidney stones from recurring and tumor lysis syndrome after cancer treatment.

Like with any medication, taking allopurinol has some risks. But allopurinol side effects are often mild and go away on their own. In rare cases, more serious side effects can occur. And these may warrant a call to your healthcare professional or immediate medical attention.

Allopurinol side effects at a glance

Mild allopurinol side effects can include:

• Diarrhea

• Nausea

• Gout flares

• Increased liver enzymes

• Drowsiness

Promotion disclosure

Allopurinol

Avg retail price

$13.99(Save 54.68%)

GoodRx discount

$6.34

Compare all prices

More rare but serious side effects can also occur. Examples of these side effects include:

• Severe skin rashes

• Kidney damage, including kidney stones

• Liver damage

• Decreased blood cell counts

Here, we’ll take a look at seven allopurinol side effects to know about and how to manage them.

1. Gastrointestinal side effects

Gastrointestinal (GI) discomfort is one of the most common allopurinol side effects. This often includes nausea and diarrhea. Abdominal pain, upset stomach, and vomiting are less common but possible.

Usually, GI side effects caused by allopurinol are mild. Still, allopurinol is best taken after meals to prevent stomach upset. It can also be helpful to stay hydrated, eat smaller, but more frequent, meals throughout the day, and avoid fatty and fried foods. But let your prescriber know if you have GI symptoms that don’t improve. They may recommend medications to help with nausea or diarrhea. 

Rare GI symptoms to keep a close eye out for are appetite loss and weight loss. Weight loss can be a sign of liver damage, a more serious side effect of allopurinol that we’ll talk about later.

2. Gout flares

While allopurinol is used to manage gout symptoms, it can also trigger gout flares as a side effect.

Gout is caused by the buildup of a natural waste product called uric acid in your joints. After you start taking allopurinol, uric acid levels in your blood rapidly decrease. But, as the uric acid crystals start dissolving, they can irritate the lining of your joints. These actions can trigger inflammation, causing a gout flare.

To minimize the risk of gout flares, your prescriber will usually start you on a low allopurinol dosage and slowly increase it over time. They may also prescribe you a second medication, such as colchicine (Colcrys) or indomethacin (Indocin), to help prevent a gout attack. You may need to continue taking that medication with allopurinol for 3 to 6 months.

Even with preventative treatment, flares may still occur. In most cases, you should continue taking allopurinol. Gout flares typically become less intense over time.

If you do experience gout flares while taking allopurinol, let your prescriber know. If your uric acid levels aren’t decreasing as much as expected, you may need a different medication.

3. Drowsiness

Allopurinol may cause drowsiness. Usually, this side effect is mild. But if you have drowsiness that prevents you from carrying out your daily activities safely, let your prescriber know.

When you first start taking allopurinol, or after a dosage increase, it’s best to avoid driving or operating heavy machinery. Wait to resume these activities until after you see how the medication affects you. If your drowsiness is extreme and doesn’t improve, your prescriber may suggest changing your treatment plan.

Combining allopurinol with alcohol or medications and substances that slow down the brain may increase your risk for drowsiness. Some examples include:

• Benzodiazepines, such as alprazolam (Xanax)

• Opioids, such as fentanyl 

• Sleep medications, such as zolpidem (Ambien)

• Diphenhydramine (Benadryl)

• Cannabis

Generally, it’s safest to avoid these medications and substances while taking allopurinol. To help avoid potential interactions, share your current medication list with your prescriber and pharmacist before starting treatment with allopurinol. Be sure to include all prescription and over-the-counter medications.

4. Skin rashes and reactions

Many medications, including allopurinol, can cause a mild skin rash as a side effect. But even a mild skin rash from allopurinol can be concerning. This is because, in rare cases, these skin rashes can lead to life-threatening skin reactions.

If you do experience a skin reaction from allopurinol, it’s most likely to occur within the first 2 months of treatment. Examples of life-threatening skin reactions include:

• Stevens-Johnson syndrome 

• Toxic epidermal necrolysis

• Drug reaction with eosinophilia and systemic symptoms (DRESS) syndrome 

Symptoms of these skin reactions may include flu-like symptoms, such as body aches, fatigue, and fever or chills. A painful, blistering skin rash may also appear. And you may experience skin peeling. In severe cases, other organs besides your skin may be affected, including the heart, kidneys, or lungs.

Less than 1% of people who take allopurinol experience skin reactions. But your individual risk is higher if you:

• Have a genetic marker called HLA-B*58:01

• Have kidney problems, such as chronic kidney disease

• Take allopurinol with certain medications, like hydrochlorothiazide and amoxicillin

The HLA-B*58:01 marker is more common in people of certain ancestry, including African, Asian (Han Chinese, Korean, Thai), and Native Hawaiian or Pacific Islander ancestry. Your prescriber may order a blood test to check for this marker before prescribing you allopurinol. In most cases, allopurinol isn’t recommended if you have the HLA-B*58:01 allele.

If you experience a skin rash while taking allopurinol, stop taking it — even if the rash is mild. And call your prescriber immediately for guidance about what to do next. If you have symptoms that feel severe, such as swelling of the lips or mouth or difficulty breathing, call 911 or seek emergency medical assistance.

5. Low blood cell levels

Taking allopurinol can decrease levels of multiple types of blood cells, including white blood cells, red blood cells, and platelets. This can increase your risk for infection, bleeding, and other symptoms like fatigue and dizziness.

This side effect can occur at any point during allopurinol treatment. The risk is greater if you take other medications that lower blood cell levels. Medications that lower blood cell levels include clozapine (Clozaril) and immunosuppressants such as the steroid prednisone (Prednisone Intensol) and organ transplant medication cyclosporine (Sandimmune, Gengraf).

While you’re taking allopurinol, your prescriber may order routine blood tests to check your blood cell levels, if you take one of these medications.

6. Liver damage

Allopurinol may increase liver enzymes in up to 6% of people who take the medication. Usually, this doesn’t cause symptoms and isn't a reason to be concerned. But in rare cases, elevated liver enzymes may be a sign of liver damage or other serious problems, like DRESS syndrome.

Symptoms of liver damage to watch for include unexplained weight loss, stomach pain, and dark urine. If you have any of these symptoms, stop taking allopurinol and contact your prescriber. Liver problems usually resolve after you stop taking allopurinol.

While you’re taking allopurinol, your prescriber may order routine blood tests to check your liver enzyme levels, especially if you have existing liver problems. If your levels are elevated, they’ll likely ask you to stop taking allopurinol.

7. Kidney damage

Allopurinol can cause kidney problems, including kidney stones, as a side effect. Kidney damage from allopurinol is rare, but you’re at a higher risk if you have an existing kidney condition. And your risk for kidney stones is higher if you’ve had kidney stones in the past.

Symptoms of kidney damage include peeing less often, confusion, and swelling in your feet or lower legs. Kidney stone symptoms include stomach pain, blood in your urine, and nausea. Let your prescriber know immediately if you experience these symptoms.

Your prescriber may check your kidney function throughout your treatment with allopurinol. You can also lower your risk for kidney problems by drinking at least 2 L (8 to 9 cups) of fluid a day.

When should you contact your care team about allopurinol side effects?

In most cases, allopurinol side effects are mild and go away on their own. But if you experience any side effects that don’t improve or worsen, let your prescriber know. 

Some side effects, such as a skin rash, require you to stop taking allopurinol and call your prescriber immediately. You should also contact your prescriber right away if you’re peeing less, feeling extremely tired, or you have unexplained weight loss.

If you develop symptoms that feel severe or life-threatening, such as swelling in the face or trouble breathing, seek immediate medical attention. Call 911 or go to the closest emergency room.

Why trust our experts?

Written by:

Alex Brewer, PharmD, MBA

Alex Brewer, PharmD, MBA, is a licensed pharmacist specializing in chronic disease and health and wellness. Upon completing his residency, he worked in the managed care field, conducting medication therapy management and adherence counseling sessions with Medicaid, Medicare, and private insurance patients, before transitioning to a career in medical writing.

Edited by:

Amy B. Gragnolati, PharmD, BCPS

Amy Gragnolati, PharmD, BCPS, is a pharmacy editor for GoodRx. Amy currently holds her pharmacist license in Georgia and California.

Reviewed by:

Daphne Berryhill, RPh

Daphne Berryhill, RPh, has two decades of experience as a clinical pharmacist. She spent most of her career in the Chicago area practicing in-home infusion.

Our editorial standards

Meet our experts