If you have a condition related to high uric acid levels, you may be interested in learning more about allopurinol. It’s a generic prescription drug used to treat the following in adults and some children:

Allopurinol can be used short term or long term depending on the condition it’s being used to treat. Allopurinol comes as an oral tablet (a tablet that you swallow). It also comes in injectable forms, but this article does not cover those forms.

This article describes allopurinol’s side effects, also known as adverse effects. For more information about allopurinol, including details about its uses, see this in-depth article.

Some people may have mild to serious side effects during their allopurinol treatment. Examples of allopurinol’s commonly reported side effects include:

  • diarrhea
  • nausea
  • gout episode (swelling and severe pain that comes on suddenly in affected joints)
  • liver enzyme increase, which in rare cases can be a sign of liver problems*
  • skin rash*

* To learn more about this side effect, see the “Allopurinol’s side effects explained” section below.

Some people who take allopurinol have mild side effects. Examples of mild side effects that people taking allopurinol have reported include:

* To learn more about this side effect, see the “Allopurinol’s side effects explained” section below.

Usually, these side effects are temporary. And some may be easily managed. But if you have any symptoms that are ongoing or bother you, talk with your doctor or pharmacist. And do not stop taking allopurinol unless your doctor recommends it.

Allopurinol may cause mild side effects other than the ones listed above. See the drug’s prescribing information for details.

Note: After the Food and Drug Administration (FDA) approves a drug, it tracks side effects of the medication. If you’d like to notify the FDA about a side effect you’ve had with allopurinol, visit MedWatch.

Some people who take allopurinol may have serious side effects, though these are rare. Serious side effects that have been reported with allopurinol include:

* To learn more about this side effect, see the “Allopurinol’s side effects explained” section below.

If you develop serious side effects while taking allopurinol, call your doctor right away. If the side effects seem life threatening or you think you’re having a medical emergency, immediately call 911 or your local emergency number.

Get answers to some frequently asked questions about allopurinol’s side effects.

Is there a lower risk of adverse effects from the 100-mg dose of allopurinol compared with the 300-mg dose?

It depends. Your doctor will usually start you on a low dose of allopurinol and increase it slowly. If you have side effects with an allopurinol starting dosage of 100 milligrams (mg) once daily, your doctor likely will not increase your dosage. Similarly, your doctor will likely not increase your dosage if you have side effects after an increase to 200 mg or higher once daily.

If you have certain medical conditions, you may have a higher risk of side effects from allopurinol. See the “Allopurinol warnings” section below to learn more about some of these factors.

To find out more about allopurinol dosing, see this article.

Does allopurinol cause any long-term side effects?

Yes, but rarely. Most of the time, allopurinol does not cause long-term side effects.

But short-term allopurinol side effects could have long-term effects if left untreated. For example, liver or blood vessel damage could lead to other long-term health conditions.

The drugmaker also reported cases of decreased bone marrow* function. These cases occurred anywhere from 6 weeks to 6 years after starting allopurinol treatment. This side effect often happened in people who also took other drugs that could cause decreased bone marrow function.

To find out about your risk of long-term side effects from taking allopurinol, talk with your doctor.

* Bone marrow is the tissue inside bones that makes blood cells.

Is muscle pain a side effect of allopurinol?

Yes, muscle pain was one of the side effects reported in studies of allopurinol, though it was rare.

Like most mild side effects, muscle pain usually goes away within a few days or weeks.

If it does not go away, muscle pain can affect your work or other activities. If you have muscle pain that lasts longer than a few days, talk with your doctor. They may adjust your allopurinol dose or check whether something else is causing the muscle pain.

Can taking allopurinol result in weight gain?

No, it’s not likely. Studies of allopurinol did not report weight gain as a side effect. But certain other side effects of the drug could lead to weight gain.

Muscle pain and joint pain can make it harder to exercise. If you develop these side effects, consider trying a low impact exercise routine.

Liver damage is one of the side effects of allopurinol, and it can cause weight gain if left untreated for a long time.

If you notice weight gain while taking allopurinol, talk with your doctor about ways to maintain a moderate weight during treatment.

How do the side effects of allopurinol and colchicine compare?

Colchicine is another drug that treats gout. (Brand names of colchicine include Colcrys, Gloperba, and Mitigare.)

Colchicine and allopurinol can cause some of the same side effects, such as diarrhea. But the two drugs work differently, so they can also cause some different side effects.

For example, throat pain was reported in studies of colchicine. This side effect was not reported in studies of allopurinol. Allopurinol may cause loss of or change in taste, which is not a known side effect of colchicine.

To learn more about colchicine, check out this article. Your doctor or pharmacist can also tell you more about this drug.

Learn more about some of the side effects allopurinol may cause.

Liver-related side effects

Liver damage was reported as a rare side effect in studies of allopurinol. Other liver-related side effects that allopurinol can cause include:

  • enlarged liver
  • death of liver tissue
  • inflammation (swelling and damage) in the liver

Symptoms of liver damage include:

What might help

While you’re taking allopurinol, your doctor will monitor you for symptoms of liver damage. They’ll also have you get blood tests to keep track of your liver function.

If you have increased levels of liver enzymes, it can be a sign that allopurinol is affecting your liver. But your doctor likely won’t change your allopurinol treatment unless you also have symptoms of liver problems or damage.

If you develop liver damage or other liver-related side effects that cause symptoms, your doctor will likely have you stop taking allopurinol. This will usually relieve symptoms of liver-related side effects.

Hair loss

Allopurinol can cause hair loss, though this side effect was rare in studies of the drug. If you notice thinning or you develop bald patches after starting allopurinol treatment, it may be related to the drug.

Other symptoms of hair loss are:

  • a widening part
  • loose hair in your brush or comb
  • more hair than usual in the shower drain

What might help

There are ways to prevent hair loss and to help hair regrow. These include:

  • taking prescription medications, such as finasteride (Propecia, Proscar)
  • using over-the-counter treatments, such as minoxidil foam* (Rogaine)
  • treating the underlying condition, if any, that caused hair loss
  • having hair transplant surgery

If you’re noticing hair loss, talk with your doctor about treatments for this condition.

* Minoxidil also comes as a tablet, but this generic drug is available only by prescription.

Thrombocytopenia

Thrombocytopenia was a rare side effect in studies of allopurinol. Thrombocytopenia refers to having a low blood level of platelets. (Platelets are a component of blood that helps with clotting.)

Other drugs can also cause a low platelet level. So if you’re taking other drugs, allopurinol may not be the cause of this side effect. This is especially true if you’re also taking cancer medication.

Symptoms of thrombocytopenia include:

What might help

If your platelet level is low, your doctor may monitor you with blood tests more often during your allopurinol treatment. They may also adjust your dosage, choose a different medication, or suggest a treatment such as a blood transfusion.

Before you start taking allopurinol, talk with your doctor about whether you have a risk of low platelet levels.

Skin rash

Allopurinol can cause skin rash that’s mild or serious. Mild skin rash was a common side effect in studies, while serious skin rash was rare. Though rare, some serious cases of this side effect were fatal.

Serious skin rashes include:

Symptoms of mild skin rash include:

Symptoms of serious skin rash include:

  • flu-like symptoms
  • severe rash
  • redness or deepening of skin color
  • blisters on the eyes, lips, mouth, or skin
  • skin pain
  • skin peeling

What might help

If you notice a skin rash while taking allopurinol, immediately stop taking the drug and contact your doctor. Even a mild skin rash can turn into severe symptoms of an allergic reaction to allopurinol, which can be fatal in extreme cases.

Be sure to tell your doctor before you start taking allopurinol whether you’ve ever had a serious skin rash from the drug. In this case, your doctor may choose a different medication for your condition.

If you have questions or concerns about this allopurinol side effect, talk with your doctor.

Allergic reaction

Like most drugs, allopurinol can cause an allergic reaction in some people.

People with certain genes are more likely to have severe allergic reactions to allopurinol.

Symptoms can be mild to serious and can include:

  • skin rash
  • itching
  • flushing (temporary warmth, redness, or deepening of skin color)
  • swelling under your skin, usually in your eyelids, lips, hands, or feet
  • swelling of your mouth, tongue, or throat, which can make it hard to breathe

What might help

Before you start taking allopurinol, your doctor may order a genetic test to see if you have a higher risk of allergic reaction to the drug. Specifically, if you are of Korean, Han Chinese, or Thai descent, your doctor may recommend this test.

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. If your doctor confirms you’ve had a mild allergic reaction to allopurinol, they’ll likely have you stop taking it.

If you have symptoms of a severe allergic reaction, such as swelling or trouble breathing, call 911 or your local emergency number right away. These symptoms could be life threatening and require immediate medical care.

If your doctor confirms you’ve had a serious allergic reaction to allopurinol, they will have you switch to a different treatment.

Keeping track of side effects

During your allopurinol treatment, consider taking notes on any side effects you’re having. You can then share this information with your doctor. This is especially helpful when you start taking new drugs or using a combination of treatments.

Your side effect notes can include things such as:

  • what dose of the drug you were taking when you had the side effect
  • how soon you had the side effect after starting that dose
  • what your symptoms were
  • how it affected your daily activities
  • what other medications you were taking
  • any other information you feel is important

Keeping notes and sharing them with your doctor will help them learn more about how allopurinol affects you. They can then use this information to adjust your treatment plan if needed.

Allopurinol may not be right for you if you have certain medical conditions. These are known as drug-condition interactions. Other factors may also affect whether allopurinol is a good treatment option for you.

Talk with your doctor about your health history before starting allopurinol. Factors to consider include those described below.

Serious reaction when previously taking allopurinol. If you took allopurinol previously and had a very bad reaction, your doctor will not prescribe the drug due to risk of harm. Serious reactions include liver damage and severe skin rash such as Steven’s-Johnson syndrome. In this case, your doctor will recommend a different medication for your condition.

Allergic reaction. If you’ve had an allergic reaction to allopurinol or any of its ingredients, your doctor will likely not prescribe it. Ask them what other medications are better options for you.

People with certain genes are more likely to have severe allergic reactions to allopurinol. Before you start taking allopurinol, your doctor may order a genetic test to see if you have a higher risk of allergic reaction to the drug. Specifically, if you are of Korean, Han Chinese, or Thai descent, your doctor may recommend this test.

Kidney problems. Allopurinol is removed from a person’s body by their kidneys. Having kidney problems can cause the drug to build up in the body, increasing the risk of side effects. If you have kidney problems, tell your doctor before you start taking allopurinol. They’ll likely monitor your kidney function more frequently while you take allopurinol and decrease your dosage if necessary.

Liver problems. Allopurinol can cause liver damage. If you have liver problems, your risk of liver damage may increase from taking this drug. Talk with your doctor about ways to monitor your liver function while you take allopurinol.

Alcohol and allopurinol

Alcohol is not known to interact with allopurinol. But your doctor may advise you to avoid certain types of alcohol to help prevent gout, which allopurinol is used to treat.

Alcohol can also contribute to dehydration, which increases the risk of kidney stones (another condition the drug is used to treat).

Allopurinol and alcohol may both cause liver damage. Consuming alcohol during allopurinol treatment may increase the risk of this side effect.

If you drink alcohol, talk with your doctor about how much may be safe to consume, if any, during allopurinol treatment.

Pregnancy and breastfeeding while taking allopurinol

It’s unknown whether allopurinol is safe to take during pregnancy or while breastfeeding.

If you’re pregnant or breastfeeding or planning to become pregnant or breastfeed, talk with your doctor about the risks and benefits of taking allopurinol during these times.

The side effects of allopurinol are usually mild and should go away in a short amount of time.

If you have any questions about the side effects that allopurinol can cause, talk with your doctor. You can also ask them about Zyloprim, which is the brand-name version of allopurinol. A generic drug and its brand-name version are expected to have similar side effects because they contain the same active ingredient. (An active ingredient is what makes a drug work.)

Here are a few questions to ask your doctor:

  • Can stopping allopurinol treatment suddenly cause side effects?
  • How can I tell if a rash from allopurinol is serious or mild?
  • How often will I need blood tests to monitor for allopurinol side effects?
  • Are there other drugs for gout that have fewer side effects than allopurinol?
  • Does allopurinol cause eye problems?

To learn more about allopurinol, see these articles:

To get information on different conditions and tips for improving your health, subscribe to any of Healthline’s newsletters. You may also want to check out the online communities at Bezzy. It’s a place where people with certain conditions can find support and connect with others.

Disclaimer: Healthline has made every effort to make certain that all information is factually correct, comprehensive, and up to date. However, this article should not be used as a substitute for the knowledge and expertise of a licensed healthcare professional. You should always consult your doctor or another healthcare professional before taking any medication. The drug information contained herein is subject to change and is not intended to cover all possible uses, directions, precautions, warnings, drug interactions, allergic reactions, or adverse effects. The absence of warnings or other information for a given drug does not indicate that the drug or drug combination is safe, effective, or appropriate for all patients or all specific uses.