Pentasa (mesalamine) is a prescription drug that’s used in certain people with mild to moderate ulcerative colitis (UC). Pentasa can cause side effects that range from mild to serious. Examples include hair loss and constipation.

Specifically, Pentasa is used in adults with mildly to moderately active UC to:

The active ingredient in Pentasa is mesalamine. (An active ingredient is what makes a drug work.) The drug comes as an extended-release capsule that you swallow.

Keep reading to learn about the common, mild, and serious side effects that Pentasa can cause. For a general overview of the drug, including details about its uses, see this article.

Some people may experience mild to serious side effects during Pentasa treatment. Examples include:

These are just a few of the more common side effects reported by people who took Pentasa in studies.

Many Pentasa side effects are mild and don’t require medical attention. Examples of mild side effects that have been reported with this drug include:

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

In most cases, these side effects should be 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. Do not stop taking Pentasa unless your doctor recommends it.

Pentasa 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 Pentasa, visit MedWatch.

Many common side effects of Pentasa are mild and go away on their own. But it’s possible that Pentasa may cause some serious side effects that require medical attention.

Serious side effects that have been reported with this drug include:

If you develop serious side effects while taking Pentasa, 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.

* To learn more about this side effect, see the “Side effects explained” section below.
† An allergic reaction is possible after taking Pentasa. But this side effect wasn’t reported in studies.

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

Hair loss

In rare cases, hair loss was reported as a side effect in studies of people taking Pentasa. But it’s not clear whether Pentasa was the cause.

Ulcerative colitis, which Pentasa is used to treat, makes it difficult for your body to absorb vitamins and nutrients. And many factors, including nutritional deficiencies and stress, can cause hair loss.

What might help

Talk with your doctor or pharmacist if you’re concerned about hair loss with Pentasa. They can recommend over-the-counter (OTC) treatments. Eating a balanced diet with foods high in vitamins B12, vitamin D, biotin, riboflavin, and iron may also help with hair loss. Examples of these foods include:

  • avocados
  • beans
  • berries
  • eggs
  • nuts and seeds
  • oysters
  • spinach
  • sweet potatoes


Constipation was reported as a side effect in Pentasa studies, but it was rare. Symptoms include:

  • having lumpy, hard stools
  • having fewer than three bowel movements per week
  • having pain or straining during bowel movements
  • feeling full even after having a bowel movement

What might help

You can usually manage constipation with OTC stool softeners or laxatives, such as:

But always talk with your doctor or pharmacist before taking OTC medications. They’ll let you know whether they’re safe to take with Pentasa.

Let your doctor know if your constipation is not relieved by OTC treatments or if you have any of the following serious symptoms:

Joint pain

Some people reported joint pain in Pentasa studies. But this side effect was not common. Joint pain caused by Pentasa is usually mild and goes away within a few days to weeks.

What might help

Mild joint pain can be relieved with OTC medications, such as ibuprofen (Advil, Motrin) or acetaminophen (Tylenol). Always check with your doctor or pharmacist before taking OTC drugs. They’ll let you know whether they’re safe to take with Pentasa.

If your pain is bothersome or is not relieved with OTC medications, talk with your doctor. They can recommend other treatments for joint pain.

Kidney damage

Kidney problems, including kidney damage, have been reported in people taking Pentasa or other drugs that contain mesalamine. Symptoms of kidney problems include:

What might help

Immediately let your doctor know if you have any symptoms of kidney problems. They’ll do lab tests to check how well your kidneys are working. Depending on the test results, they’ll determine whether Pentasa is safe for you.

Allergic reaction

Like most drugs, Pentasa can cause an allergic reaction in some people. But this side effect wasn’t reported in studies. Symptoms can be mild to serious and can include:

  • skin rash
  • itchiness
  • 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

If you have mild symptoms of an allergic reaction, such as a mild rash, call your doctor right away. They may suggest a treatment to manage your symptoms. Examples include:

  • an antihistamine you take by mouth, such as Benadryl (diphenhydramine)
  • a product you apply to your skin, such as hydrocortisone cream

If your doctor confirms you’ve had a mild allergic reaction to Pentasa, they’ll decide if you should continue 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 Pentasa, they may have you switch to a different treatment.

Keeping track of side effects

During your Pentasa 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 first 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 Pentasa affects you. They can then use this information to adjust your treatment plan if needed.

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

Can Pentasa cause weight gain?

Weight gain was not reported as a side effect in Pentasa’s studies. But other medications used to treat ulcerative colitis (UC), such as prednisone and Remicade (infliximab), may cause weight gain.

It’s also possible that symptoms of UC may lead to weight gain. Symptoms of UC, such as fatigue (low energy) and a frequent need to use the bathroom, can make it difficult for people with this condition to exercise regularly.

Bloating is also a symptom of UC. With bloating, your abdomen fills with air and gas. It looks bigger than usual, and you may feel like you have gained weight.

Talk with your doctor if you’re concerned about gaining weight with Pentasa.

Is anxiety a possible side effect of Pentasa?

It’s not likely. Anxiety was not reported as a side effect in Pentasa’s studies.

Difficulty falling or staying asleep, which can be a symptom of anxiety, was reported in Pentasa’s studies. But this side effect was not common.

It’s possible to have anxiety related to UC. According to a 2021 review of studies, people with UC are more likely to have anxiety and depression than people without this condition. This is especially true during active UC. People with UC may have anxiety around their symptoms or flare-ups.

Let your doctor or another healthcare professional know if you experience anxiety or other mental health issues related to UC. They’ll help you find ways to manage your anxiety. They can also help you access professional mental health services if you need them.

Is there a higher risk of side effects with the Pentasa 500-mg capsules compared with the 250-mg capsules?

Not really. The side effects of the Pentasa 500-milligram (mg) capsules should be the same as those of the 250-mg capsules.

The dosage of Pentasa for UC is usually 1,000 mg taken four times per day. This means you take two 500-mg capsules per dose or four 250-mg capsules per dose.

If you have trouble swallowing pills, taking several capsules in one dose may cause discomfort or nausea. In this case, the 500-mg capsules may be easier to take. (You can also check out this article for helpful tips on taking pills.)

Talk with your doctor if you have concerns about side effects of Pentasa capsules.

Pentasa may not be right for you if you have certain medical conditions. (This is known as a drug-condition interaction.) Other factors may also affect whether Pentasa is a good treatment option for you. Talk with your doctor about your health history before starting Pentasa. The list below includes factors to consider.

Kidney problems: Pentasa can cause potentially serious kidney problems. If you already have kidney problems, Pentasa can make it worse. Your doctor will check your kidney function before you start Pentasa and then often throughout your treatment. If you develop kidney problems, they’ll likely have you stop taking Pentasa.

Allergic reaction: If you’ve had an allergic reaction to Pentasa or any of its ingredients, your doctor will likely not prescribe Pentasa for you. Ask them about other medications that might be better options.

Liver problems: Some people taking drugs containing mesalamine, the active ingredient in Pentasa, have reported liver failure. These people had a history of liver problems. Let your doctor know if you’ve had liver problems before you start taking Pentasa. They’ll determine if this medication is safe for you.

Serious skin reactions: Serious skin reactions, including Stevens-Johnson syndrome and toxic epidermal necrolysis, have been reported in people taking mesalamine. (This is the active ingredient in Pentasa.) Let your doctor know right away if you have any skin reactions while taking Pentasa. They’ll likely have you stop this medication if your skin reactions are serious.

Sun sensitivity: If you have certain skin conditions, such as eczema, Pentasa may cause your skin to be more sensitive to sunlight than usual. Avoid sun exposure or use sun protection when outdoors.

Kidney stones: Pentasa can cause kidney stones. Your risk is higher if you’ve had them in the past. Let your doctor know if you have a history of kidney stones. And be sure to drink plenty of water during your Pentasa treatment to reduce your risk of developing them.

Urine color changes: Some people taking Pentasa may have changes to their urine color. This can happen when urine comes in contact with water or surfaces that have been treated with a certain kind of bleach. But if your urine is an unusual color before it comes in contact with water or a surface (as soon as it leaves your body), talk with your doctor. This symptom wasn’t reported in studies, but it has occurred since the drug was approved.

Aspirin allergy: Pentasa is a kind of salicylate drug, like aspirin. You should not take Pentasa if you have an aspirin allergy.

Alcohol and Pentasa

There are no known interactions between Pentasa and alcohol. But drinking alcohol can cause ulcerative colitis symptoms to flare up. And alcohol can worsen certain Pentasa side effects, such as headache, dizziness, and nausea.

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

Pregnancy and breastfeeding while taking Pentasa

There aren’t enough studies to know whether Pentasa is safe to take during pregnancy. If you’re pregnant or planning to become pregnant, talk with your doctor before starting Pentasa treatment.

Mesalamine, the active drug in Pentasa, passes into breast milk in small amounts. Pentasa may be safe to take while breastfeeding. But some children who were breastfed by people taking mesalamine had diarrhea. Talk with your doctor about your options for feeding your child during your Pentasa treatment. If you choose to breastfeed while taking Pentasa, be sure to monitor your child for diarrhea.

Many common side effects of Pentasa are mild and go away within a few days or weeks. But it’s possible to have serious side effects from this drug. It’s important to talk with your doctor if you have questions or concerns about Pentasa’s side effects. Here are some examples of questions you can ask them:

  • Does my risk of side effects depend on the dosage of Pentasa I take?
  • How do the side effects of Pentasa compare with those of Lialda (mesalamine)?
  • Is my risk of diarrhea higher when I first start treatment?
  • Are there foods that may increase my risk of side effects with Pentasa?

To learn more about Pentasa, see these articles:

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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.