If you have ulcerative proctitis, which is a type of ulcerative colitis, your doctor might suggest Canasa as a treatment option. It’s a prescription drug used to treat mild to moderately active ulcerative proctitis in adults.

Canasa is typically used short term to treat ulcerative proctitis. It’s unknown whether it’s safe or effective to use the drug for more than 6 weeks for this condition.

The active ingredient in Canasa is mesalamine. (The active ingredient is what makes the drug work.) Mesalamine is also the generic version of Canasa.

Canasa comes as a suppository that is inserted rectally, with a strength of 1,000 milligrams (mg), but not as an enema. For more information about Canasa, including details about its uses, see this in-depth article.

Like other drugs, Canasa can cause mild to serious side effects. Keep reading to learn more.

Some people may experience mild to serious side effects during their Canasa treatment. Examples of the drug’s commonly reported side effects include:

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

Keep in mind that not everyone who uses Canasa will have these side effects. And there are other mild to serious side effects Canasa may cause which are less common. These are discussed in more detail in the following sections.

Like most drugs, Canasa can cause mild side effects. Examples that have been reported 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 manageable. But if you have any symptoms that are ongoing or bother you, talk with your doctor or pharmacist. And do not stop using Canasa unless your doctor recommends it.

Canasa 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 Canasa, visit MedWatch.

Serious side effects are possible with Canasa. Some serious side effects reported with Canasa only occurred in people with certain medical conditions. To learn more, see the “Warnings” section below.

Serious side effects that have been reported with Canasa include:

If you develop serious side effects while using Canasa, 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.

Get answers to some common questions about Canasa’s side effects.

Can Canasa cause weight-related side effects?

No, Canasa isn’t expected to cause weight-related side effects, such as weight gain or weight loss. These were not reported in the drug’s studies.

It’s common to have weight loss caused by ulcerative proctitis, which Canasa is used to treat.

And some medications used to treat ulcerative proctitis may cause weight gain. This includes corticosteroids such as Rayos (prednisone).

But neither weight gain nor weight loss is expected with Canasa.

Does Canasa cause hair loss?

It’s possible. Hair loss was not reported in Canasa’s studies but has been reported in people using the drug since it was approved for use. But because these reports were outside the studies, it’s unknown whether Canasa or other factors caused the hair loss.

Studies have found hair loss is common in people with inflammatory bowel disease (IBD), including ulcerative proctitis. (Canasa is prescribed to treat ulcerative proctitis.) But scientists aren’t exactly sure why this is. These studies found that hair loss was less likely among people treated with drugs containing mesalamine. Canasa contains mesalamine as its active drug (the ingredient that makes it work).

If you’re concerned about hair loss, talk with your doctor.

Can Canasa cause constipation?

Canasa isn’t expected to cause constipation, but it’s possible. This wasn’t a side effect reported in studies but has been reported since the drug was approved for use. But because these reports were outside the studies, it’s unknown whether Canasa or other factors caused the constipation.

Constipation can be a side effect of ulcerative proctitis, which is the kind of ulcerative colitis Canasa is used to treat. Since Canasa can help relieve symptoms of ulcerative proctitis, it may improve constipation.

If you have constipation, talk with your doctor. They can suggest a treatment, such as an over-the-counter laxative, including Metamucil (psyllium) or MiraLax (polyethylene glycol). Be sure to speak with your doctor or pharmacist before taking a laxative so they can ensure it’s safe for you.

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

Acute intolerance syndrome

Treatment with Canasa can cause a side effect called acute intolerance syndrome. This is a kind of allergic reaction drugs containing mesalamine may cause. (Canasa contains mesalamine as the active drug. This is the ingredient that makes Canasa work.)

It is unknown how often this side effect happened in Canasa’s studies. But it has been reported in other studies of mesalamine-containing drugs.

Symptoms of acute intolerance syndrome may include:

These symptoms may occur suddenly and, in some cases, can be hard to tell apart from worsening ulcerative proctitis symptoms.

What might help

Contact your doctor if you notice symptoms listed above that may indicate acute intolerance syndrome. Your doctor will likely want to see you to investigate your symptoms further. They’ll likely recommend you stop using Canasa if they suspect or confirm you’re having acute intolerance syndrome as a side effect.

Severe skin reactions

Treatment with Canasa can cause severe skin reactions. These are a kind of allergic reaction, which can be life threatening and require emergency medical care.

Severe skin reactions have been reported in people using drugs that contain mesalamine. While these reactions weren’t reported in Canasa’s studies, Canasa contains mesalamine, so they are possible.

Severe skin reactions reported with mesalamine use include:

Symptoms of these reactions can include:

What might help

These skin reactions are life threatening and often require treatment in the hospital. If you notice symptoms of a severe skin reaction while using Canasa, immediately contact your doctor or seek emergency medical care.

Your doctor will likely have you stop using Canasa if you develop a severe skin reaction. In this situation, they can discuss alternative ulcerative proctitis treatments with you.


Treatment with Canasa may cause acne. Acne has also been linked to ulcerative proctitis, which Canasa is used to treat.

What might help

Talk with your doctor if you develop acne while using Canasa. There are many treatments for acne, and your doctor can recommend the best one for you. For example, they may recommend using topical over-the-counter drugs, such as salicylic acid or benzoyl peroxide.

Allergic reaction

Like most drugs, Canasa can cause an allergic reaction in some people. While it doesn’t appear that allergic reactions occurred in the drug’s studies, there have been rare reports since Canasa was approved for use.

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 over-the-counter antihistamine you take by mouth, such as Benadryl (diphenhydramine)
  • a topical product you apply to your skin, such as hydrocortisone cream

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

Keeping track of side effects

During your Canasa 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 a new drug or combination of treatments.

Your side effect notes can include things such as:

  • what dose of the drug you were using 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 Canasa affects you. They can then use this information to adjust your treatment plan if needed.

Canasa 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 this drug is a good treatment option for you. Talk with your doctor about your health history before starting Canasa. Below are some factors to consider.

Kidney problems. Treatment with Canasa can cause or worsen kidney problems. Be sure to talk with your doctor if you have kidney problems, such as chronic kidney disease, before starting Canasa. Your doctor can help determine whether this drug is safe for you. If your doctor prescribes this drug for you, they may also monitor your kidney health closely.

Liver problems. Canasa can cause liver failure in people with liver problems, such as hepatitis. Your doctor can help determine whether Canasa is safe for you.

Skin condition, such as eczema. Treatment with Canasa can cause your skin to be more sensitive to sunlight if you have an existing skin condition such as eczema. It’s usually safe for people with skin conditions to use Canasa, but you should limit the amount of time you expose your skin to the sun. For example, try wearing long-sleeve shirts to protect your arms or a hat to protect your face. You should also use sunscreen whenever you’re outside in the sunlight.

Heart problems. Treatment with Canasa can cause heart problems, including pericarditis (inflammation of the sac around your heart). Your doctor can help determine whether Canasa is safe for you.

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

Alcohol and Canasa

There’s no direct interaction between alcohol and Canasa, but each of them can cause dizziness. Combining the two may increase your risk of this side effect. And some studies have shown drinking alcohol may worsen ulcerative proctitis symptoms in some people.

If you consume alcohol, talk with your doctor about how much is safe to drink while using Canasa.

Pregnancy and breastfeeding while using Canasa

It’s not known if it’s safe to use Canasa while pregnant.

Canasa is present in small amounts in human breast milk. And there are limited reports of diarrhea in breastfed children when the person carrying the child was using mesalamine (the active drug in Canasa).

Talk with your doctor to learn more about treatment options for ulcerative proctitis if you’re pregnant or breastfeeding.

If you have ulcerative proctitis, your doctor may recommend Canasa as a treatment for your condition. Ask them questions to help you feel comfortable about the drug, including side effects it may cause and your risk of having them. Some examples to get you started are:

  • Do any medications I take increase my risk of side effects from Canasa?
  • If I’ve had kidney stones in the past, is it safe for me to use this drug?
  • If I spend a lot of time in the sun, should I be worried about using Canasa?

To connect with others living with inflammatory bowel disease (IBD), such as ulcerative proctitis, join the Bezzy IBD community.

For news on treatments and tips for managing your condition, sign up for Healthline’s IBD newsletter.


Do older adults have a higher risk of side effects from Canasa?



Adults ages 65 years and older have an increased risk of blood disorders, a side effect Canasa may cause. Studies of the drug did not include enough older adults to say whether side effects are more likely in this population. But since the drug was approved for use, reports suggest older adults are more likely to have blood disorders from using Canasa and other drugs that contain mesalamine. (Mesalamine is the active drug in Canasa.)

These blood disorders include low levels of the following:

Due to this risk, doctors usually want to monitor blood counts closely when older adults use Canasa. They do this by ordering frequent blood tests to check blood cell counts throughout treatment. This helps them catch any blood disorders early when they’re easier to treat and less likely to cause symptoms or problems.

Older adults may also have a higher risk of liver, kidney, or heart problems when using Canasa. If you’re an older adult considering treatment with Canasa, talk with your doctor to determine if the drug is safe for you.

The Healthline Pharmacist TeamAnswers represent the opinions of our medical experts. All content is strictly informational and should not be considered medical advice.
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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.