Side Effects of Asacol (Mesalamine)

Other brands include Pentasa, Mesasal, and Salofalk

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Asacol (mesalamine) is a 5-aminosalicylic acid (5-ASA) drug that was approved in August 1997 for the treatment of ulcerative colitis. Asacol acts topically on the intestines, suppressing the inflammation that is caused by ulcerative colitis—one of the main forms of inflammatory bowel disease (IBD).

Asacol was previously also sometimes used to treat Crohn's disease, another type of IBD. However, it has since been studied more closely and IBD specialists now believe that mesalamine is not as effective for this purpose. As a result, it is no longer recommended for routine use in treating this form of IBD. Mesalamine can be effective when Crohn's disease involves the colon or large intestine, however.

Asacol comes in various forms, including oral, suppositories, and liquid or foam enemas (rectal suspension). Because it acts topically, when the ulcerative colitis is affecting the last parts of the large intestine, the rectum, and the sigmoid colon, using a suppository or an enema may be helpful.

Topical forms are not without the potential for side effects or adverse effects, so it's worth having an understanding of what those might entail.

Asacol tablets

Asacol Is a Maintenance Drug

Asacol is classified as a "maintenance" drug, which means that it is given on a long-term basis to treat ulcerative colitis. Asacol can also be used as a first line treatment for acute mild ulcerative colitis.

Ulcerative colitis causes inflammation in the colon, among other signs and symptoms throughout the body. Asacol is used to help stop the inflammation that is present in the colon and typically starts to take effect in about two to three weeks. After inflammation is under control, patients are prescribed this drug to help prevent more flare-ups of the disease from occurring.

However, for adults with moderate to severe ulcerative colitis who have achieved remission on a biologic (e.g., Humira), small molecule (e.g., Xeljanz), or an immunomodulator medication, 2020 guidelines recommend against the use of Asacol for maintenance treatment.

This maintenance drug is known to have a relatively low incidence of side effects, although some are still possible. The following is a list of the potential side effects and adverse effects of Asacol.

Common Side Effects

Check with your healthcare provider if any of these side effects continue or are bothersome:

  • Abdominal or stomach cramps or pain (mild)
  • Diarrhea (mild)
  • Dizziness; headache (mild)
  • Runny or stuffy nose or sneezing

Uncommon Side Effects

Check with your healthcare provider if any of these side effects continue or are bothersome:

  • Acne
  • Back or joint pain
  • Gas or flatulence
  • Indigestion
  • Loss of appetite
  • Loss of hair

When to Seek Medical Attention

These side effects are less common, but warrant an urgent call to your healthcare provider:

  • Abdominal or stomach cramps or pain (severe)
  • Bloody diarrhea
  • Fever
  • Headache (severe)
  • Skin rash and itching

Seek emergency medical care right away if you experience:

  • Anxiety
  • Back or stomach pain (severe)
  • Blue or pale skin
  • Chest pain, possibly moving to the left arm, neck, or shoulder
  • Chills
  • Fast heartbeat
  • Nausea or vomiting
  • Shortness of breath
  • Swelling of the stomach
  • Unusual tiredness or weakness
  • Yellow eyes or skin

Symptoms of an Overdose

To avoid a greater risk of side effects and the possibility of an overdose, follow your prescriber's instructions.

Symptoms of overdose, which warrant immediate medical attention, include:

  • Confusion
  • Diarrhea (severe or continuing)
  • Dizziness or lightheadedness
  • Drowsiness (severe)
  • Fast or deep breathing
  • Headache (severe or continuing)
  • Hearing loss or ringing or buzzing in ears (continuing)
  • Nausea or vomiting (continuing)


Asacol is considered to be relatively safe, but people who take this drug should be aware that it has been associated with some adverse events. Some people have developed problems with their kidneys, and it's recommended that people taking this drug have their kidney function checked every so often.

Some people have also had adverse reactions that mimic the symptoms of a flare-up of ulcerative colitis, which include diarrhea, headache, and abdominal pain.

Allergic reactions are also possible. Your prescribing healthcare provider should be told about any previous allergic reactions to mesalamine or to sulfasalazine.

A Word From Verywell

Asacol is generally tolerated very well by most people, and it is often used long-term by those who have ulcerative colitis. However, as with any drug, there is the potential for adverse effects and side effects. For most people, the side effects are mild, but people taking Asacol should be aware of the potential for an allergic reaction, an adverse effect on the kidneys, or a worsening of diarrhea and abdominal pain.

Other side effects not listed above may also occur in some patients. If you notice any other effects, check with your healthcare provider. This information is meant only as a guideline. Always consult a healthcare provider or pharmacist for complete information about prescription medications.

1 Source
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Feuerstein JD, Isaacs KL, Schneider Y, et al. AGA Clinical Practice Guidelines on the Management of Moderate to Severe Ulcerative Colitis. Gastroenterology. 2020. 158(5):1450-1461. doi:10.1053/j.gastro.2020.01.006

Additional Reading

By Amber J. Tresca
Amber J. Tresca is a freelance writer and speaker who covers digestive conditions, including IBD. She was diagnosed with ulcerative colitis at age 16.