Side Effects of Asacol (Oral Mesalamine, Pentasa, Mesasal, Salofalk)

Asacol tablets
Asacol is a drug that is used to treat ulcerative colitis, one form of inflammatory bowel disease (IBD). Image © Pillbox

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, which is one of the main forms of inflammatory bowel disease (IBD). Asacol was previously also sometimes used to treat another type of IBD, Crohn's disease. However, it has since been studied more closely and IBD specialists now believe that mesalamine is not effective for treating Crohn's disease, and it is no longer recommended for routine use in treating this form of IBD. 

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 small 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 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. 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 3 weeks. After inflammation is under control, patients are prescribed this drug to help prevent more flare-ups of the disease from occurring.

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

Common Side Effects

Check with your doctor if any of the following side effects continue or are bothersome:

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

Less Frequent or Rare

Check with your doctor if any of the following side effects continue or are bothersome:

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

Always Notify Doctor

Less common

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


  • 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

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

Warnings With Asacol

Asacol is considered to be relatively safe, but it has been associated with some adverse events that people who take this drug should be aware. 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 includes diarrhea, headache, and abdominal pain. Allergic reactions are also possible, and the prescribing physician 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. For any concerns about the side effects of Asacol, contact your doctor.

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

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