Pentasa XR Side Effects: What to Expect and When to Call Your Doctor

Pentasa XR (mesalamine extended-release capsules) is generally well tolerated, but like all medications, it can cause side effects. Most people who take Pentasa XR experience only mild side effects, if any. However, some serious side effects require prompt medical attention. Here's a clear breakdown of what to expect, what to watch for, and when to call your doctor.

Common Side Effects of Pentasa XR

In clinical trials involving over 2,100 patients, the most common side effects occurring in 1% or more of Pentasa XR patients included:

• Diarrhea (3%): Ironically, since Pentasa XR is used to treat UC-related diarrhea, new or worsening diarrhea during treatment can be hard to distinguish from a disease flare. If diarrhea is new or worsening on treatment, contact your doctor.
• Headache (2%): Usually mild and may improve as your body adjusts to the medication.
• Nausea (2%): Taking Pentasa XR with food or water can help reduce nausea.
• Abdominal pain (2%): Mild abdominal discomfort is common, especially early in treatment.
• Dyspepsia / indigestion (2%)
• Vomiting (2%)
• Rash (1%): Mild skin rash. Note that Pentasa XR can increase sun sensitivity — use sunscreen and protective clothing when outdoors.

Serious Side Effects: When to Seek Immediate Help

While rare, Pentasa XR can cause serious side effects that require prompt evaluation. Stop taking Pentasa XR and contact your doctor immediately or go to an emergency room if you experience any of the following:

• Mesalamine-induced acute intolerance syndrome: Sudden cramping, severe abdominal pain, bloody diarrhea, fever, headache, and rash — can be mistaken for a UC flare but is actually caused by the medication itself. Stop Pentasa XR immediately and call your doctor.
• Kidney problems: Symptoms include decreased urination, swelling in legs or face, unusual fatigue, or blood in urine. Mesalamine can cause interstitial nephritis and, rarely, renal failure. Kidney function should be checked before starting and monitored regularly.
• Hypersensitivity / allergic reactions: Difficulty breathing, rapid heart rate, hives, or swelling of face, throat, or tongue. This can indicate a severe allergic reaction requiring emergency care. Reactions can involve organs including the heart (myocarditis, pericarditis), lungs (pneumonitis), or liver (hepatitis).
• Severe skin reactions: Stevens-Johnson syndrome (SJS), toxic epidermal necrolysis (TEN), DRESS syndrome, and AGEP have been reported with mesalamine use. Stop the medication at the first sign of blistering, peeling, or widespread rash.
• Liver failure: Reported in patients with pre-existing liver disease. Symptoms include yellowing of skin/eyes (jaundice), dark urine, or right-sided abdominal pain.
• Blood problems (in elderly patients 65+): Higher rates of agranulocytosis, neutropenia, and pancytopenia reported in patients over 65. Regular CBC monitoring is recommended in this population.
• Kidney stones: Mesalamine-content kidney stones have been reported. They are radiotransparent and may not be visible on standard X-ray or CT. Symptoms include severe flank pain or blood in urine.

Important Monitoring Requirements

Your doctor should check your kidney function (creatinine, BUN) before you start Pentasa XR and periodically while you are on it. If you are over 65, blood cell counts should also be monitored. Patients with pre-existing kidney or liver disease need extra caution.

Sun Sensitivity

Pentasa XR can increase your skin's sensitivity to sunlight. Wear sunscreen (SPF 30+), protective clothing, and avoid prolonged sun exposure while taking it. Patients with pre-existing skin conditions like atopic dermatitis or eczema may experience more severe photosensitivity reactions. For information on drug interactions that can worsen side effects, see: Pentasa XR drug interactions

The Bottom Line

Most people tolerate Pentasa XR well, with only mild, manageable side effects. The serious side effects are rare but can be severe — knowing the warning signs allows you to act quickly. Always keep your prescriber informed about any new or unusual symptoms. And if you're having trouble finding Pentasa XR at your pharmacy, medfinder.com can help you locate it near you.