Asacol (mesalamine) is an anti-inflammatory drug used to treat ulcerative colitis, proctitis, and proctosigmoiditis and is also used to prevent the symptoms of ulcerative colitis from recurring. Common side effects of Asacol include:

mild nausea,
vomiting,
stomach cramps,
diarrhea,
gas,
fever,
sore throat,
flu-like symptoms,
constipation,
headache,
dizziness,
tired feeling, or
skin rash.
Before using this or any other medication, it is recommended to consult with a doctor or healthcare provider.

Used as an anti-inflammatory agent for ulcerative colitis; not considered effective for Crohn’s disease.
Also used for ulcerative proctitis (rectal formulations, suppository, and enema only).