Enema formulations (Rowasa®) allow mesalamine to be applied directly to the left colon, reaching higher than the suppository alone. Up to 80 percent of patients with left-sided colon inflammation benefit from using this therapy once a day. Enemas are liquid and should remain in the colon for at least 20-40 minutes.

Source: http://www.ccfa.org/assets/pdfs/aminosalicylates.pdf