Fish oil (FO) and mesalazine have well-known anti-inflammatory and antioxidant effects; on the other hand, information related to combined intrarectal administration of FO and mesalazine is limited.

The present study was conducted to make comparison on therapeutic effectiveness of rectally administered FO and mesalazine in rats with trinitrobenzenesulfonic acid (TNBS)-induced colitis.

Wistar rats were randomly assigned to 5 groups as (1) Control, (2) Colitis, (3) Colitis + Mesalazine (Colitis + M), (4) Colitis + Fish Oil (Colitis + F), and (5) Colitis + Mesalazine + Fish Oil (Colitis + M + F). Intrarectally administered TNBS induced colitis. At the end of the trial, the rats’ macroscopic and histopathologic lesions were rated and tumour necrosis factor (TNF)-α, Interleukin 6 (IL6), glutathione reductase (GR), glutathione peroxidase (GP), myeloperoxidase (MPO), malondialdehyde (MDA), Superoxide dismutase (SOD), Total nitrate and nitrite, and catalase (CAT) in serum and tissue were detected.

As a result of macroscopic and microscopic examination, although separate administrations of FO and mesalazine partly decreased the damage, their combined administration decreased the damage scores significantly (p < 0.01). It was observed that separate and combined administrations of FO and mesalazine decreased the increase in the serum and tissue TNF-α and IL-6 levels caused by colitis (p < 0.05). It was observed that the serum MPO, serum GR, tissue SOD, tissue nitrite/nitrate values of both Colitis + M and Colitis + F groups were close to the control in terms of all the parameter values in Colitis + M + F group (p > 0.05). Also based on the histological results, the inflammation damage in the tissue caused by colitis in the Colitis + M + F group recovered significantly.

We found that microscopic and macroscopic damage, serum IL-6 level decreased and increased serum and tissue GP and tissue GR values in Colitis + M + F group compared to Colitis + M and Colitis + F groups. Combined intrarectal administration of FO and mesalazine may bring a new insight concerning the treatment of ulcerative colitis.