The gastrointestinal tract digests and absorbs dietary nutrients, protects the body against physical and chemical damage from contents in its lumen, provides immunity against external antigens, and keeps an optimum environment for the gut microbiota. These functions cannot be performed normally in several diseases of which the following are discussed here: irritable bowel syndrome and inflammatory bowel disease, which includes Crohn's disease and ulcerative colitis. Because these diseases are associated with oxidative stress, a host of antioxidant supplements are used for maintenance and recovery of the gut functions. However, the benefits of these supplements have not been established. The available 80 human trials were rated for levels of confidence and for benefits of the antioxidant supplements. For Crohn's disease, the supplements for which clear benefits occurred in at least 2 studies were allopurinol, Boswellia serrata (frankincense or shallaki), Artemesia species (wormwood), Tripterygium wilfordii (léi g?ng téng), and omega-3 fatty acids. Similar beneficial supplements for ulcerative colitis were allopurinol, Matricaria chamomilla (chamomile), Curcuma longa (curcumin in turmeric), and omega-3 fatty acids. There was also a clear benefit for ulcerative colitis in 2 studies where a multiherbal Chinese medicine preparation and an Ayurvedic medicine preparation were used. For irritable bowel syndrome, there was only a marginal benefit of some of the antioxidant supplements. Thus, some antioxidant supplements may be beneficial at certain stages of specific diseases. This is consistent with the current concept that antioxidants act by inhibiting oxidative stress pathways in a tissue- and environment-specific manner and not by simply acting as scavengers.