Studies have shown that IBS may be caused by the bodies own inflammatory response. The body releases histamine as part of the inflammatory response to pathogens. Those with IBS have been shown to have a higher release of histamine in the gut. It is possible that those with an autoimmune disorder may have an autoimmune system that attacks the body causing an inflammatory response. If the body then has an overactive inflammatory response it can make things even worse.
A study involving Ketotifen, a non-prescription antihistamine, showed promising results for those with IBS. Many of the participants participants reported improvement of their symptoms after taking it. Ketotifen is not easily available in the United States. Given that it is in the class of “second generation” antihistamines you can look for a substitute that is in the same class such as Loratadine(Claritin) or Cetirizine(Zyrtec). These products should also be available at your local store or pharmacy without the need to obtain a prescription first. You will probably want to ensure that there are no other active ingredients since some allergy medications include a stimulant of some sort. “Second generation” antihistamines do not pass the blood brain barrier and thus they are less likely to cause drowsiness.