gastritis. In cystic fibrosis there is insufficient evidence to justify routine use of UDCA, especially as there is a lack of available data for long-term outcomes such as death or need for liver transplantation. UDCA has also been in effective in non-alcoholic fatty liver disease, in liver bile duct-paucity syndromes. It is contraindicated in obstruction of biliary tracts such as biliary atresia. It's not effective in liver allograft rejection, and in Graft-versus-host disease involving the liver. Adverse effects Diarrhea was the most frequent adverse event seen in trial of UDCA in gallstone dissolution, occurring in 2 to 9%, which is less frequent than with chenodeoxycholic acid therapy. Bacterial conversion of UDCA to chenodeoxycholic acid may be the mechanism for this side effect. Right upper quadrant abdominal pain and exacerbation of pruritus was occasionally reported in trials in patients with PBC. Additional symptoms may include bloating, weight gain, and occasionally, thinning of hair. Mechanisms of action Choleretic effects Primary bile acids are produced by the liver and stored in the gall bladder. When secreted into the intestine, primary bile acids can be metabolized into secondary bile acids by intestinal bacteria. Primary and secondary bile acids help the body digest fats. Ursodeoxycholic acid helps regulate cholesterol by reducing the rate at which the intestine absorbs cholesterol molecules while breaking up micelles containing cholesterol. The drug reduces cholesterol absorption and is used to dissolve (cholesterol) gallstones in patients who want an alternative to surgery. There are multiple mechanisms involved in cholestatic liver diseases. Immunomodulating effects Ursodeoxycholic acid has also been shown experimentally to suppress immune response such as immune cell phagocytosis. Prolonged exposure and/or increased quantities of systemic (throughout the body, not just in the digestive system) ursodeoxycholic acid can be toxic. Anti-inflammatory effects Ursodeoxycholic acid has been shown to exert anti-inflammatory and protective effects in human epithelial cells of the gastrointestinal tract. It has been linked to regulation of immunoregulatory responses by regulation of cytokines, antimicrobial peptides defensins, and take an active part in increased restitution of wound in the colon. Moreover, UDCA's effects has been shown to have exert actions outside the epithelial cells. While some bile acids are known to be colon tumor promoters (e.g. deoxycholic acid), others such as ursodeoxycholic acid are thought to be chemopreventive, perhaps by inducing cellular differentiation and/or cellular senescence in colon epithelial cells. Chemistry Ursodeoxycholic acid is an epimer of chenodeoxycholic acid, which has