Risk factor of cholelithiasis is rapid weight loss after bariatric surgery and change in the gallbladder function which is secondary to surgery. Many observational studies support this fact, and a high incidence of 28% -71% after gastric bypass surgery (RYGB) was reported in them. According to another publication, the incidence of gallbladder diseases is 5-36% after jejunoileal bypass surgery and 2.8-36% after gastric bypass surgery.

Study format: Prospective, randomized, blinded study. 266 candidates for sleeve gastrectomy or gastric bypass surgery due to morbid obesity with BMI range of 40-50 and aged 18-65 will be studied.

The patients will be randomly divided into two groups (according to the last digit of the identity card number):

1. Treatment group will receive preventive therapy with Ursodiol 600 mg per day as of day 10 after the surgery (at the first routine visit after the surgery) and for six months or until the formation of gallstones.
2. Placebo group will receive placebo as of day 10 after the surgery (at the first routine visit after the surgery) and for six months or until the formation of gallstones.

Inclusion Criteria:

• Candidates for sleeve gastrectomy or gastric bypass surgery due to morbid obesity with BMI range of 40 to50.
• Aged 18 to 65.
• No presence of gallstones.
• Patients who have signed the consent form.

Exclusion Criteria:

• Minors,
• Pregnant women,
• Age over 65,
• Sensitive to Ursodiol,
• Gallstones,
• Biliary tract pathologies,
• Gallbladder wall thickening,
• Patients after cholecystectomy,
• Patients participating in another clinical trial.