I'll see if anything on the ingredient list looks suspect, but everyone with IBS may have different sensitivities and even something that seems safe may not agree with everyone.How many calories a day do you eat? How hard would it be to up the portions of foods you know you tolerate to get to what you need to maintain + 500 calories a day.Typically most people tolerate Ensure and while not designed specifically for weight gain, it is designed for people who are quite ill and can't eat so tends to be on the gentler side than a lot of supplements designed for healthy people.potential issues17g of fat per serving of powder, may be too much for some, but may be OK, suggest making it with whole milk which ups the fat to 25 grams.Soy protein, depends on how many soy sugars they got out if it, may cause gas in anyoneFructose, if you are fructose intolerantBarley Malt, if you have gluten sensitivityCalcium and magnesium levels are low enough and balanced enough should be OK.Vitamin C levels low enough should not be too much of a problem