Diarrhea after drinking protein shakes can be caused by an intolerance to certain ingredients. Many common ingredients in protein shakes, such as milk and lactose, can cause digestive issues in those who are intolerant or sensitive to them. Other causes include food sensitivities, artificial sweeteners, additives, sugar alcohols like sorbitol or mannitol, and high-fiber content. If you suspect your diarrhea is caused by a food sensitivity or intolerance to one of these ingredients try switching brands of shake or look for a hypoallergenic product with minimal ingredients that may not contain the ingredient you’re intolerant too. Additionally, consuming large amounts of carbohydrates alongside your shake may contribute to temporary diarrhea due to osmotic pressure created when concentrated foods enter the gastrointestinal tract quickly. In this case limiting carbohydrate intake before and/or during consumption of a shake will likely remedy the issue. Finally if none of these remedies help then it might be beneficial to consult with a doctor about further testing for Irritable Bowel Syndrome (IBS).