can collagen peptides cause diarrhea？

Adding a quality control perspective here: poor-quality raw materials can absolutely cause GI distress.
If the manufacturer uses cheap, harsh chemical acids during the extraction process instead of a clean enzymatic hydrolysis, chemical residues or high levels of sodium/sulfites can be left behind in the powder. Our GI tracts are very sensitive to these impurities. This is why you must demand a full Certificate of Analysis (COA) from your bulk supplier to ensure the raw material is highly purified and free of heavy metals and harsh processing aids.

In my experience formulating for major brands, 90% of the time consumers complain about diarrhea, the collagen isn’t the culprit—the excipients are.
Many brands want to formulate “Keto” or “Sugar-Free” flavored collagen powders, so they load them up with sugar alcohols like erythritol, xylitol, or maltitol. Those artificial sweeteners are notorious for causing severe GI distress and laxative effects. If you want a clean label product that is gentle on the stomach, stick to pure unflavored peptides or use natural sweeteners like stevia or monk fruit.

Yes, it can happen, but it’s usually dose-dependent. Because true hydrolyzed collagen peptides have extremely high bioavailability, they are absorbed very rapidly in the small intestine. If a consumer takes a massive dose (like 30-40g) on an empty stomach, it can draw excess water into the digestive tract, causing an osmotic effect which leads to loose stools or diarrhea. Advising consumers to stick to a standard 10-15g daily dose usually solves this immediately.