MBBS., MD., DM
Welcome to icliniq.com.
Protein supplements do not cause black stools. Black stools, also known as melena, occur if there is bleeding in the upper gastrointestinal tract.
There are chances of black stools if you take medications like Pepto-Bismol or Iron supplements. So, if you are not on Pepto-Bismol or Iron supplements and if there are persistent black stools which is sticky, bulky and offensive, then it requires immediate attention.
Get an upper gastrointestinal endoscopy and share your reports.