No, normally Grapefruit-Seed-Extract don't cause diarrhea. The diarrehea could be food intolerance or die off reaction. Grapefruit-Seed-Extract is consider safe against freindly flora and active against many pathogens which cause intestinal dysbiosis. Some people in this forum have tested Grapefruit-Seed-Extract using yogurt cultures, and it allow the yogurt to grow without any problem. On the other hand, caprilic acid kill the yogurt's bacterias.