I am prone to IBS and am on my third sub cut injection for herceptin. So far I have had no diarrhoea. On my fourth injection (I need 17) I will have a better idea of what herceptin's side effects are as I have finished chemo now so it will just be herceptin on its own. I do take two multiflora tablets a day, am. and pm., and Iberogast, 20 drops in the morning for gut health ... given what chemo does to your gastrointestinal lining. With chemo and herceptin my issue has mostly been constipation. Odd, as if anyone was going to get diarrhoea I would have picked me!