Hello, I m sorry you are having this. Cephalexin is a broad spectrum antibiotic. All antibiotics, especially broad-spectrum ones, can alter the microbiome of bacteria in the gut (kill the good bacteria along with the bad) and thereby cause diarrhoea. I recommend: Take a good quality probiotic (not at the same time as the antibiotic as the antibiotic will just kill it - take a couple hours apart). Call the doctor who prescribed the cephalexin to discuss discontinuing it. I would NOT recommend a different antibiotic at this point because that would likely make diarrhea worse. If your diarrhoea does not improve off of the antibiotic and with probiotics, in a few days, then you should be tested for an opportunist bacteria called c. dificile. Hope I have answered your query. Let me know if I can assist you further. Take care Regards, Dr Bonnie Berger-Durnbaugh, General & Family Physician