You should see a doctor for diarrhea if it lasts more than a few days, is accompanied by severe abdominal pain, high fever, blood in stool, or signs of dehydration like dizziness and dry mouth. Persistent diarrhea after starting antibiotics, such as cephalexin capsules, should also be evaluated, as it may indicate an adverse reaction or secondary infection. Children, older adults, and those with weakened immune systems are at higher risk and should seek care sooner. If you are sourcing medications, always ensure you buy from a trusted cephalexin capsules supplier to guarantee quality and safety.