The most common adverse effects of cefalexin, like other oral cephalosporins, are gastrointestinal (stomach area) disturbances and hypersensitivity reactions.  Gastrointestinal disturbances include nausea, vomiting, and diarrhea, the latter being the most common.  Hypersensitivity reactions include skin rashes, urticaria, fever, and anaphylaxis. Pseudomembranous colitis and Clostridium difficile have been reported with use of cefalexin. Less common and more serious side effects include bruising of the skin and yellowing of the skin or eye whites.
Signs and symptoms of an allergic reaction include rash, itching, swelling, trouble breathing, or red, blistered, swollen, or peeling skin. Overall, cefalexin allergy occurs in less than 0.1% of patients. Evidence suggests that it is seen in 1% to 10% of patients with a penicillin allergy.