Side Effects. Diarrhea, nausea, vomiting, or upset stomach may occur. If any of these effects persist or worsen, tell your doctor or pharmacist promptly. Find patient medical information for Cephalexin Oral on WebMD including its uses, side effects and safety, interactions, pictures, warnings and user ratings. You could be at higher risk for side effects from cephalexin if you have a history of liver, kidney, or colon disease, so tell your doctor about any of these conditions as well. Along with its needed effects, cephalexin may cause some unwanted effects. Although not all of these side effects may occur, if they do occur they may need medical attention. Although not all of these side effects may occur, if they do occur they may need medical attention. Cephalexin (Keflex, Keftabs) is an an antibiotic used to treat bacterial infections of the sinuses, tooth, urinary tract, and throat. Common side effects of Keflex include diarrhea, nausea, abdominal pain, headaches, and vaginitis. Dosage of cephalexin depends on the type of infection. Cephalexin treats infections caused by bacteria, including ear infections, skin infections, and urinary tract infections. Includes cephalexin side effects interactions and indications. Includes cephalexin side effects interactions and indications. Drug Information on cephalexin includes drug pictures, side effects, drug interactions, directions for use, symptoms of overdose, and what to avoid. Cephalexin is a drug from the group of cephalosporin antibiotics. Cephalexin works by killing bacteria, and therefore is used to treat infections. Cephalexin oral capsule is available as a generic drug and as a brand-name drug. Brand-name: Keflex. Cephalexin also comes as a tablet or liquid suspension that you take by mouth. Cephalexin may cause side effects. Tell your doctor if any of these symptoms are severe or do not go away: Tell your doctor if any of these symptoms are severe or do not go away: nausea.