Side effects from Cephalexin include - but are not limited to - diarrhea; dizziness; headache; indigestion; joint pain; stomach pain; tiredness. The drug can also cause yellowing of the eyes or skin; red, blistered, swollen or peeling skin; unusual bruising or bleeding; decreased urination; severe cramps; confusion. An allergic reaction to this medicine is unlikely, but seek immediate medical attention if it occurs. Symptoms of an allergic reaction include rash, itching, swelling, or trouble breathing.