hello there..thanks for writing to HCM. I read your history and understand your concern. Yes,the lack of food intake which might have resulted in hypoglycemia or low blood sugar caused you to faint and nothing to do with the medicine or sinus infection the medicine,keflex is cephalexin,an antibiotic which is usually prescribed for such infections and side effects are pretty rare.known severe side effect is a bacteria assciated diarrhea(clostridium difficile),that too is very rare. please ensure proper food and fluid intake before you consume medications and avoid alcohol and tobacco.take care.hope the clarification helps