Benadryl products contain the first-generation antihistamine diphenhydramine. Diphenhydramineis used as an allergy treatment in various forms around the world. SIDE EFFECTS: Diphenhydramine can commonly cause sedation, tiredness, sleepiness, dizziness, disturbed coordination, drying and thickening of oral and other respiratory secretions, and stomach distress. Diphenhydramine may also cause low blood pressure,palpitation, increased heart rate, confusion, nervousness, irritability, blurred vision, double vision, tremor, loss or appetite, or nausea. Diphenhydramine should be used with caution (if at all) in persons with narrow-angle glaucoma, PROSTATIC HYPERTROPHY (enlarged prostate gland), hyperthyroidism, cardiovascular disease,hypertensin and asthma.