The allergy medication Benadryl typically stays in the system for four to six hours. Most products instruct users to take the stated dosage every four to six hours as needed, while not exceeding six doses in a single day, per instructions on the package label.
The antihistamine provides temporary relief from seasonal allergies, hay fever, allergic reactions and other upper respiratory allergies and is also effective in treating runny nose and itchy or watery eyes.
Benadryl is available in capsule, tablet and liquid form and as a topical analgesic.
Common side effects of the drug include sedation, tiredness, sleepiness, dizziness, disturbed coordination, drying and thickening of oral and other respiratory secretions and stomach distress, according to the medical experts at rxlist.com.