Diphenhydramine is an antihistamine used for treating allergic reactions. Histamine is released by the body during several types of allergic reactions and--to a lesser extent--during some viral infections, such as the common cold.When histamine binds to receptors on cells, it stimulates changes within the cells that lead to the release of chemicals that cause sneezing, itching, and increased. Benadryl (diphenhydramine) is an antihistamine that reduces the effects of natural chemical histamine in the body. Histamine can produce symptoms of sneezing, itching, watery eyes, and runny nose. Benadryl is used to treat sneezing, runny nose, watery eyes, hives, skin rash, itching, and other cold or allergy symptoms. Benadryl (diphenhydramine) is a brand-name, over-the-counter product that's typically used to relieve symptoms of hay fever (seasonal allergies), other allergies, and the common cold. It's also. Find information about which conditions Benadryl oral is commonly used to treat. Taking Benadryl to help you get to sleep is okay in small doses once in a while ― but, again, it doesn’t lead to better quality sleep, Elliott said. Because antihistamines can peer pressure your body to spend more time in the lighter stages of sleep, the eight hours you clock in bed might end up feeling more like five. Benadryl (diphenhydramine) has a drying effect so over time it can cause constipation, urinary retention, dry mouth, etc. Talk to your doctor about what sleep remedy might be best for you. Medicines that interact with Benadryl may either decrease its effect, affect how long it works for, increase side effects, or have less of an effect when taken with Benadryl. An interaction between two medications does not always mean that you must stop taking one of the medications; however, sometimes it does. Speak to your doctor about how drug interactions should be managed. Does Benadryl interact with other medications? Find Lowest Prices. Read More. Overdose. Overdose. If someone has overdosed and has serious symptoms such as passing out or trouble breathing, call.