A single dose of Diphenhydramine hydrochloride, commonly known as Benadryl, is active in the body for between four and six hours on average. The drug is almost completely excreted from the body within 24 hours, as explained by Drugs.com
A single dose of Diphenhydramine hydrochloride, the primary active ingredient in Benadryl, is a water soluble, odorless, white crystalline powder. Diphenhydramine hydrochloride is absorbed by the body and reaches maximum activity within one hour, according to Drugs.com. The drug is processed in the liver and excreted through urine within approximately 24 hours.
Diphenhydramine hydrochloride is most commonly used as an antihistamine, as a treatment for motion sickness, as a sleep aid and to ease the effects of mild Parkinson's disease. The average dose for an adult when used for these purposes is between 25 mg and 50 mg three to four times a day, as reported by Drugs.com.
The drug is known to cause drowsiness and sedation, which makes it unadvisable to use when driving a car or operating machinery. People with certain medical conditions and those who take other medications should exercise caution when using Diphenhydramine hydrochloride. The effect of drowsiness is intensified by use of alcohol. In pediatric patients the drug may cause excitement and should not be administered to infants, according to Drugs.com.