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... More »

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 explai... More »

A dog becomes sleepy within 60 minutes from administration of a properly sized dose of Benadryl, according to AZVets.com. Peak levels of the active ingredient diphenhydramine are found in blood circulation within one to ... More »

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 explai... More »

An oral dose of Benadryl reaches peak blood concentrations 2 to 3 hours after consumption, and the effects usually last for 4 to 6 hours, reports the National Highway Traffic Safety Administration. WebMD suggests taking ... More »

A side effect of Benadryl is that it causes drowsiness, which can make people fall asleep. Numerous over-the-counter sleep aids contain antihistamines such as diphenhydramine and doxylamine succinate because of their pot... More »

The pediatric dosage of Benadryl, or diphenhydramine, depends on the child's weight and the formula used, according to the St. Louis Children’s Hospital. For example, the recommended dose of liquid 12.5-milligram suspens... More »

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