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 an oral dose of Bendryal about 30 minutes before bedtime, if the drug is being used as a sleep aid, or right before traveling overseas, if the drug is being used for motion-sickness.
Bendryl is a brand name for the antihistamine drug diphenhydramine. The National Highway Traffic Safety Administration reports that blood concentrations of 25 nanograms per milliliter are effective for antihistamine uses; however, drowsiness occurs at 30 to 40 nanograms per milliliter, and cognitive impairment occurs at 60 nanograms per milliliter.
An adult taking an oral dose of 50 micrograms of Bendryl typically achieves a blood concentration of 80 nanograms per milliliter within 3 hours of consumption: That is more than sufficient to cause cognitive impairment. At 24 hours after consumption, the blood level drops to under 10 nanograms per milliliter.
WebMD reports that Bendryl can be used to treat allergies and motion sickness. Due to its sedating effect, it can be used as a sleep aid for most people. Some individuals, particularly children, report experiencing excitement and restlessness instead of drowsiness in response to Bendryl. WebMD warns that individuals experiencing signs of a severe allergic reaction should seek immediate medical help.