Explore and know, how long Benadryl stays in your system?, how to get rid of Benadryl from the body, and what is the treatment for it. Diphenhydramine (Benadryl) is a medical drug classified under the group antihistamines, and it is used for the treatment of allergies, hay fever, colds, flu, and other allergic symptoms. Benadryl lasts and shows up on drug tests for different periods. Diphenhydramine stays in the system for up to two days in healthy adults. Drug tests detect Diphenhydramine in urine for up to 4 days, in blood for 3-4 days, and in saliva for 3-4 days after the last dose. Factors affecting detection time include dosage, half-life, frequency of use, drug interactions, age, and overall health condition