Question: how long does it take for liquid children's benadryl to work?
Answer: Answer. When taken by mouth, Benadryl (diphenhydramine) begins to work in about 15 to 30 minutes. However, it takes around 1 to 3 hours for Benadryl to reach maximum concentrations in the body, so you may not notice the full effects of the drug until that amount of time has elapsed.