Very drowsy effect. sarahh851. 3 years ago. I like the product itself because it completely takes away your allergies but the only problem with this product is … It doesn\’t make me drowsy but I\’m 5\’10\” and 245 so your reaction may vary. Only one tablet is required for 4-6 hours of relief and you should never take … Yes, Benadryl makes you sleepy, and those effects can last well into the next day. Diphenhydramine, the active ingredient, has an average half-life of 4 to … Antihistamine drowsiness is not a secondary effect of some drugs but a primary effect of some drugs, which essentially alters brain functioning. Benadryl can cause drowsiness and should not be used when driving. Taking large doses of Benadryl can lead to serious health issues like … Benadryl can make you sleepy for a while. #allergies #medication #doctor. 102K. 3,985. Share. Video unavailable. When overused, Benadryl can potentially cause some undesirable side effects ❌ Benadryl makes you sleepy due to the active ingredient … Why does Benadryl make you sleepy? Benadryl (diphenhydramine) is an antihistamine that can cross into the brain and block histamine, a …