Allergy medicine like antihistamines, antidepressants like amitriptyline, antianxiety like benzodiazepines and narcotics are medications that can make you drowsy.
Antihistamines, especially the 1st generation type like diphenhydramine do not discriminate which histamine receptor they block. They cross the blood brain barrier and they block histamines from binding to the receptor which affect sleep - wake cycle. Disruption of histamine in the brain causes drowsiness.
Antidepressants block neurotransmitters like serotonin and norepinephrine and these neurotransmitters induce sleep so if they are reduced it can make one drowsy.
