Diphenhydramine is an antihistamine and anticholinergic which is commonly used to combat allergies, insomnia, and cold symptoms. Once you move beyond the recommended dose range, it can have strong sedating and deliriant effects.
Those sedating and deliriant effects have led to its use as a recreational drug. However, the delirium it produces is almost always unpleasant .
At sub-deliriant doses, it can act as a sedative on its own and it can also potentiate the effects of other depressants.
Diphenhydramine = DPH; Benadryl; Nytol
Insomnia: 50 mg
Light: 150 – 250 mg
Common: 250 – 500 mg
Strong: 500 – 650 mg
Total: 6 – 8 hours
Onset: 01:00 – 02:00
After: There may be lingering effects which persist for up to 24 hours after administration.