Diphenhydramine can be taken recreationally.. it's rather weird, and a lot of people have some pretty freaky trips (just a heads up).
As to it being dangerous, so long as tylenol nor aspirin are present I wouldn't be too concerned with that. The only thing I find annoying, high dosages of diphenhydramine may leave floaters in your vision for quite some time! On really high dosages you may have nausea or puke.. but like I said I don't think anything that one really needs to be concerned about.
*Oral Diphenhydramine Dosages
(Recreational)
Threshold 10mg
Light 25-150 mg
Common 150-300 mg
Strong 250-500 mg
Heavy 400-700
LD50 (Lethal Dose) 500 mg/kg in rats
*courtesy of Erowid*