It is, but its also a tranquilizer/CNS depressant, which is why people seek it in the ER (you know, "intractable N/V, allergic to compazine, reglan, etc.") Since phenergan and benadryl have similar anticholinergic properties, the use of one to counteract the dyskinetic effects of the other doesnt make much sense, even though we know that is the mechanism for most rx of neuroleptic induced dyskinesia. My theory is that the histamine must be involved somewhere, since everything is in a balance until disrupted, and that disruption (of, dopamine, ACh, etc.) is what causes the end result.
By the way, phenergan has about 1/10th the dopamine blocking strength of thorazine.