This is the reason people take diphenhydramine with ephedrine:
Ketotifen has some proof to show it upregulates B2 adrenergic receptors, the same receptors clen acts on, as a result, people will run it with clen to increase/prolong the effects.
Now here's where the bro logic kicks in:
Since Keto is an antihistamine then all antihistamines must upregulate b2 receptors-->Ephedrine effects beta receptors/causing an eventual down regulation-->Using diphenhydramine will prolong/increase the effects of ephedrine.
Problem is, ephedrine effects a lot of adrenoreceptors, it is not selective like clen is. With the keto/benadryl deal, people are confusing correlation with causation, I can not however say that this doesn't work, just that I haven't seen any actual proof for it.
As for the sedative/narcotic effects of benadryl, I can say that when naive to the effects of diphenhydramine, it is most similar to benzo's at a dose of 25-100, it becomes hallucinogenic at higher dosages, but not the good kind of hallucinogenic. DXM is an NMDA antagonist if I remember correctly, I can say that it is mildly hallucinogenic at a dosage of 200mg, but is more of a euphoric/intoxicant, its when you start getting up into the stratosphere on dosages that things get nasty.
And yes, Lysergic Acid Diethylamide FTW