Damn 150/100mg Diphenhydramine IV!!!! And no rush!!!
It is not tolerance, I've used antihistamines IV daily for months, and even now, 50mg Diphenhydramine gives me a strong rush. Taking 75mg would make me feel very uncomfortable from the side-effects.
I don't understand omen_owen mk2. Did it burn somewhere?
As I said, Diphenhydramine usually:
-burns a lot if you shoot in an artery (so you would notice it immediatly)
-burns quite a lot when you miss the shot (but just only during a few seconds, after that, the local anesthesic properties of Diphenhydramine makes the place numb)
-when you shoot it right, in the vein, it sometimes burns all along the vein, sometimes not. (it doesn't burn if it's a big vein)
So, omen_owen mk2, was it numb all around the place you shot? If you miss the shot, as I said, you should feel completely numb/anesthesied during 15mins after.
Anyway, if you try it again, don't shoot more than 50mg. Whatever the dose you take orally, 50mg IV will produce a strong-enough rush, and you will notice it strongly.