I'd be REALLY careful about messing around with that stuff. In my opinion not only is it the CRAZIEST most UNENJOYABLE experience you're likely to obtain with anything over the counter... It's also somehow addictive. I don't know. I can't explain it. But, for some reason... People I know tried it, hated it, then the next day would take more, and like 2 minutes later be all, "Why the FUCK did I just eat those? These things suck!"
In ANY case... I'd say that it really depends on the experiences you're looking for. If you take just a couple, you may only get a light body sensation. If you take like 5, you will be very zoned out and what would normally be relaxed if there wasn't this odd background feeling of anxiety... or... something... it's a bit hard to put a finger on the exact effects on Diphenhydramine... If you take like 10 - 12 you will reach a sort of semi-deliriousness. You will find it difficult to voice your own thoughts, or even frustrating to do so. When you talk it will usually require effort to keep it at a normal volume. You will likely walk to be left alone to sit with a blanket as everything will feel somewhat cold. If you take 24, you will trip your nuts off in the worst possible way, and might have to go to the hospital... You will see and talk full conversations with people or things that aren't there... You will then realize they aren't there, and shake it off as if having conversations with hallucinations is no big deal... You will do strange things, such as start a shower, let it run for 9 hours without getting in, enter other peoples rooms at 3 am and turn their lights on, rummaging through their closet for... you can't remember... Then you wake people up to say things to them such as "Where's the jeans for the pipe!?" or "Where's George and that other girl I'm supposed to meet them for lunch." at like 4 am... Then later, they find you buck-ass naked, with boxers balled up, held over your crotch, wandering aimlessly in a guest room of the house... They ask you if you’re ok, and you scream like a freak and run scared into your room, getting into the shower finally. You awake the next day with no real clear memories, and even completely forgot about taking the shower.
That shit really is just best not to be messed with in my opinion. You're really not going to enjoy the experiences in my opinion, and again imo I don't even think it's safe to take large amounts; especially with any regularity. So... Do some research and find some other alternatives.