I feel it necessary to disagree with ultima's writeup. Of course, as usual when drugs are involved, your mileage may vary, and my sources are simply my own experiences, those of a few close friends, and everything on the internet that I could get my grubby little paws on. So, this is in no way definitive, but just thought it was worth contributing a writeup to a node I actually consider myself somewhat knowledgeable about (It was necessary so I could argue intelligibly with "Getting high on Benadryl! I say, that is absurd!")
Also, it is worth noting the dosages I was working with, since this is a strange drug that behaves differently in various situations and people and amounts. I was taking around 800 mg (32 pills) per diphenhydramine experience, and also taking a quantity of pseudoephedrine to combat drowsiness. I was approximately 180 pounds and male.
One of the benefits of diphenhydramine as a deliriant in comparison to the tropane alkaloid plants is that precise dosing is simple, as opposed to a guess of how many leaves or berries or seeds are necessary to achieve the desired effects. A miscalculation with belladonna can quite easily result in death or a trip to the hospital.
I found the best way to take it is right before bed (you can omit the pseudoephedrine then), mostly just because you wake up, and have passed through the state where coordination is significantly impaired. And I find it more enjoyable and immersive to wake up hallucinating.
The primary characteristic of the trip as I experienced was attempting to interact with aspects of reality that did not exist. The most amusing example was imagining an adventure to pick up a friend and go shopping, all the while never moving from a parking lot. This was the only constant in the trip that I noticed. There was no mood change, time distortion, just imaginings. There was a faint, but definite spiritual quality that I began experiencing midway through the day, that drove me to go for a walk in the woods(with my sitter, of course), which was a very nice experience. In contrast to most of what I've read about deliriants, I found that I remembered all or most of my trips in their entirety.
I have had no health problems (that were noticeable like ultima noded about anyway) or mental problems, but as usual, there is almost certainly some long term negative health effects that are related to long-term use of recreational doses, and perhaps prolonged medical use, or even one recreational dose.
Looking back, this was perhaps the best drug I ever used recreationally. It is an experience. However, the key is preparation and respect, since that is key to all drugs. Don't take drugs when you have responsibilities you need to take care of the next day. Have someone around that you can reality check with and to take care for you if the experience becomes too intense. If you smoke cigarettes, I find it to be very soothing, since (I believe) diphenhydramine antagonises the acetylcholine receptors which are the same receptors that nicotine agonises. Primarily, what I'm saying is use common sense. Some people prefer dimenhydrinate but that's mostly a matter of taste, and I find it better to take the diphenhydramine because the pills are smaller and less are involved.
If you feel comfortable altering your state of consciousness with chemicals, it might be worth it to you to do some research and decide if it's something you would be interested in experiencing. I advise experiencing it at least once in your life, it's a beast that I've only come across once.
Damn, I really just wanted to type Benadryl as opposed to diphenhydramine. Saying it gets old fast.