Had cancer in 2007, and had an amputation.
After trying every major opiate, we settled on daily methadone. Also they put me on dilaudid for breakthrough pain. They would also push IV DPH when I started to feel sick, with reglin.
After being on the opiates for 3 years, I stopped cold. It was very easy to stop taking it, even cold turkey.
I've been struggling to stop taking Benadryl for around 4 years. Most people can't stand the effects DPH brings. For me, it feels a hundred times better than the rush from dilaudid, with the added bonus of lasting for closer to 5 hours than dilaudid's 1 hour of relief.
It's a drug that builds a tolerance very quickly. I was taking it at doses near 250mg when I started, as I had already build up tolerance from the hospital. In order to continue being on the nod, I had to keep increasing my dose. At times I was taking up to 1200mg, split into a first does, and a boost when I felt myself coming down a few hours later.
These doses would bring intense audio hallucinations, which would sometimes be delusions until I looked for the sound and realized it was the DPH. The visual hallucinations were almost always peripheral, vanishing when I turned to look at them. I would begin sentence in the middle, as well as answering questions no one had asked. If I found myself a bit and could actually communicate with someone, mid sentence I would completely jump tracks and say things that made no sense. "Yeah man, we have some beer in the fridge, cause pieces of the couch are dirty." Shit like that.
I would also space out and loose the connection between my brain and eyes, having more what I would call Lucid dreams than hallucinations. Even though I was staring at the wall, I would be 100% inside my head. These periods could last for up to an hour.
I have attempted to drop it many many times. If I cold turkey it, I start to be violently nauseous, and tell myself that I can just take a couple to take the edge off. As soon as those couple start working and I feel the watered down effects of the larger doses, my self control goes out the window. I know most people don't believe you can become physically addicted to DPH, but that's untrue. Many times I started feeling nauseous and don't have any idea why until I remember I hadn't taken and DPH in the past couple of days.
It's also a very effective escape for me, because I'm an introvert. I take 15-20 pills twice a day, but I'm able to function at work as an IT guy, even though I usually stay buzzed the entire time I'm there.
I've spent quite a bit of time researching benadryl's effects on the body, and if there were any known long term side effects, as well as asking two different doctors. They told me that if I had to make a habit of using a drug, Benadryl was a pretty safe choice.
There is some discussion about long term use causing long term memory problems (at best) and dimentia (at worst). It may cause ulcers as well.
To close, DPH is one of those drugs that is only enjoyed by a small percentage. Trying it out won't be dangerous, but I agree with other posters about having a trip-sitter the first time to make sure you get through it without damage to yourself or others.
I'm a 5'11" male, weighing about 145 pounds (Before surgery, more like 160)
The LD50 of benadryl in rats is 500mg / Kg
This is only based on toxicity though, and doesn't take into account any problems a large dose could start in your internal organs.
Starting at a dose around 300mg is a safe way to get a benchmark. Then make small changes, no bigger than 100mg at a time. I believe the enjoyment only exists in a small window, and you have to find it for yourself.
Hope this helps,
Be safe