GHB.
I started using this with care daily in 2007, I was never aware of the physical withdrawals it could give. It is not well known since GHB is controversial for it's dose. But since I measured every dose I took properly, I never ODéd out of the more than 1000 times I have used it.
I started to ingest it daily, since it has no hang-over or any after effect, and the high itself was so bright and social to me that I just repeated doing it every evening after work.
I ended up with 10 ingestions a day, 5 times a day at work with 1/3 of the euphoric/drunken dose which was just to keep myself calm. And to put myself asleep I had to take monster amounts for which I feel ashamed.
I couldn't abandon it cold turkey, I read people died this way. What I got as withdrawals was no shit, intense fear, irregular heartbeats, sweating, extreme nervousness and seeing people in the dark who weren't there. The beginning of a delrium and thousand weird dreams a night.
So I was taking 1/3 of what I usually took, and tapered it off to baseline over a period of 2 months.
I still don't have anything bad to say about G, I had abused it. But I still worship it for it's sensational effects and bright magic feeling it gives. I can use it with no problem now by the way. Just once a month or something.
I also never had any devastating consequences, physically I never suffered, mentally extreme boredom for months. I always measured everything in a tube in where I can read every mililiter. Hardly did eye-ball dosages.
The problem is dependency, if one stops cold turkey, chances of a heart attack are real if he keeps abstaining. I tapered off with chains of very low dosages, and my body wasn't really affected, took care of it and just did my work-outs. In fact, I proceeded my entire life in the society but always had to make sure I could ingest a very little bit in secret in order to remain calm.
But I have read terrible ends about GHB addictions. The problem is that opiate users just suffer if they ain't getting their stuff. But a GHB addict is in serious danger if not getting it's chemistry. Search on the internet and you'll read about deaths related to stopping with GHB cold turkey.
I had tried crack (one night), cocaine (rarely), nicotine (rarely) and amphetamine (one night), valium (one night) alcohol (one night) prior to GHB, but none ofthem appealed to me. So it was my arrogance probably. But if I would have known it's addictive potential, I know for sure it wouldn't have happened.
It happened to me because I read a lot of matery behind GHB, but never got to know any story about GHB being addictive. Even on Erowid it was merely mentioned in the experience section those days.