I've been taking phenibut every day for almost 3 years. I started at about 300 mg/day, and gradually went up to 1.5 to 2 grams a day during the past couple months. I noticed it was becoming a problem for me, like I was getting some side effects for the first time, but the main issue I had was that I knew about the addictive nature of phenibut and I knew that I was severely down-regulating my gaba receptors, which is not smart.
So this week I decided to cut back to a gram a day for a while and the eventually try to go down more from there. I was fine on my first day of 1 gram, but for the next 3 days I felt like I was on a non-psychedelic version of LSD --- it was kind of cool in some ways, my focus was phenomenal and I didn't experience any sleepiness, including overnight. I had to take a combo of other things to get any sleep at all. I've taken the PEA (phenethylamine) + MAO-B inhibitor combination a few times, and that's pretty much what I was feeling, except it was on overdrive and it lasted all day and night.
Phenibut is thought to antagonize PEA, so it makes sense that if you take away the phenibut after a long period of use, there would be a rebound effect of too much PEA activity. So this adds to the rebound of the GABA receptors, which are used to being loaded with phenibut but are now deprived of that so that they can't function as inhibitors of other neurochemical pathways. And I will add this bit of speculation: Since phenibut is dopaminergic (by some mechanism that I don't understand), if you take away a dopaminergic drug then you're probably going to get a rebound of increased serotonin receptor activity because dopamine inhibits serotonin release (and vice-versa.)
The result of just the 3 effects I mentioned above, which are only part of the phenibut story, is that coming off of phenibut can feel like a totally disinhibited nervous system flooded with PEA and strong serotonin activity. Disinhibition would also seem to spike norepinephrine activity. The combo of all these effects is exactly like what I was experiencing. The "cure" I found for these symptoms was to go back up to 1.5 grams of phenibut. I admit it, I'm a phenibut addict, but as far as addictions go I'd say this one is among the least harmful. It hasn't had a negative effect that I know of, neither on my health nor my moods, thoughts, or behavior. It's just a problem because I got in a habit of taking too much too often, and I'm going to slowly cut back.
I've given phenibut to several noobs and I always provide them with 1.5 to 2 grams for their first experience, so that they can really get the beauty of it. None of them have developed any problem with it, but none of them are drug-seekers to begin with. They all LOVED it! And I think the reason they don't immediately go out and buy a kilo of it is that the effects are so long-lasting, lingering for as much as 24 hours even at 1.5 grams. I would never dose anyone over 2 grams, nor would I ever advise anyone to take it daily or on any regular basis at doses of more than 500 mg to occasionally 1 gram/day. As a treat, taking it at a higher dose for recreation once in a while doesn't appear to be a problem, unless you just don't happen to enjoy it.