I do not see any information about this substance on /r/Nootropics currently, so I thought I should add my own n=1 case report from trying it just now.
Cytisine is an alkaloid partial agonist of nAChRs. It is commonly used for smoking cessation as a nicotine alternative, especially in Eastern Europe ("Tabex", "Desmoxan") and Canada ("Cravv"), with a generally superior side effect profile relative to varenicline ("Chantix", "Champix") and apparently better efficacy than nicotine replacement therapy. Historically, the cytisine-containing plant Cytisus laburnum was sometimes even used as a tobacco substitute in cigarettes in Germany, Russia, and Eastern Europe.
For further information, see e.g.:
However, cytisine has a 4.8-hour half life, which is over twice as long as that of nicotine. Of course, one could see how this might be of interest if one derives some subjective benefits from occasional nicotine use.
I ordered a box of Tabex from a very reliable supplier known to this community and tried one 1.5 mg tablet sublingually, much like a nicotine lozenge. In retrospect, perhaps this may have been too much for me personally, as my use of nicotine lozenges has been very sporadic and limited to 1-2 mg at a time, separated by a few days or weeks. It is not clear.
The subjective effects began somewhat similarly to nicotine, with a sharpening of visual perception and color saturation, a slight anxiolytic effect, and mildly improved focus, though very little euphoria. Nicotine itself has always been fairly euphoric to me, and the infrequency of my use of it has enabled me to keep it that way.
However, after about 1-2 hours, I began to notice increasing dizziness, vertigo, nausea, and GI discomfort. None of this ever happens to me with nicotine. I took an antiemetic to resolve the nausea but still felt somewhat disequilibrated. (Side note: I always keep both antiemetics and anxiolytics handy when trying any new substance, and I would tend to suggest this as a useful practice in general.) After about an hour of continued variable vertigo, I decided to just take a nap to kill some time and help reset my perceptual state. I slept for about an hour and felt essentially normal when I woke up, though slightly less focused than usual-- probably just a mild rebound effect, which I sometimes experience with nicotine and various stimulants with other mechanisms of action as well, though nowhere near as bad as the rebound effects I experienced when trying various racetams. That also dissipated within an hour or two.
Nicotine itself can allegedly have the same profile of side effects, but I have never experienced any of them when using it in very small amounts. Perhaps the cytisine dose was too high. Alternately, perhaps I merely respond worse to it personally.
In any event, I cannot recommend cytisine over nicotine for nootropic purposes based on my own limited experience at least, though perhaps others may have better results.
That said, it does look very useful for smoking cessation. I vaguely suspect that schizophrenics might find it interesting as a longer-acting nicotine alternative as well, but it is not clear how common the negative side effects I experienced are, or whether they fade over time with continued use.
As with my other unusual-substance case reports, it would be interesting if anyone else who has tried cytisine could comment here with their own experiences and thoughts.