Anecdotally, there are reports of varenicline causing agression, but larger studies tended towards the finding of no effect over controls.
This got me into thinking about possible genetic polymorphisms that are implicated in aggression. Varenicline acts as a partial agonist of α4β2-nicotinic receptors -- maybe if you did a study of a prison population, you'd find those some polymorphism in this receptor among those incarcerated for a violent crime? (There isn't).
More broadly, are there any good candidate genes implicated in aggression/violence?
I had a similar experience 8 years ago, I felt rage that I have never felt before. The day I nearly punched the wall, I understood my completely uncharacteristic behaviour was induced by the drug. Then I asked everyone close if they had noticed recent changes in me, they had. I talked to my doctor and I weaned off of it, my behaviour returned to normal and now at nearly 50 years of age, this was the only such brief period of episodal rage I have ever experienced. It was intense, I've often thought of the poor souls who have comitted suicide and their loved ones who hypothesize the drug is to blame. I would believe it, given my experience.
I am female, have the low MAOA activity gene (homozygous), I also possess 2 homozygous mutations in COMT genes, these affect (slow) degradation of neurotransmitters. I also possess mutations related to dopamine receptor inefficiency, decreased BH4 production (precursor to dopamine), and folate metabolism (precursor to BH4). I am naturally and consstently one of the most even tempered people I know, I have rarely even experienced PMS and my reproductive years will soon be over. Interestly, the MAOA mutation I have has been called the "happiness" gene for woman (not-so for men, infact it's the opposite). I have 2 copies, resulting in even more self-reported happiness in day to day life, which is true for me, but was not for my time on the drug when I had anger, aggression, and rage, like never before. Incidently, it did work for me in giving up the cigerettes.