One side effect of getting off cigarettes (or removing any number of other neurotoxic chemicals like Fluoride, High Fructose Corn Syrup, Brominated vegetable oils or flours, etc from your diet) is that your brain starts to actually function the way it is designed to work.
This my be good or bad, depending on how dependent on social acceptance and "fitting in" you are. If you are the kind of person who NEEDS to be constantly validated by the people around you, and who needs to fit in, then having a fiully-functioning brain is going to cause a LOT of problems.
You're going to start noticing that the media contradict their own statements from day to day (and sometimes from minute to minute) and they act like this is OK and perfectly normal.
You're going to start noticing that politicians want to pass laws to make things illegal for YOU to do, and then they put specific exemptions in those laws so that THEY can still do those things.
You are going to see people in positions of authority--government, academia, research, law enforcement--stand up in from of cameras or crowds and just flat-out make .... up and lie to the public--like they thing we are brain-damaged 2-yer-olds. And it's not going to be little lies like about the latest boondoggle bill to fund bridges in their district, or why the military REALLY needs to have secret torture bases in Turkey. It's going to be HUGE, GLARING SCREAMING lies, that have the possibility to effect every man, woman and child in the country, and they are going to be lies that anyone with a memory span longer than a goldfish, and the research skills of a 5th grader could figure out...
So, most likely, the PRIMARY symptom and "negative side effect" of a functioning brain is that you are going to not just FEEL more awake and aware, but you are actually going to BE more awake and aware. And because of this, you will most likely start to get REALLY angry with politicians, and you are going to realize that pretty much EVERYONE in your life who is in a position of power or authority is literally lying to you with every breath. You are going to feel outrage that the people in the world who are in positions of power and authority treat the public like they are idiots. And you are going to feel isolated, that most of the people in your life are still walking round like zombies, unable and unwilling to see these things.
But if you don't need other people (most of whom are still operating their brains at half-speed to due to the neurotoxins in their diets) to agree with you to feel good about yourself, then you will find that having fully-functioning brain is good thing.
You will become more politically aware and active. You will engage people in public more, trying to get them to wake up and shake off the chains of brain-damaging chemicals/
You will start to understand what the schools are REALLY designed to do, and will take more personal, active role in your children's education.
You will become more aware of the rest of your lifestyle choices-food, clothing, housing, travel--and how they effect your health and the health of those around you.
And you are going to WAKE UP and see the world in all it's beauty, wonder, and glory, and think to yourself, "Why the heck can't everyone else see this, and why do they waste so much time plugged into the TV, video games, and the internet"?