This is what I found out from my Psych doc. Nicotine can interfere with the effectiveness of some antidepressants - I believe it is with SSRI's. It can cut down on the half-life of the antidepressant.
In layman's terms, psych meds are designed to remain in your system for a certain amount of time. For example, 350mg of seroquel taken x1 a day builds up in your system, so that when you stop taking it, it will remain in your system for about 2-3 weeks.
I'm on Seroquel, and Effexor. My doc was concerned about the level of nicotine in my system because we had discovered that the mg of effexor I was on increased my blood pressure 210/100 - even on blood pressure meds. (BTW most medical doctors aren't trained in nor knowledgeable about psych meds.) Lowering the dose of effexor and switching B/P meds and upping that doseage, my B/P is manageable.
Depending on a person's diagnosis, severity of illness, overall physical health, nicotine can have a profound effect on the mental state of a person, disrupt sleep patterns, affect appetite and so on.
I strongly suggest that if you're concerned about the effects of nicotine, talk to your medical and psychiatric doctors.
Just because someone else is doing fine with nicotine doesn't mean that you will too. If you're like me, you could be that .00000000001% of the population that might have problems.