Nic,If I remember correctly, carbon monoxide bonds very well with hemoglobin. tolerance is built quickly. It takes many hours after your last cigarette to get it all out of your system, usually overnight. So, if you wait a few hours after you wake up you will feel it as the rush and body tingle, after that, tolerance. Although one side effect I got, for the 35 years I smoked, and hence ate ibuprofen like candy, was a headache when smoking too much in a day.As opposed to what I would get sometimes the next day which I describe as a nicotine hangover. Although i am sure some of the headaches were due to excess nicotine as well.