As nicotine enters the body, it is distributed quickly through
the bloodstream and can cross the blood-brain barrier. On average
it takes about seven seconds for the substance to reach the brain
when inhaled. The half life of nicotine in the body is around two
hours[2]. The amount of nicotine inhaled with tobacco smoke is a
fraction of the amount contained in the tobacco leaves (most of the
substance is destroyed by the heat). The amount of nicotine
absorbed by the body from smoking depends on many factors,
including the type of tobacco, whether the smoke is inhaled, and
whether a filter is used. For chewing tobacco, often called dip,
snuff, or snus, which is held in the mouth between the lip and gum,
the amount released into the body tends to be much greater than
smoked tobacco. Nicotine is metabolized in the liver by cytochrome
P450 enzymes (mostly CYP2A6, and also by CYP2B6). A major
metabolite is cotinine.
from wiki.
