Nicotine is an alkaloid occurring in tobacco leaves
and extracted from tobacco refuse; it is a colorless, oily liquid. Nicotine
is used as an insecticide and to make niacin (nicotinic acid). It is one of the most toxic substances known; even the
small doses ingested by smoking causes blood-vessel constriction, raised
blood pressure, nausea, headache,
and impaired digestion.