Nicotine is an alkaloid which constitutes 0.6-3.0% of the dry weight of tobacco. It acts both as a stimulant and a relaxant. Initially, nicotine causes the release of adrenaline and may block the release of insulin and increase metabolic rate. Dopamine is also released which increases feelings of well-being.
However, tobacco smoking is linked to increasing incidence of many diseases, including various cancers and circulatory and respiratory diseases. In the UK, an estimated 100,000 people per year die from smoking-related diseases.
Nicotine is highly toxic to warm-blooded animals and can be used as a plant-derived insecticide, primarily for piercing sucking insects such as aphids and whiteflies. As it degrades quickly, it can be used on food plants nearing harvest.