Nicotine is a substance found in most members of the nightshade family of plants. Small quantities of it can be found in tomatoes, potatoes, aubergines (eggplants) and green pepper ("Capsicum", the peppers used as vegetables). Nicotine is also in the Coca plant. In higher quantities it is a very efficient poison. It acts on the nerves. It is a stimulant in lower quantities. It is the main reason people are addicted to smoking.