Nicotine is a potent parasympathomimetic alkaloid found in the nightshade family of plants (Solanaceae) and a stimulant drug. It is a nicotinic acetylcholine receptor (nAChR) agonist, except at nAChRα9 and nAChRα10 where it acts as an antagonist. It is made in the roots of and accumulates in the leaves of the nightshade family of plants. It constitutes approximately 0.6–3.0% of the dry weight of tobacco and is present in the range of 2–7 µg/kg of various edible plants. It functions as an antiherbivore chemical; consequently, nicotine was widely used as an insecticide in the past and nicotine analogs such as imidacloprid are currently widely used. In lesser doses (an average cigarette yields about 2 mg of absorbed nicotine), the substance acts as a stimulant in mammals, while high amounts (50–100 mg) can be harmful. This stimulant effect is likely to be a major contributing factor to the dependence-forming properties of tobacco smoking. The nicotine content of popular American-brand cigarettes has slowly increased over the years, and one study found that there was an average increase of 1.78% per year between the years of 1998 and 2005. Nicotine liquid can be used in vaporizers or electronic cigarettes along with a wide variety of different flavors.