Varenicline is a partial nicotinic acetylcholine receptor agonist, designed to partially activate this system while displacing nicotine at its sites of action in the brain. Varenicline is an alpha-4 beta-2 neuronal nicotinic acetylcholine receptor partial agonist. The drug shows high selectiviyty for this receptor subclass, relative to other nicotinic receptors (>500-fold alpha-3 beta-4, >3500-fold alpha-7, >20,000-fold alpha-1 beta gamma delta) or non-nicotinic receptors and transporters (>2000-fold). The drug competitively inhibits the ability of nicotine to bind to and activate the alpha-4 beta-2 receptor. The drug exerts mild agonistic activity at this site, though at a level much lower than nicotine; it is presumed that this activation eases withdrawal symptoms. Varenicline is sold under the trade name Chantix and Champix, it is indicated for use as an aid to smoking cessation treatment.

Nicotine is a natural alkaloid obtained from the dried leaves and stems of the nightshade family of pants, such as Nicotiana tabacum and Nicotiana rustica, where it occurs in concentrations of 0.5-8%. Cigarette tobacco varies in its nicotine content, but common blends contain 15-25 mg per cigarette, with a current trend towards lower levels. Nicotine is highly addictive substance, it exhibits a stimulant effect when adsorbed at 2 mg. Administration of higher doses could be harmful. Action of nicotine is mediated by nicotinic cholinergic receptors. Nicotine binds to the interface between two subunits of the receptors, opens the channel and allows the entry of sodium or calcium. The principal mediator of nicotine dependence is α4β2 nicotine receptor.

Cytisine ( aka baptitoxine and sophorine) is a naturally occurring alkaloid that can be found in several plant genera, such as Laburnum and Cytisus of the family Fabaceae. It has been found to be clinically superior to nicotine replacement therapy for the cessation of smoking. It is available in Eastern Europe under the brand names Tabex and Desmoxan and in Canada under the brand name Cravv. However certain undesirable side effects exist, Cytisine can interfere with breathing and cause death (LD50 i.v., in mice, is about 2 mg/kg) and Cytisine is also teratogenic. Cytosine is an α4β2 nicotinic Acetylcholine receptor agonist. In addition to clinical use as a smoking cessation aid, It has demonstrated anti-depressant effects in mice.