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Nicotinic and muscarinic receptors are two types of cholinergic receptors found in the nervous system. They both belong to a larger family of receptors that bind to the neurotransmitter acetylcholine (ACh). Nicotinic receptors are ionotropic, meaning they quickly open ion channels upon binding ACh, while muscarinic receptors are metabotropic, meaning that they activate intracellular signaling cascades via G-proteins upon binding ACh.

Drugs that selectively stimulate (agonize) nicotinic receptors are called nicotinic agonists. Examples of nicotinic agonists include nicotine itself and varenicline (Chantix). These compounds bind to nicotinic receptors and activate them, mimicking the effects of acetylcholine.

Drugs that selectively block (antagonize) nicotinic receptors are called nicotinic antagonists. Examples of nicotinic antagonists include tubocurarine, mecamylamine, and hexamethonium. These compounds bind to nicotinic receptors without activating them, thus preventing acetylcholine from binding and causing an effect.

Drugs that selectively stimulate (agonize) muscarinic receptors are called muscarinic agonists. Examples of muscarinic agonists include pilocarpine, cevimeline, and arecoline. These compounds bind to muscarinic receptors and activate them, mimicking the effects of acetylcholine.

Drugs that selectively block (antagonize) muscarinic receptors are called muscarinic antagonists or antimuscarinic drugs. Examples of muscarinic antagonists include atropine, ipratropium, and scopolamine. These compounds bind to muscarinic receptors without activating them, thus preventing acetylcholine from binding and causing an effect.

Nicotinic agonists can be used for smoking cessation therapy, as in the case of nicotine replacement therapy (patches, gum, etc.) and varenicline, which helps reduce cravings and withdrawal symptoms. Nicotinic antagonists can be used as neuromuscular blockers in surgery for muscle relaxation (e.g., tubocurarine) or in the treatment of hypertension (e.g., mecamylamine). Muscarinic agonists are used to treat dry mouth (xerostomia) in patients with Sjögren's syndrome or as a result of radiation therapy for head and neck cancer (e.g., pilocarpine and cevimeline). They are also used to treat glaucoma by promoting aqueous humor outflow (e.g., pilocarpine). Muscarinic antagonists are used to treat various conditions, such as overactive bladder (e.g., oxybutynin), asthma and COPD (e.g., ipratropium), and motion sickness or as a pre-anesthetic agent to reduce secretions and prevent bradycardia (e.g., scopolamine and atropine).