Carbachol is a synthetic cholinergic agent that mimics acetylcholine. It is primarily used in ophthalmology to lower intraocular pressure in conditions like glaucoma and to induce miosis during eye surgeries. Carbachol acts on both nicotinic and muscarinic receptors, promoting muscle contraction in the eye and enhancing fluid drainage. Its effects are longer-lasting than those of acetylcholine due to its resistance to degradation by acetylcholinesterase.

Acetylcholine is a key neurotransmitter that regulates muscle contraction, cognitive functions, and autonomic processes. It acts on nicotinic and muscarinic receptors. While not used directly as a drug, its effects are targeted by medications such as cholinergic agonists and acetylcholinesterase inhibitors to treat conditions like Alzheimer's disease and myasthenia gravis.