Carbachol

The workhorse, not the scalpel. Carbachol is what a researcher reaches for when the question is whether cholinergic signalling matters at all; it is the wrong molecule for any question that starts with a receptor subtype, and it is the wrong molecule to call muscarinic without a blocker in the experiment to prove it.

Systemic carbachol produces the whole cholinergic picture, because it activates both receptor families and cholinesterase does not clear it. Muscarinic activation gives salivation, lacrimation, sweating, bronchoconstriction and bronchial secretion, gut cramping and diarrhoea, miosis, bradycardia and hypotension. Nicotinic activation at autonomic ganglia and the neuromuscular junction adds fasciculation, weakness and pressor responses that oppose the muscarinic fall in blood pressure ^[5].

Atropine reverses the muscarinic half and does nothing for the nicotinic half, which is the practical reason carbachol is not used systemically. Ocular use keeps the dose local; the labelled ocular effects are miosis, brow ache, accommodative spasm and dim vision, and the intraocular solution is supplied as a single-dose vial to be discarded after use. Not every carbachol effect is nicotinic: in human skin, the sweat gland response to intradermal carbachol is antagonised by atropine and little affected by hexamethonium or tubocurarine, so the sweating is muscarinic ^[4].

Carbachol belongs to the generation of cholinergic esters built to solve one problem, which was that acetylcholine does not last. The carbamate group did it, and the drug settled into ophthalmology. The FDA approved the intraocular 0.01% solution, Miostat, on 28 September 1972 for producing miosis during surgery and for reducing the intensity of the intraocular pressure rise in the first 24 hours after cataract surgery. Systemic carbachol never established itself in the United States; every currently marketed carbachol product there is ophthalmic. In January 2026 a fixed combination of carbachol 2.75% with brimonidine 0.1% was approved for presbyopia, giving a 1970s cholinergic a new indication built on the pinhole effect of a constricted pupil.

In the laboratory carbachol is close to universal: the default cholinergic agonist in slice electrophysiology, smooth muscle work and secretion assays. That ubiquity is also its main hazard, because a result obtained with carbachol is not a muscarinic result until a blocker says so, and a great deal of literature has been written as though it were. In the clinic it is a niche ophthalmic agent that most prescribers meet only in an operating room, and the 2026 presbyopia combination has put it in front of a general audience for the first time in decades.