Which direct cholinergic agonist is known for its long duration of action?

Carbachol is recognized for its long duration of action among direct cholinergic agonists because it possesses a resistance to degradation by acetylcholinesterase, the enzyme responsible for breaking down the neurotransmitter acetylcholine. This resistance allows carbachol to maintain its activity at cholinergic receptors for an extended period.

In contrast, acetylcholine (ACh) is rapidly metabolized by acetylcholinesterase, leading to a very brief duration of action. Methacholine also has a shorter duration due to similar susceptibility to enzymatic breakdown, although it is more resistant than ACh. Cevimeline, while longer-lasting than ACh and used primarily for conditions like Sjögren's syndrome, does not have the same extended duration characteristics as carbachol. Therefore, carbachol stands out as the agent with the longest duration of action among the choices listed.