Cholinergic agents are substances that mimic or enhance the action of the neurotransmitter acetylcholine. They can stimulate the parasympathetic nervous system, which is responsible for rest-and-digest functions in the body. These agents are commonly used in medicine to treat conditions like glaucoma, myasthenia gravis, and Alzheimer's disease. There are two main types of cholinergic agents: direct-acting and indirect-acting. Direct-acting agents bind to cholinergic receptors, while indirect-acting agents inhibit the enzyme acetylcholinesterase, leading to increased levels of acetylcholine. Both types can have side effects, including nausea, diarrhea, and increased salivation.