Cholinomimetic drugs are drugs that influence the cholinergic system by mimicking acetylcholine. Direct acting drugs bind directly to acetylcholine receptors and activate them. Since acetylcholine receptors may be nicotinic or muscarinic, some drugs may be more selective, such as nicotine or methacholine. Indirect acting drugs inhibit the action of the acetylcholinesterase enzyme, increasing the concentration of endogenous acetylcholine in the synaptic clefts. This inhibition can be either reversible or irreversible. Examples of cholinomimetic drugs are neostigmine and organophosphates.