A cholinergic drug is a drug which has its effects by altering the level of acetylcholine, a neurotransmitter, in the brain. Typically, the term is used to refer specifically to refer to drugs that increase (rather than decrease) brain acetylcholine levels or facilitate (rather than impair) the use of acetylcholine. One subclass of cholinergic drug, cholinesterase inhibitors, includes those drugs which are currently marketed to treat the cognitive effects of Alzheimer's disease.