Cholinesterase inhibitors, also known as anti-cholinesterase, are chemicals that prevent the breakdown of the neurotransmitter acetylcholine or butyrylcholine. This increases the amount of the acetylcholine or butyrylcholine in the synaptic cleft that can bind to muscarinic receptors, nicotinic receptors and others. This group of inhibitors are divided into two subgroups, acetyl-cholinesterase inhibitors and butyrul-cholinesterase inhibitors (BuChE). Cholinesterase inhibitors, also known as anti-cholinesterase, are chemicals that prevent the breakdown of the neurotransmitter acetylcholine or butyrylcholine. This increases the amount of the acetylcholine or butyrylcholine in the synaptic cleft that can bind to muscarinic receptors, nicotinic receptors and others. This group of inhibitors are divided into two subgroups, acetyl-cholinesterase inhibitors and butyrul-cholinesterase inhibitors (BuChE). They may be used as drugs for Alzheimer's and myasthenia gravis, and also as chemical weapons and insecticides. Side effects when used as drugs may include loss of appetite, nausea, vomiting, loose stools, vivid dreams at night, dehydration, rash, bradycardia, peptic ulcer disease, seizures, weight loss, rhinorrhea, salivation, muscle cramps, fasciculations. While 4 cholinesterase inhibitors are approved in the United States only three are avaliable commercially. The three avaliable are rivastigmine, donepezil, and galantamine while tacrine is not. They are generally used to treat Alzheimer's disease and dementia. If a benefit occurs this is generally at two to third month after starting. It's difficult to determine which cholinesterase inhibitor has greater efficacy due to design flaws in head-to-head comparison study. Donepezil, phenserine, huperzine A, and BW284c51 are selective AChE inhibitors. Tetra (monoisopropyl) pyrophosphoramide (Iso-OMPA) and ethopropazine are selective BChE inhibitors. Paraoxon and rivastigmine are both acetylcholinesterase inhibitor and butyrylcholinesterase inhibitor. Galantamine might be less well-tolerated than donepezil and rivastigmine.