Huperzine A is a plant alkaloid derived from Club moss plant, Huperzine serrata, which is a member or the Lycopodium species. Huperzine-A is in phase III clinical trial in the USA (Alzheimer disease) and is available as a dietary supplement. It selectively and reversibly inhibits acetylcholinesterase. Huperzine A is also a NMDA receptor antagonist, which protects the brain against glutamate induced damage, and it increases nerve growth factor levels. Huperzine A is used for Alzheimer's disease, memory and learning enhancement, and age-related memory impairment. It is also used for treating a muscle disease called myasthenia gravis, for increasing alertness and energy, and for protecting against agents that damage the nerves such as nerve gases. It can cause some side effects including nausea, diarrhea, vomiting, sweating, blurred vision, slurred speech, restlessness, loss of appetite, contraction and twitching of muscle fibers, cramping, increased saliva and urine, inability to control urination, high blood pressure, and slowed heart rate. Various medications used for glaucoma, Alzheimer's disease, and other conditions (Cholinergic drugs) interacts with Huperzine A.