Huperzine A is a naturally occurring sesquiterpene alkaloid compound found in the firmoss Huperzia serrata and in varying quantities in other Huperzia species, including H. elmeri, H. carinat, and H. aqualupian. Huperzine A has been investigated for its potential as a medicine for helping people with neurological conditions such as Alzheimer's disease, but there is insufficient medical evidence for it to be recommended. Huperzine A is extracted from huperzia serrata. It is an acetylcholinesterase inhibititor and NMDA receptor antagonist. For some years, Huperzine A has been investigated as a possible treatment for diseases characterized by neurodegeneration – particularly Alzheimer's disease. As of 2013 although some research suggests it may be helpful, the evidence overall is not convincing enough for it to be recommended for use as a medicine, and little is known of its safety.