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 found, through multiple studies, to be effective as a medicine for helping people with neurological conditions such as Alzheimer's disease, but the meta-analysis of those studies concluded that they were poor quality and the findings should be interpreted with caution.