Herbal preparations (food supplements) containing extracts of the herb Huperzia serrata are sold in the Netherlands. These herbal preparations are mainly available online. According to the manufacturers, Huperzia serrata can support memory and improve concentration. Herbal preparations containing this herb turn out to be harmful to human health. RIVM advises consumers not to use herbal preparations with Huperzia serrata, especially during pregnancy. The number of people using such products is not known. People can experience acute undesirable effects on the cholinergic system. Examples include increased salivation, muscles weakness, abdominal cramps, diarrhoea, blurred vision, lacrimation and paralysis. There are also indications that Huperzia serrata is harmful to the unborn child during pregnancy. These health effects can already occur when people take the advised dose. The effects are caused by constituents of Huperzia serrrata, of which huperzine A is most studied. People using herbal preparations with Huperzia serrata extract ingest enough huperzine A to experience the harmful effects. No information is available on other constituents in Huperzia serrata, apart from an indication that eight compounds, other than huperzine A, may also inhibit the enzyme acetylcholinesterase. It is therefore possible that these eight compounds can enhance the effect of huperzine A.