The rhizomes and stipes of Dryopteris filix-mus (European aspidium or male fern), or of Dryopteris marginalis (American aspidium or marginal fern) (family Polypodiaceae); used in the treatment of tapeworm infestation, usually in the form of the oleoresin or extract, but because of its potential toxicity, its use is restricted to patients who do not respond to treatment with safer drugs such as dichlorophen, niclosamide, or quinacrine.