The root-stock of Dryopteris filix mas, a European fern. The medicinal principle of fern is an oleoresin of a bitter, nauseous taste.

It is an anthelmintic, specially destructive to the tape-worm. Although less irritating than some others of its class, it may, in overdoses, produce severe intestinal irritation, and death has been caused in an adult by taking six ounces.