One way adventurers suffer the lethargy affliction is via the Serpent-class ability in their Hypnosis skill. Another way is via the Weaponmastery ability mastered by adventurers of the Runewarden, Infernal, or Paladin classes. With this ability, HEWing either arm will give recurrent lethargy until such a time as all wrist fractures are cured.
Eating a ginseng root or a ferrum flake will cure the lethargy affliction.