Janis thorns are small, poisonous thorns used by the Sevateem as weaponry. Janis thorns are thin thorns about the length of a finger, with a poison coated tip. This poison causes first paralysis and then death in Humans.
Though there is no naturally occurring cure in nature, an anti-toxin was created by the Doctor using a bioanalyzer. (DW: The Face of Evil) His companion, Leela, a former Sevateem member, used his weapon prior to meeting the Doctor and continued to arm herself with it and occasionally use it during her travels with the Doctor, much to his disapproval. (Exactly how she is able to keep herself supplied with the thorns, even when away from her homeworld, is unknown; either she took a good supply of them with her initially, or it is possible to convert local flora into janis thorns.)