Venom is a type of natural poison secreted by arachnids, insects and other species. Several species of snakes are venomous.
A mugato's fangs contain venom that attacks its prey's nervous system, killing it after a few hours. Mako root is the only known cure. (TOS: "A Private Little War")
The venom of Regalian fleaspiders can improve a humanoid's blood circulation. (DS9: "The Ship")