Venom was natural toxin secreted by animals. Venomous animals on Earth included species of snakes, arachnids, and insects.
According to Doctor Phlox, there was no antidote for the venom carried by his Pyrithian bat. (ENT: "Chosen Realm")
A mugato's fangs contained venom that attacked its prey's nervous system, killing it after a few hours. Mahko root was the only known antidote. (TOS: "A Private Little War")
Lieutenant Worf, suffering from Barclay's Protomorphosis Syndrome in 2370, developed a venom sac beneath his jawline and began spitting the acidic compound throughout the USS Enterprise-D. When Dr. Crusher discovered the sac on his neck, Worf spit the venom in her face, severely disfiguring her. He later attacked Captain Picard in the same fashion. (TNG: "Genesis")
The venom of Regalian fleaspiders can improve a humanoid's blood circulation. (DS9: "The Ship")