Opisthoglyphous snakes (rearward grooves), found in the reptile family Colubridae, usually possess weak venom injected by a pair of enlarged teeth at the back which normally angle backwards and are grooved to channel venom into the puncture. Since these fangs are not at the front of the mouth this arrangement is called rear-fanged. To envenomate prey, a rear-fanged snake must move the prey into the rear of its mouth and then penetrate it with its fangs, presenting difficulties with large prey although they can quickly move smaller prey into position.
While the venom of most opisthoglyphous snakes is too weak to harm humans, sometimes this is not the case. Notably, herpetologists Karl Schmidt and Robert Mertens were killed by a boomslang and twig snake, respectively.