Venomous snakes include several families of snakes and do not form a single taxonomic group. This has been interpreted to mean that venom in snakes originated more than once as the result of convergent evolution. Evidence has recently been presented for the Toxicofera hypothesis however; if correct, venom was present (in small amounts) in the ancestor of all snakes (as well as several lizard families) as 'toxic saliva' and evolved to extremes in those snake families normally classified as venomous by parallel evolution. The Toxicofera hypothesis further implies that 'non-venomous' snake lineages have either lost the ability to produce venom (but may still have lingering venom pseudogenes) or actually do produce venom in small quantities, likely sufficient to assist in small prey capture, but not normally cause harm to humans if bitten.
Venomous snakes are often said to be poisonous, although this is not the correct term, as venoms and poisons are different. Poisons can be absorbed by the body, such as through the skin or digestive system, while venoms must first be introduced directly into tissues or the blood stream through mechanical means. It is, for example, therefore harmless to drink snake venom as long as there are no lacerations inside the mouth or digestive tract.
Many other snakes, such as boas and pythons may not be venomous, but their bites should be attended to medically. Their teeth may be long and sharp, capable of inflicting lacerations, with bites often introducing mouth bacteria and shed teeth into the wound.