"Poisonous snake" redirects here. For true poisonous snakes, see Rhabdophis.
Venomous snakes are snakes which have venom glands and specialized teeth for the injection of venom. Members of the families Elapidae, Viperidae, Hydrophiidae, and Atractaspididae (and some from Colubridae as well) are major venomous snakes.
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Venomous snakes use modified saliva, snake venom, usually delivered through highly specialized teeth such as hollow fangs, for the purpose of prey immobilization and self-defense. In contrast, non-venomous species either constrict their prey, or simply overpower it with their jaws.
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; 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 cause no harm to humans if bitten.
| Snake | Region | SC LD50 |
| Inland taipan | Australia | 0.025 mg/kg |
| Eastern brown snake | Australia | 0.0365 mg/kg |
| Coastal taipan | Australia, New Guinea | 0.106 mg/kg |
| Many banded krait | China, Taiwan, Burma, Laos, Vietnam | 0.108 mg/kg |
| Peninsula tiger snake | Australia | 0.131 mg/kg |
| Saw-scaled viper | Asia; Indian subcontinent | 0.151 mg/kg |
| Black Mamba | Sub Saharan Africa | 0.185 mg/kg |
| Western tiger snake | Australia | 0.194 mg/kg |
| Eastern coral snake | Northeast Mexico, Southeast U.S. | 0.196 mg/kg |
| Philippine cobra | Philippines | 0.21 mg/kg |
Lists or rankings of the world's "most venomous snakes" are tentative and differ greatly due to numerous factors, including the recentness and reliability of the data, the number of species analyzed, and the testing methods used.[3] In addition, since mice are the common indicator used to test venom from venomous snakes in LD50 tests, the LD50 results may not reflect the actual effects on humans due to the physiological differences between mice and humans. For example, many venomous snakes are specialized predators on mice, and their venom may be adapted specifically to incapacitate mice. While most mammals have a very similar physiology, LD50 results may or may not be directly relevant to humans. Sometimes, results from different tests may cause confusion as different toxicity scales are in use.[4]
While there have been numerous studies on snake venom, potency estimates can vary,[5] creating overlap and greatly complicating the task. Further, LD50 may be measured through intramuscular, intraperitoneal, intravenous or subcutaneous injections on small rodents, although the latter is the most applicable to actual bites.[3] It should also be considered that mice, the most commonly used animals in determining LD50, may react to some snake venoms differently than humans do, this may be why some species of snake show high toxicity when tested on mice, but have surprisingly low mortality rates in untreated human bites (for example, the Eastern coral snake, Philippine cobra, and even the Eastern brown snake).[6] Based on medical literature dating back from the early 1900's to the present, the most venomous or deadliest snake to humans has been the Black mamba. The species has the most rapid-acting venom of any snake[7] and it carries a 100% mortality rate in untreated cases of human envenomation.[8][9][10][11][12][13][14] Another snake with a similar reputation is the Coastal taipan, which has an untreated mortality rate of 95-99% in humans, second only to the black mamba. Although more venomous than the black mamba, the Coastal taipan's venom is slower to act and there has been cases in the medical literature in which people have survived Coastal taipan bites without any kind of medical treatment, although this is very rare. The Eastern brown snake's venom is nearly 3 times more potent than the Coastal taipan's, yet to humans the untreated mortality rate for the Eastern brown snake is somewhere between 10-20%, far lower than both the Black mamba and Coastal taipan.[15][16][12]
Many of these lists only take into account of terrestrial and arboreal snakes and neglect to list the of venom of the sea snakes. Species of sea snakes have been listed to have more toxic venom than even that of the Inland taipan and further investigations of some species' venom are needed.
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 (envenomated) 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. There are however two exceptions: the Rhabdophis snakes (keelback snakes) secrete poison from glands that it gets from the poisonous toads that it preys on; similarly certain garter snakes from Oregon retain toxins in their liver from the newts they eat.[17]
Over 600 species are known to be venomous—about a quarter of all snake species. The following table lists some major species.
| Family | Description |
|---|---|
| Atractaspididae (atractaspidids) | Burrowing asps, mole vipers, stiletto snakes. |
| Colubridae (colubrids) | Most are harmless, but others have toxic saliva and at least five species, including the boomslang (Dispholidus typus), have caused human fatalities. |
| Elapidae (elapids) | Sea snakes, Taipans, Brown snakes, Mambas, Coral snakes, Kraits, King Cobra, Death adders, Tiger Snakes, Cobras. |
| Viperidae (viperids) | True vipers, including the russell's viper, saw-scaled vipers, and puff adders and Pit vipers, including rattlesnakes, lanceheads, and copperheads and cottonmouths. |