- "Poisonous snake" redirects here. For true poisonous snakes, see Rhabdophis.
Venomous snakes 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.
Venomous snakes use modified saliva, and snake venom, usually delivered through highly specialized teeth, such as hollow fangs, for the purpose of prey immobilization and self-defense. In contrast, nonvenomous species either constrict their prey, or 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 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 ancestors 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 'nonvenomous' 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.
Most venomous snakes 
Lists or rankings of the world's "most venomous snakes" are tentative and differ greatly due to numerous factors.
The high variability of LD50 tests is a major problem. This includes the age and reliability of the data, the number of species analyzed, and the testing methods and toxicity scale used. While there have been numerous studies on snake venom, potency estimates can vary, 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. So, considering the toxicity of a species based on LD50 alone may not accurately estimate the danger of the species to humans as the efficiency of venom delivery is not taken into account. Further, results from different tests may cause confusion, as different toxicity scales are in use.
Apart from the high variability of toxicity tests, the physiological difference between the animals used and humans is another major problem of selecting the most venomous snakes. Mice are the common indicator used to test venom from venomous snakes in LD50 tests so the LD50 results may not reflect the actual effects on humans due to the physiological differences between mice and humans. Many venomous snakes are specialized predators on mice, and their venom may be adapted specifically to incapacitate mice. While most mammals have a fairly similar physiology, LD50 results may or may not be directly relevant to humans.
Moreover, many of these lists only take into account of terrestrial and arboreal snakes and neglect to list those 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.
Sometimes, toxicity of snakes is used to determine their danger to humans but this is improper. There are far more factors critical to determine the potential hazard of a snake to humans, including the distribution and behaviour of the species. For example, while the inland taipan is regarded as the most venomous land snake based on LD50, the Big Four snakes cause far more snakebites due to their frequent occurrence in highly-populated areas. Clinical mortality rate is another commonly used indicator to determine the danger of a snake as it, to a certain extent, can reflect truths other than its toxicity on mice, such as the efficiency of venom delivery, the venom yield and the behaviour of the snake when it encounters humans. The bites of black mamba and coastal taipan result in an untreated mortality rate of approximately 100%. Other species clinically known to cause high mortality rates include the common krait,  king cobra,  etc. Moreover, death time after envenomation can also reflect the danger of bites from a species to humans. Mambas, the king cobra  and the coastal taipan  are examples of snakes whose bites may result in rapid fatality if untreated. On the contrary, the inland taipan has not caused any human death.
Examples of LD50 toxicity rankings 
|The most venomous land snakes by Ernst and Zug et al. (1996)|
|The most venomous snakes (Australian Venom and Toxins database)|
Other information 
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, harmless to drink snake venom as long as there are no lacerations inside the mouth or digestive tract. The two exceptions are: the Rhabdophis keelback snakes secrete poison from glands they get from the poisonous toads they consume, and similarly, certain garter snakes from Oregon retain toxins in their livers from the newts they eat.
Families of venomous snakes 
Over 600 species are known to be venomous—about a quarter of all snake species. The following table lists some major species.
|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, and cobras|
|Viperidae (viperids)||True vipers, including the Russell's viper, saw-scaled vipers, puff adders and pit vipers, including rattlesnakes, lanceheads, and copperheads and cottonmouths.|
See also 
- Snake venom
- Big Four (Indian snakes)
- List of venomous animals
- Venomous fish
- Venomous mammals
- Poisonous amphibians
- Toxic birds
- Fry, Bryan Grieg. "Snake LD50 – discussion". Australian Venom & Toxin Database. Retrieved 2009-09-28. "Subcutaneous is the most applicable to actual bites. Only large Bitis or extremely large Bothrops or Crotalus specimens would be able to deliver a bite that is truly intramuscular. IV injections are extremly rare in actual bites."
- Mackessy, Stephen P. (June 2002). "Biochemistry and pharmacology of colubrid snake venoms". Journal of Toxicology: Toxin Reviews 21 (1–2): 43–83. doi:10.1081/TXR-120004741. Retrieved 2009-09-26.
- "What is an LD50 and LC50".
- Davidson, Terence. "IMMEDIATE FIRST AID". University of California, San Diego. Retrieved 2010-05-12.
- "IMMEDIATE FIRST AID for bites by Australian Taipan or Common Taipan".
- "Clinical Toxinology-Bungarus caeruleus". toxinology.com.
- "University of Adelaide Clinical Toxinology Resources". "Mortality rate:50–60%"
- Davidson, Terrence M. "IMMEDIATE FIRST AID for bites by King Cobra(Ophiophagus hannah)".
- Zug, George R. (1996). Snakes in Question: The Smithsonian Answer Book. Washington D.C., USA: Smithsonian Institution Scholarly Press. ISBN 1-56098-648-4.
- Dr. Bryan Grieg Fry. "LD50 menu".
- Séan Thomas & Eugene Griessel – Dec 1999. "LD50".
- Klauber LM. 1997. Rattlesnakes: Their Habitats, Life Histories, and Influence on Mankind. Second Edition. First published in 1956, 1972. University of California Press, Berkeley. ISBN 0-520-21056-5.