Venomous snakes use modified saliva, and snake venom, usually delivered through highly specialized teeth, such as (big) 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, but 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 enough to help capture small prey but causing no harm to humans when bitten.
Most venomous snakes
The Big Four snakes responsible for most fatal bites in India
The black mamba with fast-acting venom is known to be actively aggressive if threatened.
Lists or rankings of the world's most venomous snakes are tentative and differ greatly because of 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 since the efficiency of venom delivery is not taken into account. Furthermore, results from different tests may cause confusion since different toxicity scales are in use.
Apart from the high variability of toxicity tests, the physiological differences between the animals tested and humans is another major problem in categorizing 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.
Despite being the world's most venomous snake based on LD50, the inland taipan has not caused any human death.
Toxicity of snake venom is sometimes used to gauge the extent of their danger to humans, but this is inappropriate. A number of other factors are more critical in determining the potential hazard of any given venomous snake to humans, including the distribution and behavior of each species. For example, while the inland taipan is regarded as the world's most venomous snake based on LD50, the so-called Big Four snakes cause far more snakebites because they are much more abundant in highly populated areas. Clinical mortality rate (often estimated by measured toxicity on mice) is another commonly used indicator to determine the danger of any given venomous snake, but important to are its efficiency of venom delivery, its venom yield and its behavior when it encounters humans (as reflected by clinical precedents). Black mamba and coastal taipan bites, when untreated, have a mortality rate of almost 100% due to their willingness to inject fatal amounts of venom in every envenomation. Other species with clinically proven high mortality rates include the common krait at 70-80%  and the Many-banded krait from 20-30%to 77-100%, etc. The timespan between envenomation and death is another measure of how dangerous a given venomous species is to humans; e.g., a black mamba or coastal taipan bites are rapidly fatal if untreated.
LD50 toxicity rankings
The most venomous land snakes by Ernst and Zug et al. (1996)
Most venomous land snakes (Ernst and Zug et al. 1996)
Venomous snakes are often said to be poisonous, although this is not the correct term since venom and poison are different entities. Poisons are absorbed by the body, such as through skin or the digestive system, while venoms must first be introduced directly into tissues or the bloodstream (envenomated) by 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.
^ abFry, 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."
^Cecilie Beatson (November 29, 2011). ANIMAL SPECIES:Inland Taipan "The venom of the Inland Taipan is extremely potent and is rated as the most toxic of all snake venoms in LD50 tests on mice" . Australian Museum. Retrieved October 14, 2013.
^BBC Nature Wildlife. Inland Taipan page. "Australia's inland taipan is considered to be the most venomous snake in the world" . BBC. Retrieved October 15, 2013.
Garden of Eden Exotics (May 2, 2012) Dr. Bryan Grieg Fry - Interview "...The inland taipan (Oxyuranus microlepidotus) is far and away the most toxic, much more so than even sea snakes." . nyexotics.blogspot.com Retrieved October 14, 2013