Snake venoms are like soups.   Most of them contain many different components and some of these are toxic to man.  For instance, in the common tiger snake venom, Notechis scutatus, there have been at least 6 neurotoxins described.  There are several haemotoxins, interfering with activation of the clotting factors and some weak haemorrhagins.  Some of the neurotoxins also cause muscle damage.  There are also toxins which can cause a drop in blood pressure in this venom.    Most venoms have many components, some of which are known to be toxic to man.   There are also many components in most venoms that are still uncharacterised.   It is the combined effect of all the components in snake venoms that gives it its overall toxicity.

In the past, and to a lesser extent today, toxicity was measured by injecting test animals with venoms.  Many different types of animals have been used.   In the 1960's and 1970's, mice became the universally accepted test animal and comparative toxicity was expressed as an LD₅₀. This is usually measured by testing the crude venom in mice at varying doses and when a calculated concentration causes 50% of the test animals to die, it is referred to as the LD₅₀.  Today, these types of animal experiments are not performed as much because of animal welfare concerns, however they were invaluable in understanding the toxicity of both crude venoms and purified single toxins and are still the best way of measuring overall toxicity in living organism.

There are other specific tests that look at the effect of single toxins.  These can be clotting assays to measure the effect of coagulants, nerve-muscle preparations to measure neurotoxic activity, phosphilipase and phosphodiesterase assays which measure these respective agents in the venoms.  Even muscle or myoblast preparations to measure the myotoxic activity of venoms have been used.   These tests are useful to investigate the relative toxicity of purified toxins but there is sometimes a poor correlation between these results and the in vivo effects of the toxin.  There is still no alternative that surpasses the in vivo LD₅₀ for both purified toxins and crude venoms.

In 1979, the Commonwealth Serum Laboratories in Australia conducted an extensive comparative study of the toxicity of most of the Australian medically significant snake venoms and some venoms from non-Australian snakes.  Here they used mice which were subcutaneously injected with the crude venoms.   In the early 1980's, Richard Davis and I published the cobra scale, which compared venoms with that of the well known Indian cobra venom.  Cobra venom is assigned the value of 1 and the toxicity of other venoms are compared to this venom.   The table above is an expanded version of this comparison.