LD50

LD₅₀ is a meassure used to indicate how toxic a substance (e.g. a toxin) is. LD₅₀ in an abbreviation for ‘lethal dose 50%’ meaning the dose of a substance that kills 50% of individuals in a test group. It is often more informative to talk about LD₅₀ than LD₁₀₀, as there are many (high) doses that will kill 100% of a group, but only one dose that kills exactly 50%. The less toxin needed to kill 50% of a group, the lower the _LD50 and the more potent the poison or venom is.

The LD₅₀ depends on several parameters, such as how the toxin enters the body, and how it exerts it’s toxic effects. Venom from a taipan snake, for example, is incredibly toxic if given intravenously (low LD₅₀), but is theoretically completely harmless if eaten (high LD₅₀). The LD₅₀ also depends on the type of organism the drug is administered to; Penicillin is lethal to many bacteria (low LD₅₀), but generally safe for humans in a very high dose (high LD₅₀). When it comes to poisonous and venomous animals, it’s important to remember that the LD₅₀ of the animal’s venom/poison is not everything. The ammount of venom/poison in the animal is also important. For example, the venom of the black widow spider contains the toxin latrotoxin. Latrotoxin has an LD₅₀ of 4.3 µg/kg, which is significantly lower (more potent) than the LD₅₀ of the venom of the carpet viper Echis carinatus, which is 120 µg/kg. However, the black widow spider, Latrodectus curacaviensis can only deliver around 0.6 mg of venom per bite, while the viper Echis carinatus can deliver around 72 mg per bite (360 times more!) (Figure 2).

 

Table 1. Examples of LD₅₀ for different toxic substaces

Toxin	LD50	Organism	Delivery method
Ethanol (“alcohol”)	7.060 mg/kg	Rat	Oral*
NaCl (table salt)	3,000 mg/kg	Rat	Oral*
Cocaine	96 mg/kg	Mouse	Oral*
Strychnine	1-2 mg/kg	Human	Oral*
Forest cobra venom	0.225 mg/kg	Mouse	Subcutaneous**
Forest cobra venom	0.093 mg/kg	Mouse	Intravenous***
Inland taipan venom	0.025 mg/kg	Rat	Subcutaneous**
Tetrodotoxin (from pufferfish)	0.008 mg/kg	Mouse	Intravenous***
Latrotoxin (from black widow spider venom)	0.0043 mg/kg	Mouse	N/A
Batrachotoxin (from poison dart frog)	0.002-0.007 mg/kg	Human	Subcutaneous**
Botolinum toxin (botox)	0.000001 mg/kg	Human#	Oral*

The unit of LD₅₀ in this table is mg of toxin to be given per kg body weight. Values are taken from Wikipedia and SnakeDatabase respectively. ^# = Estimated LD₅₀. Delivery method terms explained: *Orally: Taken by ingestion, **Subcutaneously: Injection under the skin, ***Intravenously: Injection directly into a vein.