And just to expand on that like a pedantic shit; it is the commonly accepted method (but debatably not the best method) for measuring the toxicity of any substance, and is often in reference to tests on rodents. It is expressed in mg per kg, usually, and there are different mehods of introducing the substance into the test subject's body. For instance, the LD50 of a highly venomous snake like the black mamba is .32 milligrams per kilogram when injected into mice subcutaneously, and more like .25 (less venom to kill 50% of subjects) when injected intravenously. The Black Mamba is a very venomous snake, and among the most deadly to humans but it is not nearly the most venomous. Black Mambas are fast aggressive, large and frequently near human habitations in Africa. Envenomation by it is 100% fatal without treatment. The Many-Banded Krait has a vastly lower LD50 (as little as 0.08 when intra-peritoneal) but causes few deaths because of the comparative rarity of encounters with humans.
The LD50 of THC in rodents is around 1270.
Water is about 90 in rodents, oddly enough.
Botulinum toxin is approximately 1 nanogram per kilogram, but it is so toxic that's an estimation.
We let people inject that shit into their faces!
Theobromine, the caffeine molecule found in cocoa is about 1000.
1270 is absurdly high, and there are conflicting studies, but they all agree on similarly crazy-high (safe) LD50 values for cannabis products.
Sources;
http://www.seanthomas.net/oldsite/ld50tot.html
https://en.m.wikipedia.org/wiki/Median_lethal_dose
https://weedpress.wordpress.com/science/studies/ld50-of-cannabis/
http://www.sciencedirect.com/science/article/pii/0041008X74901264
https://medlineplus.gov/botox.html