In a nutshell Ld50 is thought to be the lethal dose for nicotine. But this assumption is based documents from over 100 years ago. In more recent times, it is well documented that, the self experiments of some people/scientists consuming much larger doses survived. Most notably, a suicide attempt by Schmidt in 1931 who attempted this with 4g (not 4mg, 4g) of pure nicotine and survived.
The Ld50 value is still accepted without scrutiny and taken as the basis for worldwide safety regulations of tobacco and other nicotine-containing products.
Nicotine is a toxic compound, and should be handled with care.