Careful with your numbers. Yes, in rats the LD50 is ~50 mg/kg. In humans, it is around 50mg per human. Huge difference.
"The LD50 of nicotine is 50 mg/kg for rats and 3 mg/kg for mice. 4060 mg (0.5-1.0 mg/kg) can be a lethal dosage for adult humans."
Sorry had to remove links. I guess I am...