“Standard textbooks, databases, and safety sheets consistently state that the lethal dose of nicotine for adults is 60 mg or less (30–60 mg), but there is overwhelming data indicating that more than 500 mg of oral nicotine is required to kill an adult.[7]” – Source
"The average cigarette contains about 10 to 12 mg of nicotine. " – Source
I guess it depends on your definition of “cigarette butt”.