Blood concentration of 0.5-1.0 mg nicotine/kg human body weight, which is the amount of nicotine in about 5 to 10 cigarettes, if all of the nicotine were absorbed, can be a lethal dosage for adults. It is impossible however to overdose on nicotine through smoking alone, although a person can overdose on nicotine through a combination of nicotine patches, nicotine gum, E-cig, and/or tobacco smoking at the same time.
Nicotine produces physical and mood-altering effects in your brain that are temporarily pleasing.