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. These effects spur your continued use of tobacco and lead to dependence or addiction. At this stage, quitting tobacco use causes withdrawal symptoms, including irritability and anxiety. Nicotine addiction brings a host of health problems. While it's the nicotine in tobacco that keeps you hooked, the toxic effects come mainly from other substances in tobacco smoke, containing thousands of chemicals.So far, no evidence indicates that nicotine causes cancer.