It is unlikely that a person would overdose on nicotine through smoking alone. The US Food and Drug Administration (FDA) stated in 2013 that there are no significant safety concerns associated with the use of more than one form of over-the-counter (OTC) nicotine replacement therapy at the same time, or using OTC NRT at the same time as another nicotine-containing product, like cigarettes. The median lethal dose of nicotine in humans is unknown. Nevertheless, nicotine has a relatively high toxicity in comparison to many other alkaloids such as caffeine, which has an LD50 of 127 mg/kg when administered to mice. At sufficiently high doses, it is associated with nicotine poisoning, which, while common in children, rarely results in significant morbidity or death.
The initial symptoms of a nicotine overdose typically include nausea, vomiting, diarrhea, hypersalivation, abdominal pain, tachycardia (rapid heart rate), hypertension (high blood pressure), tachypnea (rapid breathing), headache, dizziness, pallor (pale skin), auditory or visual disturbances, and perspiration, followed shortly after by marked bradycardia (slow heart rate), bradypnea (slow breathing), and hypotension (low blood pressure). Respiratory stimulation (i.e., tachypnea) is one of the primary signs of nicotine poisoning. At sufficiently high doses, somnolence (sleepiness or drowsiness), confusion, syncope (loss of consciousness from fainting), shortness of breath, marked weakness, seizures, and coma may occur. Lethal nicotine poisoning rapidly produces seizures, and death – which may occur within minutes – is believed to be due to respiratory paralysis.
Today nicotine is less commonly used in agricultural insecticides, which was a main source of poisoning. More recent cases of poisoning typically appear to be in the form of Green Tobacco Sickness, accidental ingestion of tobacco or tobacco products, or ingestion of nicotine-containing plants. People who harvest or cultivate tobacco may experience Green Tobacco Sickness (GTS), a type of nicotine poisoning caused by dermal exposure to wet tobacco leaves. This occurs most commonly in young, inexperienced tobacco harvesters who do not consume tobacco. People can be exposed to nicotine in the workplace by breathing it in, skin absorption, swallowing it, or eye contact. The Occupational Safety and Health Administration (OSHA) has set the legal limit (permissible exposure limit) for nicotine exposure in the workplace as 0.5 mg/m3 skin exposure over an 8-hour workday. The US National Institute for Occupational Safety and Health (NIOSH) has set a recommended exposure limit (REL) of 0.5 mg/m3 skin exposure over an 8-hour workday. At environmental levels of 5 mg/m3, nicotine is immediately dangerous to life and health.