While we know smoking tobacco is linked with certain diseases and chronic conditions that will lead to an early death, nicotine is also lethal if ingested in high doses; an oral dose (consumed rather than inhaled) of 50 to 60 milligrams of nicotine is enough to kill a 160-pound person [source: NIOSH]. For example, manufacturers recommend no one chew more than 24 pieces of nicotine gum in one day, because if you were to ingest any more nicotine than that you'd risk an overdose. Yes, you can overdose on nicotine.
Nicotine is a stimulant, as we've discussed, and when you're exposed to too much, it causes the cholinergic neurons at the cholinergic receptors in your body to get excited. It's the same response your body's nervous system has when poisoned by organophosphate insecticides such as DDT, and the same as when you're exposed to nerve agents such as sarin gas. When you're exposed to toxic levels of organophosphate, the neurotransmitter acetylcholine builds up at synapses and disrupts nerve impulses. Acetylcholine is able to imitate the electrical stimulation you'd normally associate with your vagus nerve, and all that excess acetylcholine overstimulates the neurons. Because nicotine is so similar to acetylcholine, it too binds to nicotinic cholinergic receptors and, in excess, produces the same overstimulation -- the more nicotine available in your body to bind to the nicotinic cholinergic receptors, the greater the severity of the poisoning.
Nicotine poisoning delivers a biphasic effect -- it first acts as a stimulant in the body but rapidly turns into a depressant. Vomiting is the most common symptom of nicotine poisoning and can begin as quickly as 15 minutes after ingestion. Nicotine may also cause seizures and involuntary muscle twitching, as well as abnormal heart rhythms, a slow heart rate and fluctuating blood pressure. In high concentrations, nicotine may cause death within as little as an hour, usually due to heart failure, muscle paralysis and a buildup of fluid in the lungs' air passages [source: CDC].
If nicotine poisoning is feared, call a poison control center immediately.