Final answer:
The most addictive substance with a high rate of addiction upon first use is nicotine, which is ingested by smoking or chewing tobacco. Nicotine's stimulant effects are immediate but short-lived, and it doubles as a potent insecticide.
Step-by-step explanation:
The most addictive substance, with 1 in 4 people who use it for the first time becoming addicted, is nicotine. Nicotine is a stimulant that affects the brain by mimicking the action of the neurotransmitter acetylcholine. People usually ingest nicotine by smoking or chewing tobacco. The stimulant effect of nicotine is initially strong but quickly followed by a period of depression. Besides its use in tobacco products, nicotine has been used as a contact insecticide in agriculture, underlining its toxicity. With a lethal dose estimated at about 50mg for humans, nicotine's potent nature contributes to its high addiction rates.