Nicotine! 

As the tobacco industry has known since the 1960s. 

See

Nicotine and Addiction

The Brown and Williamson Documents

John Slade, MD; Lisa A. Bero, PhD; Peter Hanauer, LLB; Deborah E. Barnes; Stanton A. Glantz, PhD

JAMA. 1995;274(3):225-233. doi:10.1001/jama.1995.03530030045033.

What Substances Are In Tobacco Smoke?

By Jennifer Kenny

1 Tobacco smoke contains numerous ingredients. These ingredients can be quite potent. For example, forty-three of the almost four thousand chemicals in tobacco can cause cancer. The most dangerous compounds are tar, carbon monoxide, nitrogen oxides, hydrogen cyanide, metals, ammonia, and radioactive compounds.

2 One major ingredient is nicotine, the addictive drug found in tobacco smoke. Nicotine itself is not considered to be cancer-causing, but it is the main reason smokers continue to smoke. Therefore, it is dangerous. Nicotine is toxic and can be deadly in large doses. It is a highly addictive drug.

3 Another ingredient found in tobacco smoke is tar. Tar refers to all the particles suspended in tobacco smoke. It contains nitrogen oxides, carbon dioxide, and carbon monoxide. Tar is brown and sticky. It stains teeth. It stains fingernails. It stains lung tissue. Tar is dangerous because it contains the cancer-causing ingredient benzopyrene.

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