Arsenic

Undoubtedly, you've heard of arsenic poisoning. You know, the kind used in killing rats! It finds its way into your cigarette by means of some of the pesticides that are used in tobacco farming. And once you've smoked a cigarette, you've just willingly consumed rat poison's main ingredient!

Also, like most other cigarette ingredients, arsenic is also carcinogenic. Yet another reason to quit smoking today itself.