What's in the smoke? When someone breathes in tobacco smoke they breath in many chemicals including: Nicotine - a stimulant drug that acts to speed up the central nervous system. Nicotine is also poisonous and contributes to the addiction to tobacco Tar - a black, sticky substance that contains most of the cancer causing chemicals in the smoke. Tar is also what makes the teeth and fingers of smokers go brown Carbon Monoxide - a poisonous gas that reduces the amount of oxygen available to the body