Carbon monoxide and tar in cigarettes are not addictive. However, they are poisonous and the reasons for other complication. They contain substances that stick and decrease the amount of oxygen in the blood.
It is mainly the nicotine. But I believe it has been proven, that cigarette manufacturers have "doubled the dose" of the nicotine in cigarettes making them more addictive or harder to quit. I have my suspicions on a couple other products, but they aren't as unhealthful as cigarretes, at least to those in the immediate vicinity of the user. Also they are just "suspicions" and I don't necessarily want to besmirch a company on suspicions alone. (I'm referring to other products, containing other drugs--Not just nicotine)