Steroids are quite good in terms of chemistry - they have loads of different uses, e.g. endogenous hormones and drugs.
Cigarettes is also good, more so medically than in terms of chemistry IMO, but you can also research about the main carcinogens in cigarettes - benzopyrenes - as well as the effects of smoking tobacco.
Is this something like the EPQ? In that case, I think I would go for cigarettes, as it's a much wider topic medically, IMO.
Haha thats ok i think its interesting too, I haven't had the time to look into it fully myself so if you do research this be sure to let me know what you find out