No one with a brain and a life ahead of them should ever smoke. It gives you a high chance of cancers, diseases, wrinkles, lowering athletic abilities, breathing problems, coughing, wheezing, pursed...
Not to be exact butbthe usual things you would find in cigarettes would be
Tar
Metals
Hydrogen Cyanide
Pesticides like Rat Poison
Radioactive compounds
Bleach
Paint stripper
there are many...