Shisha pipe health effects?

some of my friends smoke from a shisha pipe and i just want to know what effects it has on your health. i dont think they smoke tobacco, they smoke different flavours of stuff (dunno what it is, im not educated in that but they have watermelon and aniseed flavours) and i just want to know how harmful it is. they say they dont take it into their lungs, just hold in their mouth, but wll that give them mouth cancer? i just want to know a bit more about them so i can educate them on how dangerous it is.