Speaking from experience, I used to know someone who smoked weed in tobacco joints. As soon as he quit cigarettes and switched to a pipe his godawful cough dissapeared completely.

Obviously anecdotale, but I would guess the big danger is unfiltered tobacco.

I'm also wondering who is behind the weed smear-campaign at the moment. Is it just that it's silly season, or are people really trying to demonise it? I really have to think if the promised dangers were true, we would be seeing something of a mass public health outbreak at the moment. It really has been a commonplace habit since time immemorial.