Here is a list of woods acceptable for smoking.
http://www.smokingmeatforums.com/threads/woods-for-smoking.50439/
Toward the bottom of the list it says to avoid using sassafras. There's a reason though. It is either poisonous or carcinogenic, especially used long-term. I'll see if I can find that missing fact.
Found it. Sassafras creates safrole, which is known to cause cancer and liver damage. That doesn't stop people from using it though. Some folks like the taste.
There was a similar story I read a while back about a guy who loved the taste of meat smoked with hemlock in spite of the warnings. He ended up dying of stomach cancer at at 45.