There are many references, but specifically it is outlined as:
"The volatile oil of sassafras is believed to be the major active constituent of the plant. This oil contains up to 85% of the terpenoid known as safrole. Safrole causes liver cancer when given to laboratory animals in high doses for long periods of time. Sassafras bark, sassafras oil, and safrole are currently prohibited by the U.S. Food and Drug Administration from use as flavorings or food additives. Human studies are lacking to verify the efficacy of sassafras for any condition. However, one case study has been published showing that sassafras acted as a diaphoretic in an otherwise healthy woman. While the amount of sassafras that could potentially cause cancer in humans remains unknown, one cup of strong sassafras tea is reported to contain as much as 200 mg of safrole, an amount that is four times higher than the amount considered potentially hazardous to humans if consumed regularly."
The amount of File normally used in cooking... and from someone who is normally very cautious about these matters... doesn't concern me in the slightest.
"Food is our common ground, a universal experience." - James Beard