This section is from the book "A Practitioner's Handbook Of Materia Medica And Therapeutics", by Thos. S. Blair. Also available from Amazon: A Practitioner's handbook of Materia Medica and Therapeutics.
Sassafras is aromatic, astringent, and alterative, and is esteemed as a preventive of contagion by some of our foreign population, who eat onions and drink sassafras tea as soon as contagious diseases appear. Some physicians practicing among them think these agents exercise some influence in the matter. Possibly! The oil of sassafras in large doses is a narcotic poison. Most narcotics, in minute doses, exercise more or less of a stimulating influence, and probably sassafras may possess more remedial influence than we have thought. The popular idea that it "thins the blood" simply reflects a belief in its alterative properties.