Sassafras tea causes cancer and liver damage as a result of its high safrole content, and the U.S. Food and Drug Administration banned use of the tea in 1976, according to WebMD. Safrole is a deadly, poisonous substance in sassafras, a plant that people have used for medicines and flavoring.
About 200 milligrams of safrole are in a cup of tea made with 2.5 grams of sassafras, which is approximately 1.5 milligrams of safrole per pound of body weight, explains WebMD. That amount is about 4.5 times higher than the level that research indicates is toxic. Consuming about 5 milligrams of sassafras oil can cause death in an adult.