The active ingredient in it is safrole, which comes from the roots and bark of the Sassafras tree, part of the laurel family. There are three species of Sassafrass trees. Sassafras albidum is the one native to eastern North America from Floria over to Texas, up to Ontario and then east to Maine; the other 2 species are native to eastern Asia.
A Sassafrass tree can grow up to 30 to 60 feet tall (9 to 18 metres.) It is a deciduous tree that grows at the edge of forests and fields. It has a deep tap root, making it hard to transplant.
The tree has smooth leaves 3 to 7 inches long (8 to 18 cm.) The leaves can have three shapes: oval, or look somewhat like a child’s mitten, or look somewhat like the footprint of a bird in snow. Sometimes all three shapes of leaves will be on the same tree, even on the same branch. In the fall, the leaves turn yellow, red and orange.
There are female and male trees. They both blossom with small yellow flowers in April that are not very fragrant. The female trees produce blue berries on red stems.
The roots, bark and leaves of the tree are used. The ground leaves are called filé powder. They were used by the Choctaw Indians in Mississippi and Alabama as a spice. The leaves are best picked in August; some say during the full moon. The powder loses its taste and aroma when stored too long, and gets stringly if cooked too long — add it at the end of cooking.
The roots are considered best gathered in spring. The best source for roots is trees when they are still bush size, around about a yard (1 metre) tall. You scrub the roots until the bark is clean and pink, then peel off the bark and use the bark for tea. Some felt the branch bark of young branches is just as good as the roots. There two layers of bark, a thin dark outer layer, and the spongey rusty-orange coloured inner layer, which is where the flavour is and earthy smell is. You can scrape the bark off with a vegetable peeler, and then shred the bark with a grater.
Shredded Sassafrass bark is used to make a reddish-coloured tea with. You use 1 cup of shredded bark per 1 quart of water, simmer it for 10 to 12 minutes (some recipes suggest pouring boiled water over top, then steeping instead of simmering) then strain it, and sweetenwith a sweetener such as sugar or honey.
You can also buy instant Sassafras tea. The flowers can also be used for tea.
Sassafras bark used to be used in flavouring root beer, but it has now been replaced by artificial flavourings.
Sassafras can be used as a yellow dye, and for scented soaps.
The wood of the tree is very durable. Indians used it for dugout canoes.
80% of the volatile oils in the bark of the roots is safrole. Because safrole is said to be cancer-cancering, it was banned as an additive in 1960 by the American Food and Drug Administration (FDA), and Sassafras tea sale was banned in 1976.
“Safrole is reasonably anticipated to be a human carcinogen based on sufficient evidence of carcinogenicity in experimental animals”; 
“No adequate human studies of the relationship between exposure to safrole and human cancer have been reported”; 
“(FDA) Safrole is prohibited from direct addition to food or use as human food.”; 
Safrole is also found in basil, black pepper, cocoa, mace, nutmeg, and star anise.
Some say that safrole’s cancer-causing properties make cigarettes look benign; others say it’s less dangerous than the alcohol in a can of beer.
Today, safrole is used to manufacture Ecstasy, the club-goers drug.
A safrole-free extract is now being made. The extract has to be made in labs. It is not possible to do it at home. The flavour is not deemed as good.
 Above three quotes from: Safrole. In Eleventh Report on Carcinogens. By U.S. Department of Health and Human Services, Public Health Service, National Toxicology Program. 2004.
Sassafras was being grown in England by 1663.