Try to avoid traditional uses of Tansy in medicine, if at all possible.  While it may have some medicinal effects, taken in large dosages it can cause violent reactions and seizures.  The substance it contains that has been found to be dangerous is called Thujone, which, interestingly,  is the same property that has historically provided the medicinal value of the plant.  This substance is also found in wormwood.  Tansy plants themselves vary widely in the amount of Thujone they contain, and growing conditions do not seem to affect the content, suggesting individual plant genetics.  So basically, you don't know what you have in any given plant, making correct dosage calculations almost impossible.

Tansy has been used traditionally in remedies for expelling worms in children, nervous disorders, hysteria, kidney weaknesses, reducing fevers, soothing the stomach and relieving flatulence, and gout.  Externally, it has been used to treat some skin eruptions, and as poultices (pounding the leaves into a paste - use water or a Tea and add oatmeal to reach desired consistency) for rheumatism and sprains.

Once again, unless you are an experienced herbalist, leave this one alone, except in small amounts as a food flavoring.