This is Tanacetum parthenium or feverfew, one of the numerous members of the Asteraceae family. This perennial is a prolific seed producer so one plant in the yard will quickly turn into a yardful within a few years if the flowers are not regularly deadheaded and removed. New flowers will replace the old thus deadheading is also useful for keeping the yard in bloom all season long.
Medicinally, it's primary use is for the treatment of migraines.
I'm just going to say it - migraines suck. And I don't mean in an "oh, I have a headache today" kind of suck.. They can be absoluting debilitating. I suffered with them on a daily basis for years. It wasn't until I was treated for tick-borne infections that they (mostly) disappeared.
We don't understand what causes migraines. One hypothesis is the chain of events that triggers migraines may be initiated by a significant increase in serotonin from platelets. Feverfew contains a compound called parthenolide that is antiplatelet and also inhibits platelet aggregation and the release of serotonin. It is thought this compound is what accounts for feverfew's antimigraine action, although this has been a matter of debate recently.
Regardless of how it works, according to three double-blinded studies using dried, powdered leaf, feverfew reduced the severity, duration, and frequency of migraine headaches.
However, it is preferred to use fresh aerial parts of the plant (leaves and flowers). Some migraine sufferers will consume 1 to 3 fresh leaves per day for migraine prevention. Two concerns with using fresh leaves is the leaves are very bitter and they can cause mouth ulcers in sensitive people. Rather than eating the leaves directly, one could easily encapsulate the leaves to avoid both of these concerns. For acute headaches, 1 to 2 teaspoons of leaves soaked in one cup of hot water for 10 minutes can be helpful.
Traditionally, feverfew has been used in gastrointestinal issues. Given how bitter it is, this makes sense as bitters promote gastric secretions, increase appetite, and improve digestion. It was also used to stimulate menstrual flow in pre-menopausal women who were not menstruating, and it was added to baths to treat rheumatism and sprains.
Because it stimulates menstrual flow, feverfew should not be used in early pregnancy. It may also cause contact dermatitis in sensitive people and should be avoided in anyone with an allergy to this family of plants.
While I don't consider feverfew to be the best herbal medicine for migraines, if ingesting a few leaves each day keeps the migraine monster at the door until the underlying cause is found, it is certainly worth a try.