Do Medicinal Herbs Lose Potency When They Bolt?

I was wondering if medicinal herbs lost their potency once they go to bolt, particularly Feverfew? I know with most culinary herbs they aren't as good once they've flowered. Does anyone know if this also affects the plants medicinal properties?

Thanks!

This probably won't be much help -- it really depends on the plant. If you want to make a tincture with St. John's Wort, for instance, you need the flowers.