One can recover after using it. However, similar to the magic used in Eragon, if you used too much magic at once, you die. The ancient Elven race, known as the Myatha elves, wrote laws forbidding such magic, and that is why it's so rare these days
It may be based on where you draw your idea of a druid from. For me, druids are fictional beings of any race that can use magic in atonement with nature, and unlike the usage of Kura, nature magic can be easily spotted as it drains the life of nature around it to wield. Kura drains the life of the user, which is why so few use it.