YA about 4 or so young people in realm with a tree that has magic forbidden medicine.

[McKenna]

The medicine is illegal to use by all or part of the population. It leaves a mark when it is used or people are marked by authorities when they use it (I think on their hand). I wish I could remember more! I read this book around 2003-2006. Also, I think the tree was on the cover.

[Kris]

McKenna, what's the genre (which we need in the topic header) - e.g., YA fantasy, dystopia, mystery?

Can you tell us more about the main characters? Any romance?