My best friend has started smoking which I know is dangerous but she won't stop. Should I tell her parents?

You are right that smoking is dangerous.

Not only is it very expensive but it can also lead to all sorts of health related problems in later life and in fact shortens and kills many peoples lives.

First, you need to make sure that you are not tempted yourself into starting it - the risks are too high but it sounds like you are very sensible so well done.

Second, you should try to tell your friend away from those she has started smoking with of the risks and how she will regret it in the future. She has probably started due to peer pressure and thinking that it is cool to smoke even though of course you know that it couldn't be any more uncool.

If she continues to smoke once you have spoken to her then you may have to accept that you have done all you can even though you care about her and at the end of the day it is her choice (if she is above the legal age to smoke of course).

There is always a temptation to tell her parents, however you should think carefully before you do as if she finds out who told them then she will probably get mad and fall out with you.

Therefore if you tell her parents make sure they swear not to reveal their source to your best friend.