You're doing WHAT?! A daughter starts smoking.....

Before I start, let me give you some insight to the type of family we are.

I am a single mother, divorced, with 3 teens.  My oldest just graduated high school, is 18, and she will be starting college in the fall.  My other 2, twins, are 15 and going into 10th grade this coming fall. The twins are boy/girl.

I homeschooled my kids until high school, at which time I felt that it would be more beneficial for them to attend a regular school.  It's my own preference and I don't judge those that feel they can and will teach throughout the high school years.

We've always been a happy family and while I know most parents think so... my kids really are generally good kids.  I've had very few problems with them through the years and we've tackled issues that most people never have to face  -  including me being diagnosed with MS and battling cancer 3x.

NOW.... that being said... you have a general idea of how my family is.  We are all in all a pretty happy go lucky group.  Oddly enough, even my oldest's entry into high school didn't seem to produce the usual turmoil that most parents of teens seem to face and for that I was quite happy.  I guess all good things must come to an end though! LOL

My biggest issue right now is tackling the idea that my oldest, 18, has decided she will smoke.  Now we have zero smoking in our house and around our family.  We've actually been very ANTI smoking if you had to pick a position for us so this came as a total shock to me.  She claims "it calms her down" and this is why she decided to do it.  I think she's only been doing it for a few months and most all of her friends (whom are all pretty good kids themselves) do it, so I would guess that it's a combination of "being like them" and her own idea that it truly does make her "calmer".  She's been stressed with the oncoming of college and graduating, etc. and yes stressed.  She even dealt with the loss of a friend who committed suicide on December 24th this year, the year she would have graduated.  I took her to counseling and it helped her a lot but she is not one to "talk"... much like her mother.. and keeps things to herself.  She won't cry on others shoulders, bogging down her friends with her own misery and while I know it's not always a great way to look at life it's how I basically am so I can't really say much.

My question is this, when I found out I acted like I sort of suspected it anyways... which is a half lie.  I saw some butts in the bowls upstairs in her and her sisters' room and some empty packs of cigs but her friends also go up there so I assumed they allowed them to smoke.  That alone kind of boggled my mind because we all detest the smell of smoke but I figured sometimes we will allow friends to do things that normally we ourselves don't really find too keen. 

So I acted like I pretty much knew and made the comment to her that there's nothing I can do anyways but she knows she's basically an idiot for doing it.  After our own battles with cancer together I thought the last thing I would ever worry about with my kids was something like smoking that is a proven culprit for lung cancer. 

What do I do?  What do I say?  Should I talk to her longer and deeper about the subject?  Her exact comment to me of WHY she does it was "My nerves are more calm now that I'm smoking".  I guess that's a valid excuse in some sorts because the nicotine does have that effect but this really upsets me a lot.  I haven't let her know how much and haven't really sat her down at all to talk about it because to be honest I'm so ticked off that I don't want to lose control or get angry.  She KNOWS better, she's a medical student and KNOWS the effects of smoke on the body and lungs. 

Please offer some advice.  I'm open to listening to everyone and everything you can say.  I need to find an answer as to how to handle this and hopefully get her to wake up.