Ok here we go...
I have a Military background, I went to Military school and I enlisted in the Marines when I was 17.  I have always wanted to be in the Military and LE since I was a kid.
In my 2nd year of being in the Marine corps I tested dirty for Cocaine and got a General discharge.  I am working with the VA to get it changed to an Honerable discharge and since it was a mistake made ONCE when I was 19 and no other bad conduct record on my file it will be changed soon.
I has been the worst thing in my life to happen and has been eating me up in side.  Right now I am working full time, I have held a full time job since my discharge and I am in school to get my BA in Criminal Justice.  I should graduate when I am 26 so it will have been 7 years since the discharge.  Does this hurt my chances to have a career in LE.

You might be okay "IF" you can get your discharge changed to honorable.  I don't know of any dept. that will consider an applicant with anything other than an honorable discharge, but I suppose there may be a few out there.

Also,  having used cocaine just once will DQ you with a lot of depts. and most PD's don't publish what their particular DQ criteria are.  However, there are many that will hire you.   

Good luck.

I have to agree Brandon.  I am also a Marine and I have too say that until the discharge is changed your background will always be following you around intil you change it.