I see no issue here. You were given a written reprimand and a warning not to do drugs on the job. If you wish to contact an employment attorney, you may have better luck-as opposed to posting it in the criminal law section. However, I would be surprised if anything can be done.
This is not intended as individual legal advice and there is no attorney client relationship established by this answer. It is advisable that you seek individualized legal assistance. This is not a substitute for hiring an attorney.