Can i be fired for use of drugs and violating drug free workplace policy , if a drug test was never preformed?

I would say if the statement was not true you have a slander case. The employer should have at the very least looked for corroborating evidence before taking action. For instance if some person had called the police department and reported such an infraction, generally the police department must have corroborating evidence prior to taking action to arrest someone. I am sorry for your situation and wish you good luck.

A defamation lawsuit may be possible, but costly

This is an employment/labor law related inquiry and as an attorney that exclusively focuses on, and specializes in, driving under the influence related cases I strongly urge you to seek consultation with an attorney that focuses on employment / labor law related cases.