I think, The Occulist, that trying to show reputational injury enough to support an award of damages would be difficult, at best, and that would only be if all other elements necessary for a suit (either a defamation or false light action, and in Georgia these often go hand-in-hand) could be met and klc9100 were to win the suit. Any damages awarded would be for a jury to determine based on the facts presented.
While I think a discussion with an attorney may be wise for a review of the facts, I am not sure any attorney consulted will believe there is a suit here worth pursuing.
As klc9100 has noted, it is often impossible to tell why one was denied employment, unless the person in charge of hiring were to say it was due to false and defamatory statements made about klc that were communicated by the former employer, or due to false and defamatory statements circulating about klc in the community, or due to true statements communicated that could easily lead one to a false impression of klc.
Especially in klc9100's described situation, a job denial could be based on many reasons other than the failed drug test and what it can imply (ie. absenses, back-injury, the number of qualified job applicants for the position).
In addition, even if klc9100 were to meet all of the elements necessary to have an actionable case, jury awards are often not enough to cover even the costs of litigation. In one recent Georgia case, a plaintiff won her defamation suit against a novelist who based a fictional character on her, and the jury awarded the plaintiff $100,000 in damages for the reputational injury she suffered as a result - but no attorney fees were awarded. The attorney fees exceeded $100,000. The "victory," therefore, was a bit of a hollow one for the plaintiff.
IF klc9100 were to be denied employment based on false facts communicated in some manner to the hiring agent, then damages are certainly easier to demonstrate and a suit may be more worthwhile. If klc9100 were to be hired by another employer, then there is probably no reason to pursue any action against the former employer, as it would be extremely costly to pursue and there is no guarantee of success in the end.
Again, it may be worth an attorney's review, but I doubt if the facts will justify the expense of litigation. Not all defamatory statements that are made will be worth the time and expense of suing over. This probably holds true for the majority of defamatory statements that are published.