Missouri has a two year statute of limitations for defamation. If your ex-husband accused you falsely of being a drug user and abuser, and these accusations were communicated to others within the last two years, a defamation suit is something you could consider.
False accusations of the sort you describe can result in significant reputational injury, especially for someone in your profession (as you have already discovered). A lawsuit win can result in an award of damages for this reputational injury and you can be compensated for the economic losses you suffered as a result of the defamatory publications.
However, there are both pros and cons with a defamation suit. One con, of course, is that there is never a guarantee of a lawsuit win. Others are that legal costs are high and legal actions are time consuming (often taking years to work their way through the court system - which may increase your contact with your ex). And your ex-husband may not have income or assets enough to satisfy any damages awarded, should you win a suit against him.
With a defamation lawsuit, the defamatory comments that were made may be presented to a wider audience, as court proceedings and court records are (generally) open to the public. This can, ironically, result in more reputational injury. These court records can also be accessed by employers when doing background checks.
And monetary awards, no matter how large, cannot really repair a damaged reputation.
With all of that said, I still suggest you consult with an attorney in your area and review the facts of your situation. Perhaps a letter from the attorney to your ex-husband, advising him that you are seriously contemplating a defamation lawsuit if he does not stop making defamatory comments about you and interfering with your employment, may be enough to stop him from bothering you in the future.