If the statements made by these people are false, and they have communicated these false statements to others, then the statements are slanderous. If, in addition, your spouse's reputation has been demonstrably injured as a result of the statements made, then your spouse could have the basis for a defamation action against the speaker of the statements.
Reputational injury includes such things as a demonstrated loss of esteem in the community, a shunning by others, a loss of a job or a loss of income or a loss of clients or the loss of promotion - injury that is directly tied to the slanderous words spoken.
For any slander action, "witness" testimony is necessary - someone who heard the defamatory comments made and can testify as to what was said.
Fault must also be shown - your spouse must be able to prove that the people saying he/she was a drug user said it with negligence (without the due care that would be used by an average person) or with a knowledge that what was said was false. Fault is often one of the more difficult elements to prove in a defamation action.
The police complaint, on its own, cannot be used as the basis for a defamation action, as complaints filed are neither true nor false until investigated. Defamation requires a false statement of fact. If it is discovered after investigation that there is not only no evidence to support the complaint but that the people filed the complaint knowing it was false when they filed it, then the people could potentially be charged with filing a false police report.
Defenses to defamation actions are that what was said was true, what was said was believed to be true, what was said was substantially true, what was said was opinion (and then there are defenses of consent and privilege that do not seem to apply here).
Defamation actions can be extremely expensive, often costing multi-thousands of dollars.
Therefore, I suggest you and your spouse consult with an attorney in Missouri to weigh the pros and cons of any defamation action. Your spouse may want to consider the cheaper alternative of having an attorney draft a cease-and-desist letter to the people slandering him/her. The letter can advise them that if the defamatory statements are not stopped, legal action may be pursued. This may be enough to stop the problem.
