Surging drug use has contributed to an epidemic in the Pacific island nation, where one in 60 adults is estimated to be living with the virus.

The White House recently endorsed monitoring sewage for evidence of drug use. Critics fear such efforts could violate privacy and stigmatize neighborhoods.

With support from Markwayne Mullin and Robert F. Kennedy Jr., the kratom industry is pursuing a potentially lucrative policy. Mr. Mullin owns equity in a company that could benefit.