The Missouri attorney general, citing a consumer protection law normally used to prosecute fraudulent business practices, issued a new state directive this week that would severely restrict gender-transitioning treatment for both adults and minors.

In the emergency regulation announced on Thursday, Attorney General Andrew Bailey, a Republican, said it was necessary to put up “substantial guardrails” around such medical treatments because they were considered “experimental.” Those guardrails include at least 18 months of therapy with a psychologist or psychiatrist before receiving puberty-blocking drugs or surgeries.

He invoked the state’s Merchandising Practices Act in issuing the directive, saying that the attorney general “is charged with protecting consumers, including minors, from harm.”

“The regulation is necessary due to the skyrocketing number of gender-transition interventions, despite rising concerns in the medical community that these interventions lack clinical evidence of safety or success,” said Mr. Bailey, who was appointed in November. He is running for a full four-year term in 2024.