Supporters of the Arkansas bill say it would protect young people from undergoing irreversible medical treatments, and the text of H.B. 1570 claims — contrary to the consensus of medical professionals — that “the risks of gender transition procedures far outweigh any benefit at this stage of clinical study on these procedures.”

Medical research shows the opposite.

In a 2019 statement opposing laws that restrict minors’ access to gender-affirming treatment, the American Academy of Child and Adolescent Psychiatry said, “Blocking access to timely care has been shown to increase youths’ risk for suicidal ideation and other negative mental health outcomes.”

More broadly, in an official position paper in 2018, the American Psychiatric Association said there was “significant and longstanding medical and psychiatric literature” demonstrating “clear benefits of medical and surgical interventions” for transgender people.

Sam Brinton, vice president for advocacy and government affairs at the L.G.B.T.Q. suicide prevention organization the Trevor Project, said people who contacted the group for help during mental health crises often cited discrimination and public expressions of anti-trans sentiment.

“When that discrimination is given a bill number, it can be devastating,” Mx. Brinton said, citing research indicating that young trans and nonbinary people who reported experiencing discrimination based on their gender identity were twice as likely to have attempted suicide, and that those who reported having at least one “gender-affirming space” — which could be a doctor’s office — were 25 percent less likely to have attempted suicide in the past year.

Mr. Hutchinson’s veto was striking not only because he is a Republican, but also because just last month he signed bills that allowed doctors to refuse to treat people based on religious or moral objections and that barred transgender women and girls from competing on women’s sports teams in high school or college. (Such measures have become popular among conservative lawmakers, who have introduced them in more than two dozen states this year.)

He argued that H.B. 1570 was “overbroad, extreme and does not grandfather those young people who are currently under hormone treatment,” and said, “The state should not presume to jump into the middle of every medical, human and ethical issue.”