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Republican lawmakers in South Dakota have passed a bill that could make it a crime for doctors to support transgender children and teenagers.

The bill, which passed in a 46-23 vote, will now go to the state senate where it could become law – making it illegal for doctors to give medically necessary care to trans children and teenagers.

Doctors in South Dakota who prescribe puberty blockers or cross-sex hormones to under 16’s with gender dysphoria could face a $2,000 fine and a year in prison under the proposed law.

“By blocking medical care supported by every major medical association, the legislature is compromising the health of trans youth in dangerous and potentially life-threatening ways,” said Libby Skarin, policy director for the ACLU of South Dakota, in a statement.

“Discrimination against a marginalised group is a distraction from the state’s real needs and hurts us all. Transgender young people live in our state and need to feel like the government represents them, too. The more we legislate solutions in search of problems, the more our communities suffer.”

South Dakota is the first US state to pass a bill like this, which takes away parents’ rights to make decisions about their children’s healthcare, although Republican-backed bills targeting trans youth are currently being proposed in a dozen other states.

Dr Alexis Chavez, medical director at LGBT+ youth suicide-prevention organisation The Trevor Project, said the proposed bills “not only contradict reality and majority medical opinion in the United States — they would also put young lives in jeopardy”.

The medically necessary care being criminalised includes puberty blockers, a safe, reversible and “life-saving” treatment that pauses puberty for trans teens so that they can wait until they’re older to make bigger decisions about transition-related healthcare.

Puberty blockers are most effective if taken before or at the beginning of puberty. The onset of puberty happens before the age of 16 for many people.

It would also be illegal for doctors to provide under 16’s with gender-confirmation surgery or hormone treatments, although these treatments are relatively rare for under 16’s in the US.

The bill was sponsored by Republican Fred Deutsch, who recently apologised for comparing doctors treating trans kids to Nazis.

When the bill passed, Deutsch tweeted: “After some of the heaviest lobbying I have seen in years, South Dakota‘s bill to protect gender confused children passes 46 to 23 .”

Chase Strangio, a transgender lawyer for the ACLU, sent a tweet aimed at reassuring trans youth in South Dakota.

“To trans youth in South Dakota and beyond: We know you know who you are,” Strangio said.

“You don’t need to be protected from your own agency but from the powerful adults who claim to know what is best for you and your bodies. We will fight with you against these horrible bills.”