The US Supreme Court has ruled against a Colorado state law banning "conversion therapy" for LGBTQ minors, siding with a Christian therapist who challenged it on the grounds of free speech.
At issue is the constitutionality of a 2019 Colorado law that prohibits licensed practitioners from conducting "conversion therapy" on patients under 18.
Proponents of the treatment claim to change the sexual orientation or gender identity of LGBTQ people.
The therapy has been discredited by major medical organisations, including the American Psychiatric Association and the American Medical Association, and is banned in more than 20 US states and much of Europe.
Research has shown that it is ineffective and even harmful, leading to depression and suicidal thoughts.
But in an 8-1 decision, the court ruled in favour of Kaley Chiles, a licensed mental health counsellor who invoked her Christian faith and challenged the law, arguing that it violated her First Amendment right of free speech.
"Colorado's law addressing conversion therapy does not just ban physical interventions. In cases like this, it censors speech based on viewpoint," wrote conservative Justice Neil Gorsuch in the decision on behalf of the majority of the court.
"As applied to Ms Chiles, Colorado's law regulates the content of her speech and goes further to prescribe what views she may and may not express, discriminating on the basis of viewpoint," he argued.
The First Amendment, Mr Gorsuch wrote, is a "shield against any effort to enforce orthodoxy in thought or speech in this country".
As a result, the Supreme Court sent the case back to the lower courts to review their decisions.
Ruling opens 'can of worms' - dissenting judge
Only liberal Justice Ketanji Brown Jackson expressed dissent, accusing her colleagues of opening "a dangerous can of worms" by undermining states' ability to regulate medical practices that "risks grave harm to Americans' health and wellbeing."
"The Constitution does not pose a barrier to reasonable regulation of harmful medical treatments just because substandard care comes via speech instead of scalpel," she wrote.
Ms Chiles' lawyer, James Campbell, of the conservative Alliance Defending Freedom, hailed the ruling in a statement as "a significant win for free speech, common sense, and families desperate to help their children".
After taking office for his second term in January of last year, President Donald Trump said the US government would only recognise two genders - male and female - and signed an executive order restricting gender transition medical procedures for people under the age of 19.
In June, the Supreme Court voted 6-3 to uphold a Tennessee state law banning hormone therapy, puberty blockers and gender transition surgery for minors.
Conversion therapies are banned, at least partially, in many countries, with the support of health organisations such as the Royal College of Psychiatrists in Britain.
The United Nations has called for a global ban, describing the therapies as discriminatory, humiliating and a violation of individuals' bodily integrity.
