The NHS has announced plans to start offering puberty blockers as part of a £10.7 million clinical trial, prompting warnings of an “unethical experiment on children”.

Puberty blockers were “indefinitely” banned across the UK last year for use in children identifying as transgender, due to fears they are unsafe and harm bone and brain development.

Under the planned trial, run by a team at Kings’ College London, children seeking help from NHS gender services will be eligible for the drugs, if their parents and doctors agree it is suitable. The “Pathways” trial will closely monitor participants for two years, including with regular brain scans.

Puberty blockers suppress the release of sex hormones to pause the physical changes of puberty, such as breast development or facial hair. Over the past two decades, they have become widely used around the world for the treatment of children with gender dysphoria.

In 2024, the NHS stopped using puberty blockers in response to the Cass Review, which found they had safety risks and that there was “remarkably weak evidence” to support them.

In her final report, Dr Hilary Cass recommended a new trial to look into puberty blockers. This NHS-commissioned study, due to last until 2031, was confirmed by the National Institute for Health and Care Research (NIHR), on a contract worth £10,694,902. The trial is yet to pass ethical approval, although the NHS expects it to begin this year.

Cass welcomed the new trial, saying it “aims to fill some of the gaps in our knowledge about the outcomes of different interventions and address some of the uncertainty about the impacts and efficacy of puberty-suppressing hormones”.

However, several doctors and campaign groups have raised concerns about the ethics of the trial, arguing that existing evidence proves puberty blockers are harmful, and damage brain and bone development as well as long-term fertility and sexual function.

One psychiatrist warned the trial risks “repeating the same ethical failures” of the now-closed Tavistock and Portman NHS gender identity development service (Gids), which was accused of rushing hundreds of children on to puberty blockers from 2011 onwards.

Dr David Bell, a psychiatrist who was a whistleblower at the Tavistock trust, said he found it “extraordinary” that the NHS-supported trial was going ahead, adding that “again an experiment is being carried out on children”.

“If we accept that puberty blockers were an experiment that failed many many children, how can we justify conducting a trial when we know that a significant number of children will be harmed?”, he asked.

“There are a number of reasons why it is unethical. Once children are started on puberty blockers it’s extremely hard for them to come off them. By starting puberty blockers children are in effect being put on a medical pathway to gender transition which will include, for many, progression to surgery.
“The prescribing of puberty blockers introduces physical harms to a physically healthy child. There is significant evidence that puberty blockers seriously impact on bone density.”

Helen Joyce, the director of advocacy at charity Sex Matters, said: “We have good reason already to think these drugs are harmful, and that the benefits are limited or non-existent.

“It’s as if the NHS was planning a trial of lobotomies long after concerns first started to be raised — in fact, even worse because the test subjects are children. It seems that £10 million of public money is going to be spent on this unethical experiment.”

The study will involve tracking the well-being of children who attend a network of new NHS gender clinics for children, including those not on the puberty blocker trial. It is being led by Professor Emily Simonoff, the head of child and adolescent psychiatry at King’s College London.

The research will also monitor whether puberty blockers affect young people’s thinking and brain development, using various activities and brain scans. Researchers will speak to young people about their experiences, as well as obtaining the views of parents and staff.

James Palmer, the NHS national medical director for specialised services, said: “This suite of research will examine the evidence for a range of clinical care, including the use of puberty-suppressing hormones, following advice from the Cass Review.

“Independent researchers led by King’s College London will now progress this joint NHS and NIHR project, which will be subject to strict ethical and regulatory approval and follow stringent safeguards in scientific research.

“In the meantime, the NHS continues to expand and improve services for children and young people with gender dysphoria and incongruence, in line with recommendations from the Cass Review.”

The trial summary does not specify the age of participants, and the NHS clinics have no minimum age for referrals. Puberty blockers are usually given in early or mid-puberty, which typically starts at the age of 10 or eleven.

Behind the story

Three months ago, Wes Streeting announced an indefinite ban on the use of puberty blockers in the UK. The health secretary said that the drugs pose an “unacceptable safety risk” and that “we do not yet know the risks of stopping pubertal hormones at this critical life stage”.

It followed the Cass Review, published last April, which found that there was “remarkably weak” evidence around puberty blockers and warned that the drugs may harm bones and fertility, as well as “temporarily or permanently” disrupting brain development.