Details of a new UK clinical trial to assess the risks and benefits of puberty-blocking drugs in children who question their gender have been announced.

It follows the banning of the drugs for gender treatment last year after a major review raised concerns about the lack of clinical evidence over their safety for under-18s.

Researchers from King’s College London say the trial will involve around 220 children under the age of 16 who are going through puberty, and will examine the impact of the drugs on their physical, social and emotional wellbeing.

Some clinicians and campaigners question whether the trial is ethical.

Prof Emily Simonoff, study leader and professor of child and adolescent psychiatry at King’s College London (KCL), told BBC Radio 4’s Today programme that young people and their parents attending services for gender distress “tell us that they don’t know what to do – they look at the information that’s out there and they don’t know what’s best for them”.

But she said the study was not expecting a “one size fits all finding”.

“We are looking very much at the balance between, possibly, benefits for mental health and quality of life, and any possible risks or harms.”

She added this would include monitoring people’s physical health, and would also be the first ever study looking at the impact on brain development.

Puberty blockers, also known as puberty suppressing hormones (PSH), are drugs used to delay or prevent puberty happening.

They were used to treat some young people with gender incongruence – when someone’s gender identity doesn’t match the sex they were registered at birth – or with gender dysphoria, when it causes significant distress.

As a result of the uncertainty over the safety of the drugs, highlighted by the Cass review into gender care, led by paediatrician Dr Hilary Cass, doctors can now only offer the treatment to under-18s as part of a research study.

Last year, the government brought in a UK-wide indefinite ban on the drugs being prescribed privately or by the NHS to children and young people questioning their gender identity.

The new clinical trial, called Pathway, will involve children who are currently accessing gender services and have a diagnosis of gender incongruence.

They will all have reached puberty, but will be younger than 16 – and will have to meet strict criteria, undergo intensive medical and psychological screening before they are allowed to start taking puberty blockers.

A team of specialist NHS doctors must have a full picture of the young person’s wellbeing before deciding if they think they are suitable for the treatment.

The young person will also have to show they have a good enough understanding of the potential impact of taking puberty blockers to give their consent, and their parent or legal guardian will need to agree. They will be provided with ongoing psychological support.

To explore the impact of the drugs, the researchers plan to start one group on the treatment straight away and another group 12 months later. The children in these groups will be chosen randomly.

The KCL researchers said there would be no minimum age for taking the drugs, but puberty normally starts around the age of 11 for girls and 12 for boys.

The trial will look at issues such as bone density, brain development and mental health and wellbeing over time.

The research team said the trial had been given ethical approval and was expected to start in January, with five to six children recruited every month. The first results should be available in around four years.

Alongside this, a larger observational study involving 3,000 children will research different types of support and how effective they are.

The puberty blockers trial has already proved controversial, with campaigners threatening legal action.

Keira Bell, who took the Tavistock gender clinic to court in 2020 after she was given puberty blockers and cross-sex hormones as a teenager, said the trial should be halted immediately. If it is not, she says she and another campaigner will start judicial review proceedings at the High Court.

She said it was “disgusting” that children were being put on the drugs when they had already been banned because they were “unsafe”.

In her case, the High Court ruled that under-16s were “unlikely to be able to give informed consent” to puberty blockers, but this was later overturned by the Court of Appeal which ruled that doctors can judge whether young people can give consent to the treatment.

Some clinicians from the Clinical Advisory Network on Sex and Gender, which campaigns for rigorous science and improved treatment options for gender-questioning people, have also questioned whether the trial can be carried out ethically.

Study lead Prof Simonoff said: “We know there are ongoing societal discussions about gender transition, but this research is focused solely on informing and improving healthcare by better understanding how to support the physical and mental health of young people with gender incongruence.”

A spokesperson for charity Stonewall, which campaigns for the rights of LGBTQ+ people, said all young people should have access to the very best medical care, guided by evidence.

“We urge the government and policymakers to invest in delivering excellent healthcare for trans young people and to make sure the voices of trans young people and their families are at its core.”