A new research trial to study the impact of puberty blockers in children experiencing gender incongruence (a mismatch between birth sex and gender identity, which may cause considerable distress) has been granted ethical approval and will start recruiting participants soon. The trial will only be open to those under 16 who are patients at specialist NHS Children and Young People’s Gender Services. A second study, comparing the brain health of those receiving puberty suppression with those who are not, will also be undertaken. One of the concerns raised about puberty blockers has been their potential impact on brain development and brain function, given that adolescence is a crucial time of rapid accrual of new thinking abilities.
New prescriptions for puberty blockers for under 18s have been illegal in the UK since 2024. A government ban, first introduced in May 2024, was made permanent in December, following the four-year independent Cass Review of young people’s NHS gender identity services. Former President of the Royal College of Paediatrics and Child Health (RCPCH), Dr – now Baroness – Hilary Cass found that this was an area of healthcare for which there is “remarkably weak evidence”. She recommended that a puberty blocker trial be set up as part of a “full programme of research”. Participation in the new “Pathways Trial” will be the only lawful way for a young person to get puberty blockers in the UK for gender-related treatment.
To be eligible for the trial, young people will also have to have started puberty and participated in “other forms of care for gender incongruence”. The research team told a briefing organised by the Science Media Centre that they anticipate the youngest female participants to be aged 10 or 11, with boys being as young as 11 or 12. Both the young person’s own clinical care team and a National Multi-Disciplinary Team (NMDT) will have to agree to the young person’s participation and the “leading clinician” must consider there to be “a reasonable prospect of benefit” from the suppression of puberty. Consent from one parent will also be required. All children taking part must discuss fertility preservation.
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Young people will be excluded from the trial if they have “unstable physical health requiring active intervention”, if they have unstable mental health which impairs their ability to provide informed consent, if they are unable to provide informed consent more generally, or if they have a family or home situation affecting their ability to abide by the conditions of the research. Any young person who has already taken either puberty blockers or masculinising or feminising hormones (testosterone or oestrogen, respectively) for the treatment of gender incongruence will also be ineligible.
Participants will be randomised between two groups: one will receive the puberty suppressing hormones immediately; the other group will begin the treatment after a one-year delayed start. This will provide “comparative information”, argue the research team, led Professor Emily Simonoff, Professor of Child and Adolescent Psychiatry at King’s College London’s Institute of Psychiatry, Psychology & Neuroscience (IoPPN). Information will also be collected from a matched group of young people who are receiving psychosocial support (talking therapies) but not medical interventions. All participants will receive psychosocial support.
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It is unclear how this design will allow the research team to judge to what extent any observed benefits or harms are due to the medical intervention, as opposed to the other support being provided alongside. This was one of many difficulties in the trial undertaken by the Tavistock’s Gender Identity Development Service (Gids) and University College London Hospitals in the 2010s. The study “identified no changes in psychological function, quality of life or degree of gender dysphoria.” It is also unclear what will happen to the children taking part in the trial after they have undergone two years of puberty suppression, as per the design. The study protocol says that participants will spend “24 months in randomised trial with potential for non-randomised open extension according to clinical compassionate care.” It is unknown whether those who are younger when they start the trial will stay on the blockers for many years – potentially putting their long-term bone health at risk – or if they will be allowed to commence treatment with hormones at an age younger than 16. This is not currently allowed by the NHS.
Professor Simonoff told a press briefing that she expected a “very small minority of the young people” attending NHS children’s gender services to join the trial. The researchers estimate 226 young people could be recruited to the clinical trial over three years. While these estimates may be accurate, those put forward by Gids in the past grossly underestimated both the demand for its service as a whole and the number of young people wanting to suppress their puberty.
The study protocol confirms that the “primary objective” is “to determine the short/medium-term benefits and risks” of puberty blockers in children and young people with gender incongruence. While this will include looking at a young person’s quality of life and mental health, as well as the “impact on cognition and brain development and physical effects including bone mineral density”, longer term effects of the drugs will only be studied “pending further funding”. But it is precisely regarding the long-term effects on neurocognitive development, psychosexual development and bone health where the gaps in the evidence base lie. It is not clear that this new NHS-funded study will fill those gaps. While the close monitoring of children taking part in the trial will continue for just two years from their first assessment, all will have an annual follow-up for the study period. It plans to recruit participants for 40 months. The study team say that young people will also be asked whether they consent to longer-term follow-up “through linkage with national health registries and routinely collected NHS digital datasets”.
So much is still uncertain. One thing that is clear: today’s announcement prompts just as many questions, as it does answers. The research team say it will take at least four years before the trial yields any results.