An NHS trial of puberty blockers has been halted after health officials raised concerns about “long-term biological harms” to children.
Up to 250 girls and boys as young as ten were due to be recruited to join the clinical trial of hormone-suppressing drugs run by King’s College London. However, on Friday the UK’s medical regulator wrote to King’s raising concerns and arguing for a minimum age of 14. The Department of Health said that, as a result, “preparations for the trial have now been paused”, although it could still go ahead if the issues are resolved.
The study, due to be the biggest trial yet of puberty blockers, was due to involve children being put on the drugs for up to two years and monitored until early adulthood, with their brain development, quality of life and physical and mental health compared with those not given them. The trial had been facing a legal challenge from campaigners who said it was unethical.
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The letter from the Medicines and Healthcare products Regulatory Agency (MHRA) highlighted concerns including the “very likely” loss of fertility and permanent bone damage to children. The drugs pause the physical changes of puberty, such as breasts, periods or facial hair.
The MHRA said: “We would like to discuss the inclusion of a minimum age for participation in the trial … since potentially significant and, as yet, unquantified risk of long-term biological harms is present to participants and biological safety has not been definitively demonstrated in this proposed cohort, at the very least, there should be a graded … approach starting with those aged 14 as the lower limit of eligibility.”
It highlighted the infertility risk, stating: “It is widely understood that reduction in gametogenesis will very likely cause infertility in these young people. These are medically consequential effects for a future adult individual that may involve infertility treatment and significant medical interventions.”
Wes Streeting, the health secretary, had been facing pressure from dozens of MPs to halt the study. Streeting banned puberty blockers for under-18s in gender medicine in 2024 over fears they were harmful. They were given to children at the Tavistock clinic since 2011, but the NHS stopped using them in 2024 after the Cass review found there was little evidence to support their use.
Protesters opposed to the use of puberty blockers campaigned outside the Department of Health and Social Care last year
WIKTOR SZYMANOWICZ/FUTURE PUBLISHING/GETTY IMAGES
In her final report Dr Hilary Cass warned of safety risks and recommended a trial into puberty blockers to fill an evidence gap about how best to treat gender dysphoria in children.
The Department of Health and Social Care said: “We have always been clear about the red lines regarding this trial — ensuring the safety and well- being of the children and young people involved and always being led by the clinical evidence. The MHRA has now raised new concerns — directly related to the wellbeing of children and young people — and scientific dialogue will now follow with the trial sponsor.”
It said “preparations for the trial have been paused”, adding: “[It] will only be allowed to go ahead if the expert scientific and clinical evidence and advice conclude it is both safe and necessary.”
The MHRA said: “It is part of the usual process that clinical trials are kept … under review and for us to have active scientific dialogue with the trial sponsors. The safety and wellbeing of the participants to be recruited … is paramount, particularly in view of the age of the children [who] may be involved.”
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King’s said: “The wellbeing and health of young people with gender incongruence and their families has been, and will remain, our priority, and we will continue to work with the MHRA to support their further review of the trial, which has been designed by world-leading academics with scientific rigour at its core.
“That rigour and ongoing scientific discussion is important for any clinical trial, particularly one as complex as [this], which aims to build an evidence base that can help young people and clinicians to make better-informed decisions in the future.”
Claire Coutinho, the shadow equalities minister, said: “This is a huge win. The medicines regulator has rightly paused the puberty blocker trial over serious concerns about children’s age and lifelong fertility. Wes Streeting has ignored warnings from the Conservatives, campaigners and health experts for months. It should never have taken the regulator to step in and tell him that putting 200 children at risk of sterilisation was wrong.
“Whatever choices someone makes as an adult, they are not decisions for a child. Wes Streeting must now end this trial for good, and the Conservatives will continue to fight until that happens.”