The NHS will begin recruiting children as young as 11 for its puberty blocker trial from August, despite Britain’s medicines regulator previously warning of “potentially significant” long-term biological harms including a very likely risk of fertility loss and permanent damage to bone health.
MHRA concerns over long-term biological harms
The Medicines and Healthcare products Regulatory Agency (MHRA) raised its concerns in correspondence with King’s College London, which leads the NHS Pathways study, before the trial was suspended indefinitely in February. The regulator warned that participants faced a “very likely” risk of infertility and irreversible harm to bone density, and highlighted the “potentially significant and, as yet, unquantified risk of long-term biological harms” associated with the treatment. The MHRA had originally proposed introducing a minimum age of 14 for participants, but the revised protocol agreed between the regulator and King’s College London now sets the threshold at 11 for girls and 12 for boys.
Puberty blockers — formally known as gonadotrophin-releasing hormone analogues — suppress the physical changes associated with puberty, including menstruation, breast development and facial hair growth. The MHRA’s concerns also encompassed potential impacts on cognitive development, though the regulator stressed that the risks were not based on the emergence of any new scientific evidence, according to King’s College London. The regulator requested further discussions before the study could proceed, and on Friday confirmed those concerns had been addressed following agreement on a modified protocol.
The MHRA’s repeated emphasis on “unquantified risk” underscores the scientific uncertainty surrounding the treatment’s effects on young bodies. The regulator’s correspondence made clear that the long-term biological harms — including to bone health, fertility and cognitive development — remained both significant and poorly understood.
Trial restart and revised protocols
King’s College London confirmed it had reached agreement with the MHRA on revised trial protocols. Researchers said the updated trial includes strengthened safeguards but stressed there had been “no major changes to the design or conduct” of the study. The trial aims to recruit up to 250 children and young people receiving treatment through NHS gender services, making it the largest study of its kind in the UK. Participants will receive puberty blockers for up to two years and will be closely monitored for potential side effects, including changes to bone density and vaginal bleeding. Families will also receive enhanced information about fertility preservation, including egg and sperm freezing.
The study cannot begin until an ongoing judicial review challenging the approvals granted by the MHRA and the Health Research Authority has concluded. The NHS stopped routinely prescribing puberty blockers in 2024 following the Cass Review, which concluded there was insufficient evidence to support their use. The review found the evidence base for puberty suppression to be unclear and recommended that such treatments should only be given in the context of a clinical trial. Following the Cass Review and advice from the Commission on Human Medicines (CHM), which deemed the use of puberty blockers in the current prescribing environment an “unacceptable safety risk,” the drugs were banned for routine use in under-18s.
Ethical debate intensifies
Campaigners and cross-party politicians have previously called for the trial to be abandoned, arguing there is no ethical basis for conducting such research on children. Helen Joyce, director of advocacy at Sex Matters, criticised the decision to restart the study. “It is already clear that puberty blockers are no solution to childhood gender distress, and that they cause unacceptable harm,” she said. She described the protocol changes as “trivial” and questioned whether children as young as 11 could meaningfully consent to the potential long-term risks.
Campaigners including the Bayswater Support Group, Keira Bell — a former patient who detransitioned and was a lead claimant in a previous legal challenge concerning the Tavistock Centre — and psychotherapist James Esses have been involved in legal challenges against the trial, arguing that it fails to safeguard vulnerable children and that the benefits do not outweigh the risks. The Cass Review also warned that puberty blockers could place children on a pathway towards further medical intervention with cross-sex hormones. The study’s commencement is contingent on the conclusion of the ongoing judicial review challenging the approvals granted by the MHRA and the Health Research Authority.