A clinical trial assessing the risks and benefits of puberty-blocking drugs in children who question their gender has been paused after safety concerns were raised by the UK medicines watchdog.

The Medicines and Healthcare Products Regulatory Agency (MHRA) is set to hold talks with King's College London, whose researchers are leading the trial, next week to address concerns, the government said.

Details of the trial, which would involve more than 220 children aged between about 11 and 15, were announced in November after getting approval from the MHRA and Health Research Authority.

An MHRA spokesperson said the "safety and wellbeing" of participants - yet to be recruited - was "paramount".

Puberty blockers for people under-18 questioning their gender identity were banned in 2024.

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

The clinical trial, called Pathways, was set to involve children who are distressed about their gender and currently accessing gender services.

It would examine the impact of the drugs on their physical, social and emotional wellbeing.

But the MHRA has suggested the minimum age limit of those taking part should be increased to 14, and that there should be more detailed monitoring of bone density and a more rigorous consent process for participants.

In a letter published on the government website on Friday, the MHRA expressed concern over the current age limit for participants 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/stepwise approach starting with those aged 14 as the lower limit of eligibility," it reads.

In a separate statement on Friday evening, an MHRA spokesperson said it was "part of the usual process that clinical trials are kept continuously under review and for us to have active scientific dialogue with the trial sponsors".

"We have raised some concerns related to the wellbeing of participants and scientific dialogue will now follow with the trial sponsor," they added.

The Department of Health & Social Care (DHSC) said participants would not be recruited until the issues raised had been resolved between the MHRA and the trial clinicians.

"We have always been clear about the red lines regarding this trial – ensuring the safety and wellbeing of the children and young people involved and always being led by the clinical evidence," a spokesperson said.

They added that the trial would "only be allowed to go ahead if the expert scientific and clinical evidence and advice conclude it is both safe and necessary".

A spokesperson for King's College London said the wellbeing and health of young people with gender incongruence and their families was its "priority".

It added that "rigour and ongoing scientific discussion" was important for a clinical trial, "particularly one as complex as Pathways".

Some doctors have questioned whether the trial is necessary.

Campaigners are also taking legal action against the MHRA, the government and others involved in the trial.

They claim it is unethical and that children cannot give fully informed consent to a treatment which might affect their future fertility.

A 2024 review by Dr Hilary Cass found gender medicine had been operating on "shaky foundations" when it came to evidence for treatment, with the ban for puberty blockers for under-18s introduced after the review raised concerns about their safety

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