Dr Hilary Cass has criticised the medicines regulator for blocking a clinical trial into puberty suppressors for children, saying it had made “completely bizarre” claims, hadn’t produced any new evidence and seemed to be responding to “political pressure”, not science.
The Pathways trial was paused after the Medicines and Healthcare Products Regulatory Agency (MHRA) published a letter to the research team on Friday, raising fresh concerns about potential long-term harms to children.
The MHRA had previously approved the study, which aims to examine the impact of puberty suppressing hormones on about 220 children aged from 11 to 15 who clinicians believed might benefit from the drugs.
But in its letter, which came after threats of legal action from parents and criticism of the trial from gender-critical campaigners such as JK Rowling, the regulator said children should only be allowed to take part after the age of 14.
Cass told The Observer she was “disappointed” by the MHRA’s intervention. “There are no new research findings and the MHRA hasn’t presented any new evidence,” she said. “It feels to me like they are responding to political pressure rather than to science.”
The trial was launched after Cass’s 2024 review of gender services, which said research was needed since there was little evidence whether or not puberty-suppressing hormones helped children with gender dysphoria. After the paediatrician’s report, the NHS stopped use of the drugs at gender identity clinics, although they continue to be used for children whose puberty begins abnormally early.
‘It feels to me like they are responding to political pressure rather than to science’
‘It feels to me like they are responding to political pressure rather than to science’
Dr Hilary Cass
Cass said the trial was necessary because “we genuinely don’t know if some children will benefit or have no benefit”.
By raising the age limit to 14, it would cut out children who might benefit, which “will make the results invalid”, Cass said. “It would make the design really, really flawed and you should not be subjecting children to a flawed study.”
The MHRA raised issues including the potential long-term effects of the drugs on fertility and bones, and whether children who might experience vaginal bleeding could report their symptoms.
“There were some bizarre things [in the letter], that children won’t be able to tell you if they’re bleeding. Well, anyone who can’t tell you if they’re bleeding can’t consent to this treatment. That seemed completely bizarre.”
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Cass said the MHRA had put “no references” in its letter and had shown no evidence it had consulted paediatricians or endocrinologists who had prescribed drugs or knew about bone health issues.
“I’m not dismissing it out of hand. They may have valid points to make, but they haven’t made them adequately in this letter.
“I have not changed my position an inch since I wrote my report, and yet, suddenly, people from the gender-critical side of the debate seem surprised or discomforted that I'm supporting a trial,” she said.
“But I called for a trial two years before the report, and I said in the report that everything that we do to these young people needs to be done in the context of a proper research programme, because otherwise we can't improve what we're doing for them.”
In December, the Bayswater Support Group – 800 parents of trans and non-binary children – began legal action aiming to halt the Pathways trial, and the MHRA said it was considering whether the action raised “further matters which impinge on safety or efficacy”.
The MHRA’s decision was criticised by trans groups and welcomed by gender-critical campaigners. Chay Brown of TransActual, a transgender rights group, said: “The suggestion to introduce a minimum starting age of 14 is ludicrous. Many young people have already begun puberty at 14, and irreversible pubertal changes are already under way.”
Claire Coutinho, the shadow equalities minister, described the pause as a “huge win”. The Bayswater Support Group said the trial “must not be amended or restarted”, adding that children “cannot meaningfully consent to life-altering interventions”.
The Department of Health and Social Care said 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”.
Photograph by Getty