Baroness Cass is wrong about the puberty-blocker trial
It’s hard to overstate the importance and influence of the Cass Review, published in April 2024. It made global news for its stinging criticisms of the Gender Identity Development Service at the Tavistock Clinic (since closed), its systematic reviews demonstrating the lack of any evidence underpinning “gender-affirmative” treatment, and the evidence that most children who had been subjected to such treatment were vulnerable in multiple ways, including having serious mental-health problems, being on the autistic spectrum or having suffered abuse, trauma or family breakdown. Cited around the world, it started mainstream criticism of the excesses of gender medicine. Before Cass, only those who had been watching carefully understood that child gender medicine was a scandal; since Cass that has become widely, if not universally, accepted.
But the report had limitations, most seriously – as we said in our initial commentary – its failure to tackle the school-to-clinic pipeline and its endorsement of a trial of puberty blockers to provide the evidence base it recognised was lacking. The puberty-blocker trial was one recommendation of 32 in the report, and the only outright bad one. There was no detail on how such a trial might be designed; many experts thought it would never happen because ethical approval was simply not possible.
And yet Baroness Cass has thrown her considerable prestige and authority behind plans, now far advanced, to carry out such a trial. Here is why we think she is wrong.
School-to-clinic pipeline
While Baroness Cass was carrying out her review, we raised the problem of the promotion of trans ideology and widespread acceptance of “social transition” in schools, meaning children were being treated as if they had changed sex in school rules and policies. Many parents say that it was in school that their child first picked up the idea that they might be trans, and it had become commonplace for schools to transition children behind their parents’ backs. The Cass Review acknowledged the role of social contagion in trans identification, and suggested that social transition might be unwise. But it showed no understanding of the pernicious way that doctors’ recommendations of social transition had been converted into a “doctor’s orders” edict to schools, which then extended to children being supported to socially transition in school without any doctor’s involvement.
The review also failed to grapple with the fact that what an individual child wants is not a matter for them alone. Even if, hypothetically, puberty blockers and social transition are “right” for a particular child, this cannot be accommodated in schools without unacceptably infringing on the rights of other children. This means social transition supported by puberty blockers and the promise of cross-sex hormones cannot ethically be offered to a child as a viable and coherent package.
Schools exist to educate children while keeping them safe, not to provide complex and risky psychosocial treatments. A gender-confused child’s classmates are not there to validate the child’s professed identity by accepting the falsehood that he or she has changed sex. Doctors have no right to propose to their patients that this should happen, even if that is what those patients want.
The missing evidence base
As originally envisaged, the review was supposed to include evidence gleaned from following up the children who had previously been prescribed puberty blockers by the NHS, thought to be around 2,000 of those seen at the Tavistock. This “data-linkage study” would not have been definitive on outcomes because the control group (children seen but not treated) would not be statistically comparable with those who received the drugs. But it would have provided excellent data on safety, revealed whether the children were still distressed, probably provided useful information on desistance and detransition, and in general been the most meaningful data on outcomes from medicalising gender distress beyond a year or two anywhere in the world.
But the adult gender clinics refused to cooperate with the Cass review, and refused to hand over their data. In the end the review was published without that information. The data-linkage study is still nowhere to be seen. And yet in December 2024 the secretary of state for health, Wes Streeting, announced that NHS England was commissioning a new trial of puberty blockers. And in November 2025 news broke that the trial had received ethical approval.
If the data-linkage study had concluded by now, it would have established the existence or otherwise of some of the most immediate harms thought to be caused by puberty blockers, in particular whether they raise the risks of early-onset osteoporosis, which is so rare among people in their teens or 20s that even a couple of cases would send a strong signal about treatment safety. It would also help establish whether puberty blockers are helpful in reducing suicides, which are also, thankfully, extremely rare at that age. By now we would have data on chronic and acute health problems, emergency admissions, diagnoses, prescriptions and eligibility for incapacity benefit for adults who had received puberty blockers as children, which would give a clear picture of whether or not they are thriving.
In short, we would know a lot more about whether the treatment pathway that starts with puberty blockers and almost always continues to cross-sex hormones creates serious risks to physical health, and whether those who travel this path are unusually unhealthy in early adulthood. If the answer to either of these questions was Yes, any further use of these drugs would be indefensible.
Baroness Cass has repeatedly criticised the adult gender clinics for refusing to cooperate. She knows that puberty blockers are not a standalone treatment: her report said so. She knows they hugely increase the risk of future, irreversible medical intervention – that they do not, in the words chosen by journalist Hannah Barnes for the title of her whistleblowing book about the Tavistock, offer children “time to think”. Before the advent of puberty blockers most gender confusion resolved in childhood; afterwards, most persisted. Puberty blockers are an almost inevitable precursor to giving oestrogen to boys and testosterone to girls. Any fair assessment of their benefits, risks and harms therefore has to include those of cross-sex hormones. She also knows that the previous trial at the Tavistock clinic opened the door for routine prescribing before its results were published, and that those results belatedly showed no benefit for children’s mental health.
The planned trial will compare answers to a questionnaire about wellbeing between children who receive puberty blockers at the start of the trial, and children who are required to wait a year. That can provide no useful information about medium- or long-term risks. It is likely to create an artificial difference between the trial’s two groups, since participants may well believe false claims by the trans lobby that puberty blockers are “life-saving”. Those who receive the treatment straight away are likely to be elated; those forced to wait devastated.
Recently, Baroness Cass has defended the trial as a harm-reduction measure. Children denied puberty blockers on the NHS, she says, may turn instead to online providers who do not monitor their wellbeing and may be selling less safe alternatives. But the trial has not been designed to test harm reduction. And no data is being collected to find out whether or not overall harm is in fact being reduced.
Despite all this, instead of putting her considerable authority and prestige behind the push for the data-linkage study, which would give useful information without exposing any more children to a treatment regarded as so risky that neither the NHS nor private health providers can lawfully prescribe it, Baroness Cass instead supports a new trial. That the new trial is so poorly designed that it cannot possibly provide a meaningful contribution to the evidence base her review said was lacking makes this even more inexplicable.
Reckoning with the Supreme Court
In April 2025, a few months after plans for the new trial were announced, the Supreme Court ruled in the case of For Women Scotland (FWS) that in the Equality Act, sex means sex. Two weeks after that judgment was handed down, Sex Matters, together with LGB Alliance, Transgender Trend and Genspect, wrote to Sir James Mackey, then the chief executive of NHS England, and the health secretary.
In that letter we spelled out the implications of the ruling for the care of gender-distressed children:
“Children under the care of all gender services, both NHS and private, must now be told that they will never be able to access spaces or services for the opposite sex, no matter what legal or medical steps they take now or in the future. Such children will, throughout their lives, need to use either the spaces and services designated for their own sex or an alternative provided for both sexes (mixed-sex or single-user) if available.
…
“We have always argued that it was unethical and a breach of human-rights principles to treat young people’s mental distress with major, unproven physical interventions with known harms (including to their adult sexual function and fertility), and to promise them that they could intrude on other people’s privacy, safety and dignity as well as put their own safeguarding at risk by using spaces for the opposite sex.
“Now that the Supreme Court has ruled out the use of opposite-sex spaces, no promise can be made that medical treatment will enable a person to fit in or go unnoticed while using opposite-sex facilities. The use of these drugs on children too young to understand this is ethically untenable.”
This year Keeping children safe in education, statutory safeguarding guidance for schools, has been updated to take account of the judgment. Schools have been expressly warned that by law they must record every child’s sex accurately, and must not ever allow any child to use toilets (or changing-rooms where they exist) for the other sex. Even if a school considers using preferred pronouns for trans-identifying children, or allowing them to wear the opposite sex’s uniform (we think both these policies are indefensible on safeguarding grounds), it is now beyond doubt that schools cannot lawfully pretend any child is the opposite sex.
As we also said in that letter, this undercuts the entire rationale for treatment with puberty blockers:
“The drugs are intended to block gender-distressed children’s natural puberty so that if they persist in cross-sex identification and progress to cross-sex hormones and perhaps surgery, they will “pass” better as the opposite sex. But passing as the opposite sex ceases to be a desirable goal to present to children when they will never be permitted to use spaces or services for the opposite sex, and may in future be restricted in employment in roles such as police officer or nurse which can involve intimate contact with others in situations where sex matters for those others’ human rights. Instead it becomes a major, permanent social problem for those young people, since their future lives and freedoms will be seriously curtailed by being legally barred from using the other sex’s spaces while appearing anomalous in those for their own sex.”
Not only are puberty blockers a treatment based on a false premise, but they cannot ethically be offered to any child. Quite apart from the health risks, no child (or parent on their behalf) can consent to destroying their future ability to live normally in society as an adult.
As we wrote when news broke last year that the trial had received ethical approval:
“The Department of Health, NHS England, the National Institute for Health and Care Research and the Research Ethics Committee which approved this study have allowed it to go ahead by ignoring the conflict with the Equality Act and feeding the unrealistic hopes of children who are not old enough to understand or consider these constraints on their desires.
“Recruiting children into a study that is based on a mis-selling of rights and which forecloses their chances for adult sexual function and fertility is unconscionable.”
That a clinician and parliamentarian of Baroness Cass’s stature continues to press for this trial risks undermining all the good work she has done.