‘First, do no harm” is the sacrosanct principle that is supposed to underpin modern medicine. But history is littered with examples of medics breaching this doctrine. Last week, the publication of Hilary Cass’s final report on healthcare for gender-questioning children laid bare the devastating scale of NHS failures of a vulnerable group of children and young people, buoyed by adult activists bullying anyone who dared question a treatment model so clearly based on ideology rather than evidence. Cass is a renowned paediatrician and her painstakingly thorough review was four years in the making. She sets out how the now-closed NHS specialist gender clinic for children abandoned evidence-based medicine for a wing and a prayer. Significant numbers of gender-questioning children – it’s impossible to know exactly how many because the clinic did not keep records, itself a scandal – were put on an unevidenced medical pathway of puberty-blocking drugs and/or cross-sex hormones, despite risks of harm in relation to brain development, fertility, bone density, mental health and adult sexual functioning. What drove this? The medical pathway is rooted in a belief that many, perhaps even most children questioning their gender will go on to have a fixed trans identity in adulthood, and that it is possible to discern them from those for whom it is a temporary phase. But studies suggest that gender dysphoria resolves itself naturally in many children. It is often associated with neurodiversity, mental health issues, childhood trauma, discomfort about puberty, particularly in girls, and children processing their emerging same-sex attraction; a large number of children referred to the Gender Identity Development Service (Gids) were gay. Putting these children on a medical pathway does not just come with health risks, it may also pathologise temporary distress into something more permanent. Cass is also clear that socially transitioning a child – treating them as though they are of the opposite sex – is a psychological intervention with potentially lasting consequences and an insufficient evidence base, that transitioning in stealth may be harmful, and says that for pre-pubertal children this decision should be informed by input from clinicians with appropriate training. There is a conundrum at the heart of the report. Cass finds a childhood diagnosis of gender dysphoria is not predictive of a lasting trans identity and clinicians told the review they are unable to determine in which children gender dysphoria will last into adulthood. If this is indeed impossible, is it ever ethical to put a young person on a life-altering medical pathway? If there are no objective diagnostic criteria, on what basis would a clinician be taking this decision other than a professional hunch? The report recommends a total overhaul in the NHS’s approach to caring for gender-questioning children and young people: holistic, multidisciplinary services grounded in mental health that assess the root causes of that questioning in the round and take a therapeutic-first approach. Puberty blockers will only be prescribed as part of an NHS research trial and she recommends “extreme caution” in relation to cross-sex hormones for 16- to 18-year-olds; one might expect this to be contingent on it being possible to develop diagnostic criteria for gender dysphoria that will last into adulthood. Cass’s vision is what gender-questioning children deserve: to be treated with the same level of care as everyone else, not as little projects for activists seeking validation for their own adult identities and belief systems. But it is going to be immensely challenging for the NHS to realise, and not just because of the parlous underfunding of child mental health services. There will be resistance among captured clinicians wedded to quasi-religious beliefs; it is astounding that six out of seven adult clinics refused to cooperate with the review on a study to shed more light on those the NHS treated as children. A senior NHS researcher at one trust told me the opposition to taking part in an uncontroversial methodology to inform better outcomes came not from the board but from some clinicians in their service, and this was unheard of in other parts of the NHS. Cass has also commented on the intense toxicity of the debate. The fact that she says medical professionals were scared of being called transphobic, or accused of practising conversion therapy, if they took a more cautious approach in a climate where activists and charities like Stonewall were quick to level accusations of bigotry at people flagging concerns, and that NHS whistleblowers were vilified by their employer, has not only prolonged the avoidable harm that will have been caused to some young people but will make it difficult to recruit clinicians to the new service. Cass has warned ministers about the risks of the criminal ban on conversion therapy activists are pushing for; the definitional challenges risk criminalising exploratory therapy and could further increase fear among clinicians. The former chief executive of Stonewall has already endorsed the view that the Cass model is itself conversion therapy. Given what it says about social transition, the implications of the Cass review go beyond the NHS to schools and children’s services, where there are similar pockets of ideological capture. As we report today, the parents of one child whose school facilitated their social transition without their knowledge have given Brighton council two weeks to withdraw the trans toolkit it has endorsed for use across all its schools, or face legal action in light of legal advice from the country’s leading equality and human rights lawyer, Karon Monaghan KC, that the toolkit is itself unlawful and advises schools to act unlawfully. She sets out how it gets the law devastatingly wrong in several areas, including on safeguarding in relation to the wellbeing of gender-questioning children who want to socially transition. On single-sex spaces and sports, it wrongly advises that a child’s chosen gender identity should override their sex, which is likely to lead to unlawful discrimination against other pupils, particularly girls. This influential toolkit is in use by schools in at least several other local authorities; the parents have published the advice in full to enable other parents to challenge schools on its unlawfulness. The Cass review is an immense achievement; it has stripped the heat out of one of the most contested areas of modern medicine and restored the role of evidence back to its rightful place. But there is a long way to go yet in unpicking the influence of a contested and controversial – yet in some cases, deeply embedded – adult ideology about gender in the way children are supported by the NHS, children’s services and schools. Sonia Sodha is an Observer columnist