Puberty blockers have “unknown” and “life-changing consequences” for trans children, and could impact their fertility, the Court of Appeal has heard.

An NHS Trust, which runs the UK’s only gender identity development service for children, began its appeal to challenge a landmark legal ruling over the use of puberty-blocking drugs for children on Wednesday.

The legal challenge has been launched after the High Court ruled in December that children under 16 with gender dysphoria can only consent to the use of hormone-blocking treatments if they are able to understand the "immediate and long-term consequences" of the drugs.

However, judges added it was "highly unlikely" that a child aged 13 or under would be able to consent to the treatment, and that it was "doubtful" that a child of 14 or 15 would understand the consequences.

On the first day of the appeal hearing aiming to overturn the High Court ruling, Fenella Morris QC, for The Tavistock and Portman NHS Foundation Trust, said the legal ruling "reset almost half a century of established law" and caused "serious distress to many young people and their families".

The landmark legal ruling regarding puberty blockers was made after Keira Bell – a 24-year-old woman who began taking puberty blockers when she was 16 before later "detransitioning" – brought a case against the Tavistock and Portman NHS Foundation Trust.

A mother of an autistic teenager who is on the waiting list for treatment, only known as Mrs A, supported Ms Bell in their successful legal challenge.

However, the Court of Appeal also heard that puberty blockers are “experimental” drugs that have “unknown consequences”, both physically and psychologically. 

Keira Bell

Jeremy Hyam QC, for Ms Bell and Mrs A, argued that the appeal should fail as it did not find any "material errors" in the original ruling.

He told the Court: “What puberty blockers do is they stop puberty at the appropriate age with unknown consequences. Nobody knows that is the evidence, nobody knows what the consequences of stopping puberty for a child are.”

Mr Hyam also told the Court that “it’s clear that taking puberty blockers is very likely in many cases, at least 80 percent of cases” to lead to a child progressing to taking cross sex hormones. 

He added that the consequences of this are “significant and life-changing” and could result in “a life-time of medication and lifetime of perhaps surgical intervention if that’s chosen”.

“Cross-sex hormones are very likely to be taken. The child must understand what is involved.”

Ms Morris had previously told judges that puberty blockers are said to be "fully reversible" in international guidelines relied upon by the trust.

The barrister also said that children or young people who are considering going on to puberty blockers are told about the effects on their fertility that may be caused by later stages of transition.

Arguing that it is not inevitable that a young person will continue on to cross-sex hormones after puberty blockers, she added: "The question is whether a child is counselled about fertility implications at the next stage and you can see from the material … all of that is explained at the initial stage.

"There is no suggestion anywhere that this is one pathway … there is no shying away from explaining to children and young people what the possibilities are."

The hearing before Lord Chief Justice Lord Burnett (sitting with Sir Geoffrey Vos and Lady Justice King) is expected to last two days, with judgment given at a later date.