What are puberty blockers?
Puberty blocker drugs are prescribed to some children who are experiencing gender dysphoria, to temporarily stop their bodies developing.
The NHS describes gender dysphoria as "a condition where a person experiences discomfort or distress because there's a mismatch between their biological sex and gender identity".
The drugs suppress the release of the hormones oestrogen and testosterone, which start being produced in much bigger quantities during puberty.
The hormones tell your body to develop things like breasts, periods, facial hair or a deeper voice.
Suppressing oestrogen and testosterone slows or stops these things developing. The blockers are also used to treat conditions which cause premature puberty in much younger children.
Legal action is being launched against the NHS over the prescribing of the drugs.
Why are they used?
The Gender Identity Development Service (Gids), based in London and Leeds, says that pausing puberty is designed to give a young person with gender dysphoria more time to consider their options - while not having to go through the additional distress of their body changing in a way they do not want.
When someone stops taking blockers, their puberty should resume.
The limited evidence that exists suggests that most young people taking puberty blockers for gender dysphoria do not stop taking them, though, and many go on to take cross-sex hormones.
Cross-sex hormone therapy involves taking either oestrogen or testosterone and is a treatment only available to over-16s on the NHS in England.
By pausing puberty and the development of things like breasts or facial hair, someone who goes on to have cross-sex hormone therapy may avoid having more invasive surgical treatment like having their breasts removed (mastectomy) later on.
Why have they been controversial?
Legal action being launched against the Tavistock and Portman NHS Trust, which runs the gender identity clinic, focuses on whether children can give informed consent to treatment with puberty blockers.
Puberty hormones are linked to changes not just in the body but also in the brain.
Gids says that it is not yet known whether puberty blocker treatment "alters the course of adolescent brain development".
It also says that the full psychological effects of the blocker are not yet known.
Some early data from one study showed some taking the drugs reported an increase in thoughts of suicide and self-harm but it was unable to say whether it was the drugs or something else causing the increase.
Experts on clinical trials criticised the design of the study but said the data warranted further investigation.
NHS England says, in its clinical guidelines, that research evidence around the long-term impacts of puberty blockers is "limited and still developing".
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Although they are considered by the NHS to to be a "fully reversible" treatment, since puberty can be restarted, the blockers may also have longer-term health consequences.
The court case against the Tavistock and Portman NHS Trust will also claim puberty blockers may effect someone's fertility and sexual functioning, although the evidence around this is limited.
One of the people bringing the court case, known as Mrs A, is the mother of a 15-year-old with autism who is on the waiting list for treatment at the Gids.
Gids says that there seems to be a "higher prevalence of autistic spectrum conditions in clinically referred, gender dysphoric adolescents than in the general adolescent population".
Increase in referrals
There has been a large increase in children being referred to Gids in recent years.
Gender Identity Development Service annual referrals
While there is no clear explanation for this increase in referrals, Gids puts it down to greater awareness.
Large increases in referrals have also been seen at gender identity clinics elsewhere, for example in the US and Canada.
There has also been a shift in the past few years, with people assigned female at birth making up the bulk of the increase in young people wanting to transition.
Referrals by sex assigned at birth
Who can they be prescribed to?
Puberty blockers when used to treat gender dysphoria can only be initially prescribed by a specialist, rather than a GP.
They are prescribed by the NHS to children or young people who have already started puberty. This process begins often years before any physical signs of puberty appear and has to be assessed by a hormone specialist.