I thought I'd write this as I see so many questions about why they are so contraversial and there is so much info being thrown about that it can be overwhelming.
So a brief history and how they were taken on by the Gender Clinics.
The main crux of the issue is that they are GnRH analogues. GnRH- which are gonadotropin-releasing hormones are released by the pituatory glands and GnRH analogues stop this process from happening. They control and regulate the development of the sex hormones in both males and females.
So they were used decades ago to 'treat' homosexuals when homosexuality was illegal. If a homosexual was arrested for having gay sex for example, the option was to go to jail or agree to take this drug. One key person who this happened to was Alan Turing, who later committed suicide. They have been used as well to 'treat' paedophiles.
They are also used to treat 'precocious puberty' in very young children, and are used for a very short time due to the known affects of the drug. It is also used for endometriosis.
Sidebar: the FDA in the USA reports 25,000 adverse effects, and it is linked to at least 1,500 deaths. And that is for use 'on book' ie being used for the things is it regulated to be used for.
So back in 2003, when the main cohort of people wanting to 'transition' was males, somewhere in a clinic in Amsterdam, it was suggested that these GnRH analogue drugs could be used in males who were being treated for 'gender dysphoria' because the drugs would suppress the physical development of the voice, the growth of a beard, the jawline and height and what grown up men who had 'transitioned' wanted to look at was allowing other men to pass as women more easily when they grew up.
They tried this drug on Patient Zero, and drug seemed to work.
Sidebar: a 22 year follow up study on Patient Zero reports that he hadn't managed to sustain any long term relationship, was ashamed of his genitals, but was assessed as a positive outcome because he had a job. It was reported that 'the concept of regret in transition isn't the most helpful framework'.
In 2004 meanwhile, at the GIDS in the Tavistock in London, Sue Evans reported that this was now being prescribed and there was no evidence as to the success of these drugs and a 16 year old boy had been referred for these drugs after 4 sessions. She blew the whistle but it was completely ignored.
Sidebar: it is impossible to do a blind or double blind study on puberty blockers as if a placebo is given, puberty will continue to develop so it becomes obvious very quickly.
It appears that across the board, the drugs were being used, off book ie they have never been approved for use to block puberty as a treatment for 'gender dysphoria' and the use ramped up over the years. The useage was also expanded to girls.
In 2011, the clinic in the Netherlands did a follow up of 70 people who had been prescribed these drugs as teenagers. They seemed to have done two surveys on them. The Utrecht Gender Dysphoria scale was used, which was initially done in their actual sex and the follow up which they used the opposite sex scale to measure the success, [the survery switcheroo]. They also used another 'Body Image Scale] which was also swapped out for the opposite sex scale. It is unclear how long any of these people had been on the Puberty Blockers, and unclear if they ever asked about any other side effects of the drugs.
Out of the people that responded, 15 dropped out [didn't respond]. They said they wanted to see who had gone on to full surgery but in other reports I found that these HAD gone on to full surgery, so who knows. They found that issues had been had by the other 55, of which some didn't fulfil the surgery requirements because they now were obese, or had diabetes, some refused to complete the survey and others couldn't be operated on due to 'short penis length'. It is unclear how many of the original 70 proceeded to surgery or whether it was 'successful' or not. It is unclear how much input the Dutch Clinic had with anything after the puberty blockers. It in unclear if there were any actual girls in this cohort.
Sidebar: if you start searching for treatment of homosexuals using Lupron the internet takes you down a road, and the end of which is that 'Lupron is no longer available'. However there are many names for the same drugs now. If you start searching for treatment of precocious puberty or endometriosis you get a swathe of reports on the damage and side effects and catastrophic issues from people whose lives have been destroyed by this drug, even after just a few weeks treatment. If you however search for puberty blockers, there is just the 'safe, effective, reversible' information from a huge range of clinics and what I would consider reputable medical insititutions.
The reason that the Cass report took 4 years is the impossibility of actually finding out anything concrete from any studies, it is like trying to nail oily spaghetti to a jelly wall using prawn crackers. The internet resists any cross referencing from one 'treatment' and named drug across different treatment protocols and links between one report saying the drug has massive catastrophic side effects and it should be a last and final treatment to another saying it is completely safe and reversible.
I've synthesised a fair amount of this information from the interview between Stella O'Mally and Sasha Ayed and the two medics who developed the Dutch Protocol, and using their actual words.
Michael Biggs did a deep dive into this protocol here which establishes that the use was documented as a standard treatment in 2001 by the Harry Benjamin International Gender Dysphoria Association which later became WPATH. Two years before the Dutch Clinicians said that the drug regime began being used.