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Cass Review on puberty blockers: no evidence that they buy time to think

Puberty blockers — GnRH analogues such as leuprorelin and triptorelin — have been used since the Dutch Protocol to suppress puberty in young people with gender dysphoria. The Cass Review (April 2024) writes that nearly all young people who start them go on to hormones, that there is therefore no evidence that blockers buy time to think, and that the evidence on the effect on mental health is insufficient or inconsistent (source, paras 14.25 and 14.28).

What are puberty blockers?

GnRH analogues suppress the production of sex hormones. They are licensed for early-onset puberty and are also used for some cancers and gynaecological conditions in adults. According to the Cass Review, use in gender dysphoria is a different situation: the normal rise in hormones that should occur in the teenage years, and that matters for psychosexual and other development, is blocked (paras 14.6 and 14.7).

What does the Cass Review say?

The University of York's systematic review of puberty suppression in young people with gender dysphoria assessed 50 studies. One was of high quality, 25 of moderate and 24 of low quality. The researchers concluded that high-quality research is lacking and that no conclusions can be drawn about the effect on gender dysphoria, mental and psychosocial health and cognitive development (source). The final report adds:

• Several studies show that blockers suppress puberty, and also that bone density is compromised during suppression (para 81 of the summary).
• No changes in gender dysphoria or body satisfaction were demonstrated. Evidence on psychological and psychosocial wellbeing, cognitive development, cardiometabolic risk and fertility is insufficient or inconsistent (para 82).
• Because nearly all young people who start blockers go on to hormones, the report says there is no evidence that blockers buy time to think; there is also concern that they may affect the development of sexuality and gender identity (para 83).
• Little research exists on the effects on brain development: the review found one cross-sectional study on executive function, with worse results after more than a year of treatment (paras 14.39 and 14.40).

How many go on to hormones?

In the English early intervention study (Carmichael et al., 2021), 98 percent (43 of 44) of those who started blockers went on to cross-sex hormones. In a Dutch study (Van der Loos et al., 2023), 93 percent of 707 eligible young people went on, and in another Dutch study from 2022, 98 percent (paras 13.23 and 14.24). In a British discharge study (Butler et al., 2022), 7.4 percent (16 of 217) of those under 16 at referral stopped blockers. Because a treatment intended for one group (mainly boys gender incongruent since childhood) has been given to another (mainly girls with dysphoria from early to mid-puberty), the Review says it is hard to know what share of them would have resolved their distress in other ways (paras 14.33 and 14.34).

The NHS decision and international developments

The NHS stopped routinely prescribing puberty blockers to minors in March 2024; in December 2024 the UK government made the ban on supply via private prescribers indefinite (source). The Tavistock clinic GIDS closed on 31 March 2024. In Sweden, Socialstyrelsen wrote in December 2022 that the risks of puberty blockers probably outweigh the expected benefits at group level and that treatment is possible only in exceptional cases (source); see also SBU. The Norwegian commission Ukom (March 2023) recommended defining this treatment as "utprøvende behandling" (source); see also Finland and Denmark. In the Netherlands, the Health Council called the strength of evidence for the effects low to very low on 30 June 2026, but saw no reason to organise care differently (source).

The Dutch Protocol: a study of 70 young people

The Cass Review describes the Dutch study by De Vries et al. (2011) of 70 young people who started blockers between 2000 and 2008. They were part of a group of 111 referred young people and were chosen because they were the first ready for the next step, hormones; the outcomes of the other 41 were not reported. During suppression, body dysphoria did not change while behavioural and emotional problems decreased; 59 to 73 percent completed the questionnaires after treatment (paras 2.10 and 2.11). The UK early intervention study could not replicate the improvement in mental health (para 14.27). See criticism of the VUmc protocol and the Dutch rollout.