Regret doesn't seem to set in until about 7 years after. Studies that claim a positive mental outcome are short-term, poor quality, and often self selecting. Good quality large scale research suggests increased suicide risk.
Straub et al, 2024.
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The Djeune study found higher all cause mortality, including suicide, in their research.
Once the euphoria wears off, and obviously as younger patients mature, the reality of what has been done with drugs or surgery sets in. Unfortunately we can't measure regret levels easily as these patients just disappear off the radar, stop picking up prescriptions etc. Clinics aren't interested in what happens long term. The Tavistock did zero follow ups in 20 years. They had no data as to whether giving kids puberty blockers and later hormones improved their mental health or not.