RISKS OF GENDER-AFFIRMING HORMONES
IN ADOLESCENTS
The known physical risks of gender-affirming hormones in adolescents are relatively limited and are managed with routine monitoring: the main ones are a temporary dip in bone density with puberty-pausing medication, a raised red blood cell count with testosterone, and a small increase in blood-clot risk with estrogen. The larger and more public debate is not mainly about proven harm; it is about how strong the evidence is for the benefits. Independent reviews have rated that evidence as low-certainty, while major US medical organizations regard the care as safe and effective when it is properly provided. Both sides agree that decisions should be individualized and closely monitored.
This page uses “puberty-pausing medications” for puberty blockers after the first mention; the broader category is sometimes called gender-affirming care, transition-related care, or supportive care, and those terms describe the same thing. It covers what is known about risks and how the evidence is debated, not doses or treatment decisions, which belong to a young person and their care team.
CONTENT NOTE
This is information, not medical advice. Whether this medication is appropriate for a particular young person is a decision for that young person, their family, and their qualified healthcare providers, based on individual circumstances.
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The physical risks are limited and monitored. The main ones are a temporary bone-density dip with puberty-pausing medications, a raised red blood cell count with testosterone, and a small increase in blood-clot risk with estrogen.
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“Low-certainty evidence” is not the same as “proven harmful.” The central scientific dispute is about how strong the evidence for benefits is, not about demonstrated widespread harm.
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Expert bodies differ in emphasis. US medical organizations endorse the care as safe and effective when appropriately provided; several European reviews, including the UK’s Cass Review, have urged more caution, citing weak evidence.
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Long-term data are limited. Much of the adverse-effect data comes from adults, and long-term adolescent data are still thin, a point both sides make.
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Monitoring is standard. Regular blood tests and follow-up are designed to catch the known effects early.
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Decisions are individualized. The care is not considered right for everyone, and it is meant to be tailored with providers and families.
KEY TAKEAWAYS
WHAT ARE THE RISKS OF GENDER-AFFIRMING HORMONES FOR ADOLESCENTS?
There are two different questions inside this one.
The first is about direct physical risks, which are relatively few and are tracked with routine monitoring.
The second is about the strength of the evidence for the treatments’ benefits, which is where the genuine expert disagreement lies.
Keeping the two apart is the key to reading this topic clearly, because a debate about evidence certainty is often reported as if it were proof of harm, and it is not the same thing.
WHAT THE RESEARCH SHOWS
Sources: bone density, JAMA Pediatrics, van der Loos et al. (2023); erythrocytosis, Annals of Family Medicine (2023); clot risk, Frontiers in Endocrinology meta-analysis (2021).
THE EVIDENCE DEBATE: CERTAINTY VS. HARM
The public controversy is mostly about a different question: how good is the evidence that these treatments help? Independent evidence reviews have answered “not as strong as we would like.” The UK’s Cass Review (NHS England, 2024), working from systematic reviews it commissioned, concluded that the evidence for benefits in young people is low-certainty, and the UK subsequently restricted puberty-pausing medications for gender-related care to clinical trials. Importantly, the Review still stated that medical transition is the right outcome for some young people, and framed its findings as being about the quality of the studies rather than proof that the treatments are harmful.
Major US medical organizations weigh the same evidence differently. The American Academy of Pediatrics reaffirmed its supportive policy in 2023 and commissioned its own systematic review, and the American Medical Association and American Psychological Association hold that the care is safe and effective when appropriately provided. When a 2025 report from the US Department of Health and Human Services concluded the evidence was low-quality and emphasized potential harms, the AAP and AMA rejected it as methodologically flawed and not reflective of clinical consensus. UK medical bodies, for their part, largely welcomed the Cass Review, though some clinicians and researchers criticized parts of it and argued it should not be used to guide care elsewhere.
WHAT IS KNOWN, AND WHAT ISN'T
What is well established: the direct physical risks above, and the fact that they are monitorable. What remains uncertain: long-term outcomes, because most adverse-effect data come from adults and long-term adolescent studies are limited. That uncertainty is exactly what the evidence reviews flagged, and it is why both those urging caution and those supporting access agree on two things: that care should be individualized rather than automatic, and that it should be closely monitored over time.