Side Effects of Testosterone for Trans Teens: A Cited Resource for Families

TransHealthHub

Sep 9

4 min read

Key Takeaways

Side effects of testosterone for trans teens fall into two groups that are reported as one: the changes the treatment is meant to produce, and the unwanted effects clinicians watch for.
Acne is the most common unwanted effect. Among 46 teens with clear skin at the start of treatment at one U.S. clinic, 25 developed acne.
Thickened blood, called erythrocytosis, is what drives routine lab work. It appeared in 9.5% of 158 adolescents in a 2025 study at an Australian gender service, usually in the second year.
Some intended changes are permanent. The Endocrine Society names voice deepening and clitoral growth as irreversible.
Monitoring is scheduled. The Endocrine Society advises visits every three to six months until dosing is settled, then every six to twelve months.
The American Academy of Pediatrics recommends access to this care and, in the same policy, calls the long-term research limited.
Searches may use masculinizing hormone therapy or gender-affirming hormone therapy, which describe this same treatment.

The short answer

Most side effects of testosterone for trans teens are manageable and tracked on a schedule. Acne is the most common, thickened blood is what drives regular lab work, and both are familiar enough that clinicians plan for them before treatment starts. Several intended changes, notably voice and facial hair, do not reverse, which is why they are discussed in advance rather than counted as side effects at all.

At TransHealthHub, we think the most useful thing a parent can hold onto is that this is monitored care, not a prescription and a shrug.

Expected changes are not the same as side effects

Separating the two lists clears up most of the confusion. The Endocrine Society, in its guidance on transgender and gender-diverse children and adolescents, lists the effects of testosterone as facial and body hair, increased muscle bulk, periods stopping, clitoral enlargement, vaginal dryness, voice deepening, acne, and scalp hair loss. Clitoral growth and voice deepening are irreversible.

Most of that list is the point of the treatment. Acne, scalp hair loss, and thickened blood are the parts nobody wants, and they are what monitoring is built around.

What research shows about side effects of testosterone for trans teens

Effect	What adolescent studies found
Acne	25 of 46 teens with clear skin developed it; 70% incidence proportion at two years
Erythrocytosis	9.5% of 158 adolescents; two-thirds of those cases in the second year
Serious somatic events	Not commonly reported in the small adolescent cohorts published so far

On acne, Lena Chu and colleagues reviewed records for 60 patients under 18 at the Children's Healthcare of Atlanta pediatric endocrinology clinic who started testosterone between 2016 and 2019, publishing in Endocrine Practice in 2023. Of the 46 with no acne at the start, 25 developed it, and the study reported a 70% incidence proportion at two years. Patients using progestin were more likely to develop acne than those who were not, 92% versus 33%.

On blood counts, Ashleigh McLean and colleagues studied 158 adolescents at a pediatric gender service in Australia, published in Transgender Health in 2025.

Erythrocytosis developed in 9.5%, and in two-thirds of those cases it appeared in the second year, which is why monitoring continues past the first year.

What clinicians monitor

The Endocrine Society advises that young people on this treatment see their provider every three to six months until adult dosing is settled, then every six to twelve months, to check hormone levels and look for both wanted and unwanted effects. Acne is treated much as it would be in any teenager. Our page on the risks of hormones in adolescents covers the wider monitoring picture, and our guide to hormone therapy for trans youth covers how treatment is started.

What is still uncertain

The adolescent evidence base is thinner than the adult one. Writing in Transgender Health in 2025, David Inwards-Breland noted that studies here rely on racially and ethnically homogeneous samples, that participants skew white, and that data on more diverse populations remain limited.

That gap sits alongside a clear position from U.S. pediatricians.

The American Academy of Pediatrics policy statement, reaffirmed in 2023, backs access to comprehensive, developmentally appropriate care while calling the research on long-term risks limited. Both belong in the same sentence: sparse data argues for monitoring, not against treatment. It also means percentages from small single-clinic cohorts show what to watch for, not what any one teen will experience.

Frequently asked questions

Which one is most likely? Acne, by a wide margin in the adolescent studies so far.

Why the repeated blood tests? Testosterone can raise red blood cell concentration, and the adolescent data show this often emerges in the second year.

Are any effects permanent? Yes. Voice deepening and clitoral growth do not reverse, which is why they are discussed before treatment rather than after.

Do side effects mean stopping? Not usually. Most are managed with treatment or a dose adjustment by the clinical team, which is a conversation for them rather than a decision made at home.