In transgender men, or trans masculine people (FTM), the most common medication used for transition is testosterone. Administration of testosterone (via transdermal, intramuscular, subcutaneous, or oral routes) lowers serum estradiol levels, raises serum testosterone levels, and results in the development of typical male secondary sex characteristics. Irreversible changes include: deepening of the voice, increase in facial and body hair growth, clitoral enlargement (clitoromegaly), and thickened facial bone structure. Reversible changes include amenorrhea, male-pattern fat distribution, increased muscle mass, vaginal atrophy, and male-pattern baldness. Some trans men also describe changes in emotions (e.g., inability to cry, increased anger) as well as increased libido. Adverse effects can include elevations in blood pressure, polycythemia, worsening of lipid profile, elevations in glucose, elevations in transaminases, acne, and effects on fertility (although testosterone is not an effective contraceptive as it does not interrupt ovulation, so pregnancy can still occur).
Finasteride can also be used to prevent male-pattern baldness in transgender men, as it only blocks dihydrotestosterone (DHT), not testosterone itself; however this will likely slow or decrease secondary hair growth, and may slow or decrease clitoromegaly as well.
In transgender women or trans feminine people (MTF), the most commonly used medications are estrogens and anti-androgens. Administration of estrogen (via oral, sublingual, transdermal, intramuscular, or subcutaneous routes) lower serum testosterone levels, raises serum estradiol levels, and results in the development of typical female secondary sex characteristics including: breast growth, softer skin, decreased muscle mass, and female-pattern fat distribution. These effects are largely reversible. Estrogen can also cause testicular and penile atrophy (ultimately resulting in potential erectile dysfunction and infertility). Some trans women also describe changes in emotions (e.g., more tearful) as well as decreased libido. Adverse effects can include increased risk for thrombosis, elevations in blood pressure, elevations in prolactin (rarely including development of a prolactinoma), migraines, elevations in transaminases and effects on fertility (although estrogen is not an effective contraceptive). Estrogens will NOT heighten voice pitch, decrease facial hair, change facial bone structure, or reverse male-pattern baldness. Other methodologies would need to be employed (e.g., voice training, electrolysis or laser hair removal, facial feminization surgery, hair restoration, etc).
Anti-androgens (i.e. spironolactone, bicalutamide, flutamide, finasteride) are also commonly used in trans women who have not had an orchiectomy. These medications block the effects of testosterone, resulting in decreased erectile function and allowing estrogen to develop typical female secondary sex characteristics. Finasteride, however, specifically targets dihydrotestosterone (DHT), not testosterone, so it is not as effective at lowering total testosterone levels.
GnRH Agonists (i.e. Lupron) could also be used instead of Anti-Androgens to block endogenous testosterone production. Lupron is typically given intramuscularly every couple months and is very effective at blocking total testosterone levels. However, it can be difficult to obtain insurance coverage for it, and is otherwise fairly expensive out of pocket.
Progesterones activate the androgen receptors slightly, so may be used to improve libido and mood. in some cases, it may be indicated to maximize breast growth, though this is likely happening via weight gain. Of note, some studies show a possible increased risk for VTE, cardiovascular disease and/or breast cancer with use.
Not all transgender patients will want to take medications for gender transition and the risks, benefits and alternatives should be discussed with each individual along with their personal goals for transition to determine the right course.