Progesterone
Progesterone is the part of feminizing hormone therapy people argue about most. Some swear it helped their breasts fill out, their mood or their sleep. Many prescribers won't routinely offer it, because the evidence that it does any of those things is thin. Both groups are often right about what they've seen, and neither has the studies to settle it.
This page explains what progesterone is, how micronised progesterone differs from the synthetic progestins, what people hope for from it, what has actually been shown, and what to ask if you want to try it.
I'm not a doctor, and this is one of the areas where my own vantage point settles the least. The women I've coordinated through surgery split roughly between people who were on it and people who'd never been offered it, and I couldn't have told you which was which from anything else about them.
On this page
What progesterone is
Progesterone is the other main hormone made by the ovaries. In a menstrual cycle it rises in the second half, after ovulation, and it's central to pregnancy. Estrogen is the hormone of breast growth in puberty; progesterone's role in adult breast tissue is more about the final stages of development.
That second fact is where the argument comes from. The theory goes that estrogen alone leaves breast development incomplete, and adding progesterone lets it finish. It's a reasonable theory. It hasn't been shown in trans women in studies large or long enough to count.
Micronised progesterone vs progestins
Micronised progesterone (sold as Utrogestan in the UK and Prometrium in the US) is chemically the same as the body's own progesterone. It's usually a capsule taken at night, because it makes many people drowsy.
Progestins are synthetic relatives. Medroxyprogesterone acetate is the best known. They act like progesterone in some ways and differently in others. Cyproterone acetate is technically one too, which is part of why it raises prolactin; the anti-androgens page covers it.
The distinction matters because much of the worry about progesterone comes from the Women's Health Initiative trials in postmenopausal cis women, which found higher rates of breast cancer and blood clots with estrogen plus a synthetic progestin. Whether those findings apply to micronised progesterone, or to younger trans women, is uncertain. Most prescribers who do use progesterone for feminization now choose the micronised form for this reason.
What people hope for, and what's shown
People usually want progesterone for one of four things: fuller breast development, better mood, better sleep, or higher libido. Each has advocates and anecdotes. None has good evidence from controlled studies in trans women.
The few small studies that exist haven't found a clear effect on breast size, and they're too small and short to rule one out either. The Endocrine Society's guideline doesn't recommend routine use, citing the lack of evidence and the potential risks, and WPATH's Standards of Care take a similarly cautious view. Some specialists now offer a trial to people who ask, especially later in transition, on the basis that the risk of the micronised form looks low and the effects people describe are consistent enough to be worth testing in an individual.
That's where things honestly stand. If someone tells you progesterone is essential, or that it's dangerous, they're claiming more certainty than exists.
If you want to try it
Agree with your prescriber in advance what you're looking for and how long you'll give it. Taking photos and measurements, and writing down how you sleep and feel before you start, gives you something to judge by beyond memory.
Side effects
Micronised progesterone commonly causes drowsiness, which is why it's taken at bedtime. Some people find it lowers their mood or makes them irritable, sometimes the opposite of what they were hoping for. Bloating, breast tenderness and changes in libido in either direction are also reported.
Progestins, and progesterone to some extent, may add to the risk of blood clots on top of estrogen, particularly in people already at higher risk. Risks and side effects covers clot warning signs. Progesterone can also affect cholesterol and blood sugar, which is part of why prescribers want routine blood tests.
If your mood drops after starting progesterone, tell your prescriber. It's one of the more common reasons people stop, and it usually lifts once they do.
How to raise it with a prescriber
Some prescribers offer progesterone readily, some only on request, and some not at all. It often depends on the clinic's policy rather than the individual.
Useful questions: Do you prescribe micronised progesterone, and on what basis? What would make you stop it? Are there reasons it's a worse idea for me, such as a clot history, smoking or a mood condition? A prescriber who can explain their answer, whichever way it goes, is doing their job. If they won't prescribe it and you'd like a second view, getting hormones covers other routes.
Frequently asked questions
Does progesterone help breast growth?
It's widely believed to, and it hasn't been shown in controlled studies in trans women. The evidence is too thin to confirm or rule it out. See what people hope for, and what's shown.
Is progesterone safe?
Micronised progesterone looks lower risk than the synthetic progestins behind older safety concerns, but it isn't risk free, particularly for people with a clot history or mood problems. See micronised progesterone vs progestins and side effects.
Why won't my doctor prescribe progesterone?
Major guidelines don't recommend routine use because the benefits haven't been shown. Some prescribers will offer a trial on request; others follow clinic policy. See how to raise it with a prescriber.