Progesterone

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 it's used for, and what the evidence shows

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.

Part of why the question is so hard to settle is timing. Breasts keep developing on estrogen alone for two to three years or more, so if you add progesterone at eight months and see growth at eighteen, there's no way to tell which did it. Mood and sleep are harder still to judge, because they move with everything else in your life. That's one reason some prescribers who are open to it suggest waiting until growth on estrogen alone has slowed, so any change afterwards means more.

Progesterone also has a mild dampening effect on the brain's signal to the testes, so some prescribers find it helps testosterone suppression a little. It isn't a replacement for an anti-androgen, and how much it helps varies.

So the balanced picture is this: progesterone isn't essential, the micronised form looks low risk for most people, and it's a reasonable thing to try with a prescriber on a clear plan.

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 clot risk on top of estrogen, particularly in people already at higher risk; the warning signs are in risks and side effects. Progesterone can also affect cholesterol and blood sugar, which your routine blood tests already cover. Progesterone levels themselves usually aren't measured; prescribers judge it by how you get on.

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.

Who needs a closer look first

Some people are more likely to be steered away from progesterone, or offered it only with closer watching:

• A past blood clot, or a strong family history of them, because progesterone may add to the clot risk estrogen already carries.
• Smokers, for the same reason.
• A history of depression or mood problems that progesterone has worsened before, or that is currently hard to manage.
• Breast cancer in you, or a strong family history, where any extra hormone is approached carefully.
• Significant liver disease.

None of these is an automatic no for every prescriber, and none applies to most people. They're the reasons a prescriber might say "not for you" rather than "not at this clinic", and it's worth knowing which kind of answer you're getting.

Trying it, and judging it

If you and your prescriber decide to try it, the usual approach is micronised progesterone taken at bedtime, on a plan agreed in advance for how long you'll give it before deciding. Many prescribers think in months rather than weeks, because breast changes are slow and mood effects can take a while to show their direction.

Some people take it every night; some take it in a cycle, for part of each month, on the idea that this is closer to how the body uses it. There's no good evidence that either pattern is better for feminization, and prescribers differ. What matters more is that you agree the pattern, the length of the trial, and what would count as working, before you start.

Stopping is simple. Progesterone doesn't need tapering in the way some medicines do, and any drowsiness or low mood it was causing usually lifts within days. Tell your prescriber either way, so your records show what you tried and what you noticed.

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 it's used for, and what the evidence shows.

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.

Should I start progesterone at the same time as estrogen?

Most prescribers who use it suggest waiting until breast growth on estrogen alone has slowed, partly so any change afterwards can be judged. See what it's used for, and what the evidence shows.

Can I stop progesterone if I don't like it?

Yes. It doesn't need tapering, and side effects such as drowsiness or low mood usually lift within days of stopping. Tell your prescriber. See trying it, and judging it.