Micronized progesterone is a form of the hormone progesterone that has been ground into extremely tiny particles to help your body absorb it more effectively when taken by mouth. It is chemically identical to the progesterone your ovaries naturally produce, which is why it’s often called “bioidentical” progesterone. This distinguishes it from synthetic alternatives like medroxyprogesterone acetate (MPA), which have a different molecular structure and can behave differently in the body.
How Micronization Works
Progesterone on its own is poorly absorbed through the digestive tract. The micronization process breaks the hormone into microscopic particles, dramatically increasing its surface area. This allows more of the hormone to pass through the intestinal lining and into the bloodstream. Without micronization, most of an oral progesterone dose would be broken down by the liver before it ever reached circulation. The FDA notes that oral bioavailability of progesterone is specifically increased through this process.
Some oral capsule formulations suspend the micronized progesterone in peanut oil to further aid absorption. This is worth knowing if you have a peanut allergy, since you’d need an alternative formulation. Micronized progesterone also comes as vaginal capsules, inserts, and gels, which bypass the digestive system entirely and deliver the hormone more directly to the uterus.
What It’s Used For
The FDA has approved oral micronized progesterone (sold under the brand name Prometrium, among others) for two main purposes: preventing abnormal thickening of the uterine lining in postmenopausal women taking estrogen, and treating secondary amenorrhea (when periods stop for reasons other than pregnancy or menopause).
In menopause hormone therapy, estrogen taken alone can cause the uterine lining to grow excessively, raising the risk of endometrial cancer. Adding progesterone counteracts this effect. Women who still have a uterus and use estrogen therapy are typically prescribed progesterone alongside it for this protective purpose.
Vaginal micronized progesterone also plays a role in obstetric care. The American College of Obstetricians and Gynecologists (ACOG) notes that vaginal progesterone may be considered for patients with a history of preterm birth who have a shortened cervix during a singleton pregnancy. However, ACOG updated its guidance in 2023 to clarify that vaginal progesterone should not be offered to prevent recurrent preterm birth when the cervix is not shortened, as the evidence does not support effectiveness in that scenario.
Why It’s Considered Different From Synthetic Progestins
The distinction between micronized progesterone and synthetic progestins like MPA matters most when it comes to long-term safety. A systematic review and meta-analysis found that progesterone combined with estrogen was associated with a 33% lower relative risk of breast cancer compared to synthetic progestins combined with estrogen. Women using estrogen with natural progesterone showed no statistically significant increase in breast cancer risk, while those using estrogen with synthetic progestins showed a trend toward higher risk.
These two types of progestogen also appear to affect blood lipids, clotting factors, and insulin differently, which could translate into different cardiovascular profiles. However, no head-to-head studies have directly compared cardiovascular outcomes between the two. That gap in the evidence means the cardiovascular question remains open, even though the breast cancer data leans in favor of micronized progesterone.
The Sedative Effect and Why It Happens
One of the most distinctive side effects of oral micronized progesterone is drowsiness. This isn’t a coincidence or a minor quirk. When your liver processes oral progesterone, it converts a portion into a metabolite called allopregnanolone. This compound acts on the same brain receptors that respond to anti-anxiety medications like benzodiazepines. Allopregnanolone is a potent modulator of GABA receptors, the brain’s primary system for calming neural activity.
This sedative quality is strong enough that most prescribers recommend taking oral micronized progesterone at bedtime. For many women, this side effect is actually welcome, as it can improve sleep quality. Vaginal formulations produce far less allopregnanolone because the hormone bypasses the liver, so drowsiness is much less of an issue with that route. If sedation is a problem for you, switching from oral to vaginal delivery is a common solution.
Available Formulations
Micronized progesterone comes in several forms depending on the intended use and region:
- Oral capsules: Prometrium is the most recognized brand in the United States. In the UK, Utrogestan and Gepretix are common. Generic versions are also widely available in 100 mg and 200 mg doses.
- Vaginal capsules and inserts: These deliver progesterone directly to the uterus and are often preferred in fertility treatments and preterm birth prevention, since they minimize systemic side effects like drowsiness.
- Vaginal gels: Another option for localized delivery, often used in assisted reproduction protocols.
The choice between oral and vaginal forms depends on the clinical goal. Oral capsules are standard for menopause hormone therapy. Vaginal forms are more common in pregnancy-related uses and when avoiding the sedative metabolite is a priority. Both deliver the same bioidentical hormone, but the way your body processes it differs significantly based on the route.