Yes, Prometrium is micronized progesterone. It contains progesterone that is chemically identical to the hormone your body naturally produces, processed through a technique called micronization that shrinks the particles small enough to be absorbed through your digestive system. It comes in 100 mg and 200 mg capsules.
What Micronization Actually Does
Progesterone on its own is poorly absorbed when swallowed. The molecule is fat-soluble, and in its natural crystal form, it passes through the gut without much of it reaching your bloodstream. Micronization solves this by grinding the progesterone particles down to less than 10 micrometers, dramatically increasing the surface area exposed to your intestinal lining. This allows the hormone to be absorbed into the lymphatic system more efficiently, largely bypassing the liver’s first-pass metabolism that would otherwise break it down before it could take effect.
The progesterone in Prometrium is derived from plant sources (soybeans and Mexican yam roots) but has an identical chemical structure to the progesterone your ovaries produce. This is what distinguishes it from synthetic progestins, which are lab-created compounds that mimic some of progesterone’s effects but have a different molecular structure.
How Prometrium Differs From Synthetic Progestins
The distinction between micronized progesterone and synthetic progestins matters clinically. In a direct comparison with medroxyprogesterone acetate (one of the most commonly prescribed synthetic progestins), women taking the synthetic version reported more vaginal bleeding and breast tenderness. Neither type affected mood in the study, which ran counter to a longstanding belief that progesterone causes depression. The researchers concluded that some women may prefer micronized progesterone because of its lighter side effect profile.
This difference is part of why many prescribers have shifted toward micronized progesterone for hormone therapy in recent years, particularly for postmenopausal women who need uterine protection while taking estrogen.
FDA-Approved Uses
Prometrium has two approved indications. The first is preventing endometrial hyperplasia, a thickening of the uterine lining that can become precancerous, in postmenopausal women taking estrogen who still have a uterus. For this use, the typical regimen is 200 mg nightly for 12 days out of every 28-day cycle. The second approved use is treating secondary amenorrhea, the absence of menstrual periods in women who previously had them. That protocol uses 400 mg nightly for 10 days.
Off-Label Use in Pregnancy
Micronized progesterone is also widely used outside its FDA-approved indications, particularly in early pregnancy and fertility treatment. Vaginal micronized progesterone is prescribed for luteal phase support during IVF cycles and for women at risk of miscarriage. In a large analysis of clinical trials, women with vaginal bleeding in early pregnancy and a history of one or more prior miscarriages had a 75% live birth rate with progesterone compared to 70% with placebo. The benefit was more pronounced for women with three or more previous miscarriages: 72% live birth rate with progesterone versus 57% without it.
These pregnancy-related uses typically involve vaginal administration rather than the oral capsules, since vaginal delivery concentrates the hormone directly in the uterus while producing fewer systemic side effects like drowsiness.
Taking Prometrium: Food and Timing
If you’re prescribed oral Prometrium, eating when you take it roughly doubles absorption compared to taking it on an empty stomach. The label recommends taking it as a single dose in the evening, partly because of this food interaction (most people eat dinner in the evening) and partly because the capsules can cause drowsiness or dizziness, especially during the first few days. Bedtime dosing minimizes those effects.
The drowsiness is actually related to one of progesterone’s natural metabolites, which acts on the same brain receptors targeted by certain sleep medications. This is a feature of the bioidentical hormone itself, not a side effect of the drug formulation, and it tends to become less noticeable over time.
The Peanut Oil Issue
One important detail about brand-name Prometrium: the capsules use peanut oil as a suspension agent. The FDA label states plainly that Prometrium should never be used by anyone with a peanut allergy. If you have a peanut allergy and need micronized progesterone, generic versions and compounded formulations are available that use different oils, such as sunflower or olive oil. This is worth flagging to your pharmacist, since brand and generic versions are not always interchangeable on this point.
Capsule Identification
The 100 mg capsules are round and peach-colored, imprinted with “SV.” The 200 mg capsules are oval and pale yellow, imprinted with “SV2.” Both are soft gelatin capsules, which is how the micronized progesterone stays suspended in the oil base inside.