Prometrium is made from progesterone derived from a plant source, specifically wild yam. The progesterone is extracted and synthesized from a compound called diosgenin found in wild yam root and tubers, then processed into a form that is chemically identical to the progesterone your ovaries naturally produce. The capsules also contain peanut oil as a key inactive ingredient, which is an important detail for anyone with a peanut allergy.
The Active Ingredient: Plant-Derived Progesterone
The sole active ingredient in Prometrium is progesterone, classified as USP (United States Pharmacopeia) grade. According to its FDA-approved labeling, this progesterone “is synthesized from a starting material from a plant source and is chemically identical to progesterone of human ovarian origin.” That last part matters: unlike some synthetic hormone medications that use modified versions of progesterone, Prometrium contains what’s often called “bioidentical” progesterone because its molecular structure perfectly matches what your body makes on its own.
The plant source is wild yam (Dioscorea species). Wild yam root and tubers contain a compound called diosgenin, a natural plant sterol that serves as a chemical precursor to progesterone. In a laboratory setting, diosgenin is converted through a series of chemical steps into pharmaceutical-grade progesterone. It’s worth noting that eating wild yam or taking wild yam supplements will not give you progesterone. Your body cannot make that conversion on its own. The transformation requires industrial chemical processing.
What Micronization Does
Prometrium is specifically described as “micronized” progesterone, which refers to how the progesterone particles are physically processed before being put into capsules. Micronization breaks the progesterone into extremely small particles, which increases the total surface area of the drug. This improves how well your body absorbs the hormone when you take it by mouth and increases its bioavailability, meaning more of the active ingredient actually reaches your bloodstream rather than passing through your digestive system unused. Micronization also reduces certain metabolic and vascular side effects compared to non-micronized forms.
Inactive Ingredients and Peanut Oil
Beyond the progesterone itself, the capsules contain several inactive ingredients that hold the drug together, help with absorption, and give the capsules their color. The most notable is peanut oil, which serves as the suspension medium for the micronized progesterone inside each capsule.
The full inactive ingredient list for the 100 mg capsule includes: peanut oil, gelatin, glycerin, lecithin, titanium dioxide, and two colorants (D&C Yellow No. 10 and FD&C Red No. 40). The 200 mg capsule uses the same base ingredients but swaps FD&C Red No. 40 for FD&C Yellow No. 6, which accounts for the difference in capsule color.
If you have a peanut allergy, this is a critical detail. The presence of peanut oil means Prometrium is not safe for people with peanut hypersensitivity. Alternative progesterone formulations that don’t contain peanut oil do exist, so your prescriber can offer a substitute.
How the Two Strengths Differ
Prometrium comes in two strengths: 100 mg and 200 mg. The 100 mg capsules are round and peach-colored, branded with a black “SV” imprint. The 200 mg capsules are oval and pale yellow, branded with “SV2.” The difference between them is simply the amount of micronized progesterone inside. The capsule shell and oil base are essentially the same formulation, with only the coloring agents changed slightly between the two doses.
Why the Plant Source Matters
Historically, many hormone medications were made from animal sources. Prometrium’s plant-derived origin is one reason it became popular as an alternative. Because the final product is molecularly identical to human progesterone, your body’s enzymes and receptors interact with it the same way they would with progesterone produced by your own ovaries. This is different from synthetic progestins (like medroxyprogesterone), which have an altered chemical structure and can produce a different side effect profile. When your doctor or pharmacist refers to Prometrium as “natural” progesterone, they’re referring to this chemical identity with human progesterone, not to the fact that it comes from a plant, since significant laboratory processing is still involved in its manufacture.