Bioidentical Hormones vs. Synthetic: What's the Difference and Are They Safer?

What does "bioidentical" actually mean?

"Bioidentical" describes a hormone whose molecule is identical to one the human body makes. For menopause, that mainly means 17β-estradiol (the main estrogen before menopause), progesterone, and sometimes estriol or testosterone. Because the structure matches, these hormones bind receptors and are broken down the way your own hormones are.

"Synthetic" or non-bioidentical hormones have different structures. The two most familiar are conjugated equine estrogens (a mix of estrogens from pregnant mares' urine) and synthetic progestins such as medroxyprogesterone acetate. Those were the hormones used in the large Women's Health Initiative (WHI) trials, whose early results in 2002 shaped decades of fear about hormone therapy (Cagnacci 2019).

Two common misconceptions are worth clearing up. First, "bioidentical" is not the same as "natural": these hormones are made in a lab, usually from plant starting materials in soy or wild yam. Your body cannot convert yams or wild yam cream into estradiol or progesterone. Second, "bioidentical" is not the same as "compounded." Many bioidentical hormones are standard, FDA-approved prescriptions.

FDA-approved vs. compounded bioidentical hormones

ACOG's 2023 clinical consensus found a lack of high-quality data on the safety and effectiveness of custom-compounded hormone therapy and advises that it should not be prescribed routinely when FDA-approved formulations exist. When a patient chooses it, ACOG says clinicians should explain the lack of FDA approval and the risks specific to compounding. It also cites testing in which some compounded products were as much as 26% below their labeled estradiol strength or 31% above their labeled progesterone strength, though most were within 10% (ACOG 2023). A National Academies committee, convened at the FDA's request, reached a similar conclusion about insufficient evidence to support routine use (NASEM 2020).

Many women don't know this. In US survey data, 86% of women were unaware that compounded hormone products are not FDA-approved (Pinkerton 2015).

By the numbers

Compounded hormones by the numbers

86%of women unaware compounded hormones are not FDA-approvedSource: Pinkerton 2015

26%below labeled estradiol strength in some compounded products testedSource: ACOG 2023

31%above labeled progesterone strength in some compounded products testedSource: ACOG 2023

Are bioidentical hormones safer than synthetic ones?

The honest answer is "in some specific ways, possibly; in general, not proven." Here is what the evidence supports:

• Route matters more than "bioidentical." Estrogen through the skin (patch or gel) was not linked to higher clot risk in large UK data, while oral estrogen was. Oral estradiol carried somewhat lower risk than conjugated equine estrogens (Vinogradova 2019).
• Progesterone vs. progestins. In the French E3N cohort, estrogen combined with micronized progesterone was not associated with increased breast cancer risk (relative risk 1.00), while estrogen with other synthetic progestins was (1.69) (Fournier 2008). This is observational, mostly short-term data, not a randomized trial, and longer use may still carry some risk.
• The core estrogen risks still apply. The Menopause Society's position is that risks depend on type, dose, duration, route, timing of initiation, and whether a progestogen is used (Menopause Society 2022). A bioidentical label does not remove the need for a progestogen if you have a uterus, or the contraindications such as breast cancer or past clots.
• Compounded products are the least studied. Their trials are mostly small, short and without a control group, and there is no requirement to report adverse events (ACOG 2023).

A note on progesterone creams

If you have a uterus, progesterone's main job in hormone therapy is protecting the uterine lining from estrogen. Oral micronized progesterone is the FDA-approved, well-studied option for that. Progesterone absorbed through the skin from a cream gives low and variable blood levels, and researchers have long debated whether that is enough to protect the lining reliably (Stanczyk 2005). If you use a cream containing progesterone, your clinician should explain this, and any unexpected bleeding or spotting should be reported and checked.

Why do some women choose compounded hormones?

Reasons women give include wanting one product instead of two, a strength or form not sold commercially, an allergy to an ingredient in a commercial product (such as peanut oil in some progesterone capsules), or a preference for a cream. These can be legitimate reasons, and compounding exists to meet individual needs. What doesn't hold up is marketing that compounded hormones are safer, "natural," or able to be precisely matched to your levels by saliva tests. ACOG does not recommend hormone-level testing to dose compounded therapy.

Pellets deserve special caution because they cannot be removed once placed and can produce high hormone levels; see Biote vs. compounded hormones.

What about estriol and testosterone?

Two bioidentical hormones show up mostly in compounded prescriptions. Estriol is a weaker estrogen that is not in any FDA-approved US product, though vaginal estriol is used in other countries for dryness and recurrent urinary infections; "biest" formulas mix it with estradiol. It is not risk-free just because it is weaker, as our guide to bioidentical estriol explains. Testosterone has no FDA-approved product for women in the US. International consensus supports it only for low sexual desire causing distress after menopause, at doses that keep blood levels in the normal female range; see low testosterone in women.

Who should avoid bioidentical hormones?

How to choose between your options

Step by step

Choosing between your options

1. 1Start with the goal.Hot flashes and night sweats need systemic estrogen. Dryness and pain with sex alone often need only low-dose vaginal estrogen.
2. 2Ask about FDA-approved bioidentical options first.Transdermal estradiol plus oral micronized progesterone is a common, well-studied pairing, and generics are often covered by insurance.
3. 3If you prefer compounded,Ask why it suits you better, how the pharmacy tests potency, how the uterine lining will be protected, and how follow-up works.
4. 4Plan follow-upAround symptoms and side effects, usually after about 3 months and then at least yearly, rather than around hormone tests.

Amie's prescription, Amie HRT+, is a compounded cream that combines bioidentical estradiol and progesterone, with the strength set by a licensed clinician after an online review. It is not FDA-approved. The clinician decides whether it is appropriate, and FDA-approved products from your own doctor remain a strong alternative.

The bottom line