Bioidentical vs. Synthetic Hormones: A Side-by-Side Comparison

Bioidentical vs. synthetic hormones: what's the actual difference?

The difference between bioidentical and synthetic hormones is the shape of the molecule. A bioidentical hormone is built exactly like one your body makes. A synthetic hormone is built differently, even if it acts on the same receptors. Both kinds are manufactured in a lab, and both kinds can be FDA-approved.

If you want the background on what “bioidentical” means and how compounded products are regulated, start with our guide to bioidentical hormones. This article puts the specific molecules side by side and looks at what the studies show when they are compared.

Which hormones are bioidentical and which are synthetic?

Conjugated estrogens are made from pregnant mares' urine and consist mainly of estrone sulfate and equilin sulfate, along with other estrogens (Premarin label). They are often called synthetic, though strictly they are “non-human” rather than made from scratch. Either way, they are not bioidentical. If you take Premarin and are weighing a switch, see Premarin alternatives.

Why this comparison matters: the WHI trials

Most of what people fear about hormone therapy comes from the Women's Health Initiative (WHI), which tested two synthetic regimens: conjugated equine estrogens (CEE) alone in women without a uterus, and CEE plus medroxyprogesterone acetate (MPA) in women with one. The combined arm was stopped early in 2002. After about 5 years it showed a higher risk of breast cancer (hazard ratio 1.26), blood clots in the lungs, stroke and heart events, alongside fewer fractures and colorectal cancers (Rossouw 2002).

The estrogen-alone arm told a different story. In long-term follow-up, CEE alone was linked to lower breast cancer incidence than placebo (HR 0.78), while CEE plus MPA was linked to higher incidence (Chlebowski 2020). That contrast is one reason researchers have focused on the progestogen. The WHI did not test estradiol or micronized progesterone, so the trials can't tell us directly how bioidentical regimens compare.

What does the evidence say about blood clots?

Here the comparisons are the clearest, though they come from observational studies rather than randomized trials:

• Estradiol vs. CEE by mouth. In UK records of more than 80,000 women with clots, oral estradiol carried lower risk than CEE: about 15% lower for estrogen-only products and 17% lower for combined products. CEE plus MPA had the highest risk of any regimen, about twice that of non-users (Vinogradova 2019).
• A US health-plan study found that oral CEE users had about twice the odds of venous thrombosis compared with oral estradiol users (odds ratio 2.08) (Smith 2014).
• Route matters even more. Estrogen through the skin, which is always estradiol, was not linked to higher clot risk in the UK data (Vinogradova 2019). In the French ESTHER study, oral estrogen was linked to about four times the odds of a clot, while transdermal estrogen was not; micronized progesterone was not linked to clots, but one group of synthetic progestins (norpregnane derivatives) was (Canonico 2007).

By the numbers

Estrogen type, route and clot risk

~15%lower clot risk with oral estradiol than CEE (estrogen-only products)Source: Vinogradova 2019

2.08odds ratio for venous thrombosis, oral CEE vs. oral estradiolSource: Smith 2014

About 4×odds of a clot with oral estrogen; no rise with transdermalSource: Canonico 2007

What does the evidence say about breast cancer?

• Progesterone vs. progestins. In the French E3N cohort of 80,377 women, estrogen with micronized progesterone had a relative risk of 1.00 (no increase), while estrogen with other synthetic progestins had 1.69 (Fournier 2008).
• Duration still matters. An international expert review concluded that estrogen with oral micronized progesterone does not raise breast cancer risk for up to 5 years, but that limited evidence suggests some increase with longer use, and that counseling on combined therapy should cover breast cancer risk whichever progestogen is used (Stute 2018).
• Every systemic type carries some risk. A worldwide analysis of more than 100,000 women with breast cancer found that every type of systemic hormone therapy, but not vaginal estrogen, was linked to some excess risk, higher for estrogen plus progestogen than estrogen alone and rising with longer use (Collaborative Group 2019).

By the numbers

Breast cancer risk by progestogen (E3N cohort)

1.00relative risk with estrogen plus micronized progesteroneSource: Fournier 2008

1.69relative risk with estrogen plus other synthetic progestinsSource: Fournier 2008

Put simply, a bioidentical regimen may be the gentler choice on breast and clot risk, but it is not risk-free. The Menopause Society notes that risks depend on type, dose, duration, route, timing and whether a progestogen is used (Menopause Society 2022).

Bioidentical vs. synthetic: head-to-head summary

What about bioidentical vs. synthetic testosterone?

For women, testosterone therapy almost always uses the bioidentical molecule, given as a cream or gel, often a product made for men at a fraction of the dose or a compounded preparation. The main synthetic option, methyltestosterone, is taken by mouth. International guidance says oral testosterone worsens HDL and LDL cholesterol and is not recommended. The same guidance says the only evidence-based reason to use testosterone in women is low sexual desire causing distress (hypoactive sexual desire disorder) after menopause, and its recommendations don't extend to pellets, injections or compounded products (Davis 2019).

There is no FDA-approved testosterone product for women in the US, so any option is off-label or compounded (ACOG 2023). More detail is in our guide to low testosterone in women.

FDA-approved vs. compounded: the other comparison that matters

The bioidentical-vs-synthetic question often gets mixed up with a second one: FDA-approved vs. compounded. They are separate. Bioidentical estradiol and progesterone come in many FDA-approved forms. Compounded bioidentical hormones are made by a pharmacy for one person and are not FDA-approved or reviewed for safety, effectiveness or dose consistency. ACOG advises against routine use of compounded hormones when an FDA-approved option exists, and doesn't recommend saliva or blood tests to dose them (ACOG 2023).

So the comparison with the most evidence behind it is FDA-approved estradiol (often through the skin) plus FDA-approved micronized progesterone. Compounded products may suit some women for practical reasons, such as one cream instead of two products or an allergy to an ingredient, but they have less data, not more. See Biote vs. compounded hormones for how pellets fit in.

Which should you choose?

1. Start with your symptoms and history. Hot flashes and night sweats call for systemic estrogen; vaginal dryness alone often needs only low-dose vaginal estrogen.
2. Ask about route. If clot risk is a concern, ask whether a patch, gel or spray is right for you.
3. If you have a uterus, ask about the progestogen. Micronized progesterone is one well-studied choice; your clinician may recommend another for good reasons.
4. If you are on a synthetic regimen that suits you, there is no need to switch in a hurry. Talk it through at your next review.

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 fits you, and FDA-approved bioidentical products from your own doctor are a strong alternative.

The bottom line