By Menopause Reviewed Editorial Team | Last reviewed: May 2026
In the summer of 2002, the Women's Health Initiative published a paper in JAMA that effectively ended two decades of routine hormone prescribing for menopausal women. The study, which had randomized more than 16,000 women to combination estrogen-progestin therapy or placebo, was stopped early. The data and safety monitoring board determined that the hazard ratio for invasive breast cancer had crossed a pre-specified boundary: 1.26, with a confidence interval that just touched statistical significance.
News coverage was immediate and sweeping. Major medical organizations revised their guidance. HRT prescriptions fell by more than 50 percent within three years. Millions of women stopped therapy without clinical guidance. Many spent the following decade in unnecessary suffering, told that their hot flashes, insomnia, and vaginal atrophy were an acceptable price for reduced cancer risk.
What is now clear, more than 20 years later, is that the study's findings were more complicated than the headlines suggested, that the specific combination and route of hormones used in WHI were not representative of modern prescribing practice, and that the wholesale abandonment of hormone therapy caused its own category of harm. This article examines what the WHI actually found, what the evidence looks like in 2026, and where the compounded "bioidentical" hormone industry fits into that picture.
What "Bioidentical" Actually Means
The word "bioidentical" is used in two distinct ways that are frequently conflated, sometimes deliberately.
In its pharmacological sense, "bioidentical" means that a hormone molecule is structurally identical to the hormone produced by the human body. By this definition, several FDA-approved, commercially manufactured hormone therapies are bioidentical. Estradiol, the primary form of estrogen manufactured by the ovaries, is the active ingredient in transdermal patches (Vivelle-Dot, Climara), gels (EstroGel, Divigel), sprays (Evamist), vaginal rings (Femring), and oral formulations (Estrace). Micronized progesterone (Prometrium) is chemically identical to the progesterone produced by the corpus luteum.
In its marketing sense, "bioidentical" typically refers to custom-compounded hormone preparations prepared by a compounding pharmacy, often marketed as "natural" or "tailored" alternatives to conventional therapy. These compounded bioidentical hormones (cBHT) frequently include combinations of estrogens (estradiol, estriol, estrone), testosterone, DHEA, and pregnenolone in individualized doses, sometimes based on salivary hormone testing. They are not FDA-approved. They have not been evaluated in randomized controlled trials for safety or efficacy as formulations. And they are not regulated under the same manufacturing quality standards as pharmaceutical drugs.
The conflation of these two categories is one of the central problems in public discourse about hormone therapy. FDA-approved bioidentical estradiol and micronized progesterone are not the same thing as custom-compounded cBHT. The former has decades of safety and efficacy data. The latter does not.
The WHI Study: What It Actually Found, and What It Didn't
The Women's Health Initiative comprised two parallel trials: one combining conjugated equine estrogen (CEE) plus medroxyprogesterone acetate (MPA) in women with a uterus, and one using CEE alone in women who had undergone hysterectomy.
The combination arm was stopped in 2002. The hazard ratio for breast cancer was 1.26 (95% CI 1.00-1.59). This means that, compared with placebo, the risk of developing invasive breast cancer was nominally 26 percent higher. In absolute terms, this translated to roughly 8 additional breast cancers per 10,000 women per year of use.
Several critical caveats were underreported at the time and are now central to interpreting the findings.
Age and timing. The mean age of WHI participants was 63, with a range of 50-79. Most had been menopausal for more than a decade when they enrolled. Only 3.5 percent were aged 50-54 at baseline. The trial therefore tested hormones in older postmenopausal women and women with symptomatic conditions, not in newly perimenopausal women seeking symptom relief. The "timing hypothesis," subsequently articulated by JoAnn Manson and colleagues and supported by the Kronos Early Estrogen Prevention Study (KEEPS), holds that hormone therapy initiated within ten years of menopause onset (or before age 60) has a meaningfully different risk-benefit profile than therapy initiated in later postmenopause.
The hormones used. CEE is derived from the urine of pregnant mares and contains multiple estrogen compounds not endogenous to humans. MPA (medroxyprogesterone acetate) is a synthetic progestin, not progesterone. Neither is what would be prescribed to a 47-year-old woman seeking perimenopausal symptom relief in a contemporary clinical setting.
The breast cancer finding itself. Subsequent analyses of the WHI found that CEE alone, in the hysterectomy arm, actually reduced breast cancer incidence by 23 percent and breast cancer mortality by 40 percent. A 2023 critical review in Menopause journal noted that when the CEE+MPA breast cancer finding was adjusted for multiple outcomes and sequential monitoring (as per standard statistical protocol), the association was no longer statistically significant.
The global index. The WHI global index, which included stroke, pulmonary embolism, colorectal cancer, and hip fracture alongside breast cancer and coronary heart disease, was the basis for the "more harm than good" conclusion. That conclusion does not hold when the analysis is restricted to women in the 50-59 age group or those within ten years of menopause onset; in those subgroups, the benefit-risk ratio is favorable.
What the Science Looks Like in 2026
The 2022 Hormone Therapy Position Statement from the North American Menopause Society, published in the journal Menopause, represents the most current comprehensive synthesis of the evidence. Its core conclusions are worth stating plainly.
Hormone therapy remains the most effective treatment for vasomotor symptoms (hot flashes and night sweats) and for genitourinary syndrome of menopause (GSM). For women under 60 or within ten years of menopause onset who have no contraindications, the benefit-risk ratio is favorable for treatment of bothersome vasomotor symptoms and for prevention of bone loss. The risks differ by type of hormone, dose, duration of use, route of administration, timing of initiation, and whether a progestogen is used.
For women who initiate hormone therapy more than ten years after menopause onset or at age 60 or older, the benefit-risk ratio becomes less favorable because of greater absolute risks of coronary heart disease, stroke, venous thromboembolism, and dementia. This does not mean HRT is contraindicated in these women; it means the decision requires more careful individualized risk-benefit assessment.
The NAMS statement also clarifies that there is no mandatory duration limit for hormone therapy in women with persistent symptoms, good tolerability, and an individualized assessment supporting continued use. The previous practice of prescribing HRT for a maximum of five years was not evidence-based and caused unnecessary discontinuation.
Compounded BHRT: Where the Controversy Actually Lives
The market for custom-compounded "bioidentical" hormones has grown substantially over the past two decades, driven in part by the post-2002 collapse of conventional HRT prescribing, in part by direct-to-consumer marketing, and in part by clinicians who concluded that individualization of hormone regimens required compounding. Annual spending on cBHT in the United States now runs into the billions of dollars.
The Endocrine Society's 2019 Scientific Statement on compounded bioidentical hormones concluded that "no evidence supports the popularized notion that custom-compounded bioidentical hormones have fewer risks when compared with FDA-approved hormone treatments." The statement further noted that the widespread availability of FDA-approved bioidentical hormones produced in monitored facilities demonstrates a high quality of safety and efficacy, and therefore "there is no rationale for the routine prescribing of unregulated, untested, and potentially harmful custom-compounded bioidentical HTs."
The specific concerns about cBHT cluster into three categories.
Dosing variability. Compounded hormone preparations are not subject to the same potency and uniformity standards as pharmaceutical manufacturing. Studies examining cBHT pellets in particular have found blood hormone levels substantially outside the intended therapeutic range, including hormone concentrations far above the anticipated level.
Absence of efficacy and safety data. No compounded BHRT formulation has been evaluated in a large, randomized controlled trial. Claims that they are more effective, better tolerated, or safer than FDA-approved alternatives are not supported by evidence.
Salivary hormone testing. Many cBHT prescribers use salivary hormone testing to individualize dosing. Neither NAMS nor the Endocrine Society endorses salivary testing for this purpose; serum levels of sex hormones are the validated clinical standard. Salivary estradiol does not reliably correlate with serum levels or with symptom burden.
This is not an argument that conventional pharmaceutical companies are beyond criticism, or that every woman's hormonal situation is identical. Individualized prescribing is appropriate and important. The argument is narrower: that the claims made for custom-compounded BHRT, specifically that it is safer, more natural, or more effective than FDA-approved options, are not supported by evidence, and the absence of quality control introduces risks that do not exist with regulated products.
Routes and Formulations
Modern hormone therapy is not one product. Route of administration meaningfully affects risk profile.
Transdermal versus oral estrogen. Oral estrogens undergo first-pass hepatic metabolism, which increases the production of clotting factors and C-reactive protein and may modestly increase venous thromboembolism (VTE) risk. Transdermal estrogen bypasses hepatic metabolism; multiple observational studies and the ESTHER study published in Circulation (2007) found that oral but not transdermal estrogen was associated with increased VTE risk (odds ratio 4.2 for oral versus 0.9 for transdermal, compared with non-users). A 2010 meta-analysis in Current Opinion in Obstetrics and Gynecology confirmed that pooled VTE risk ratios were 1.9 for oral and 1.0 for transdermal estrogen users. Transdermal delivery also avoids estrogen-induced increases in triglycerides and sex hormone-binding globulin seen with oral formulations.
Progesterone versus progestins. Micronized progesterone (identical in structure to endogenous progesterone, sold as Prometrium in the US and Utrogestan in the UK) appears to have a more favorable risk profile than synthetic progestins, particularly regarding breast tissue stimulation and thrombotic risk. The ESTHER study found that micronized progesterone and pregnane derivatives did not increase VTE risk, while norpregnane derivatives (a class of synthetic progestins) did. Observational data from the E3N cohort study in France found a lower relative risk of breast cancer with micronized progesterone compared with synthetic progestins when combined with estradiol.
Vaginal estrogen. Low-dose vaginal estrogen (cream, ring, tablet, or suppository) for GSM has minimal systemic absorption. It does not require the addition of a progestogen, does not carry the same risk profile as systemic HRT, and can be used at any age and for extended duration if needed. It is substantially underutilized.
Who Is HRT Contraindicated For
Hormone therapy is not appropriate for all women. Absolute or near-absolute contraindications include:
- Current or recent (within 12 months) breast cancer diagnosis; history of hormone-receptor-positive breast cancer requires specialist guidance.
- Active liver disease or hepatic impairment (for oral or transdermal systemic estrogen; vaginal estrogen may be used with specialist guidance).
- Unexplained or undiagnosed vaginal bleeding.
- Recent myocardial infarction or stroke (within 6-12 months); history of stroke or TIA requires individualized risk assessment.
- Active or history of venous thromboembolism, particularly with oral estrogen (transdermal may be considered with specialist guidance in selected cases).
- Known or suspected estrogen-sensitive malignancies beyond breast cancer (certain endometrial cancers).
- Untreated hypertension (treatment and stabilization before initiating HRT is appropriate).
Relative contraindications, where individual risk-benefit assessment is warranted, include: family history of breast cancer, hypertriglyceridemia, fibroids (symptom-dependent), endometriosis history (requires progestogen co-administration), and migraine with aura (where some evidence suggests elevated stroke risk with estrogen, though data are mixed).
The presence of a contraindication does not mean symptoms cannot be treated; it means the approach to treatment changes. Non-hormonal options with evidence for vasomotor symptoms include paroxetine, venlafaxine, gabapentin, and the recently FDA-approved neurokinin B antagonist fezolinetant (Veozah), which acts directly on the KNDy neurons that drive thermoregulatory dysregulation.
What to Ask Your Provider
If you are considering hormone therapy, these questions will help you have a more productive conversation:
- "Given my age, time since perimenopause onset, and medical history, what does my individual benefit-risk assessment look like?"
- "Would transdermal estrogen and micronized progesterone be appropriate for me, given the VTE and breast cancer risk profile?"
- "What are the non-hormonal options if I choose not to use HRT?"
- "How will we monitor my response and when would you recommend reassessing?"
- "If I have tried compounded hormones, how do we evaluate whether they have been effective and safe?"
A NAMS-certified menopause practitioner can help navigate this assessment. Find one at npp.menopause.org.
Sources
- Rossouw JE, et al. Risks and benefits of estrogen plus progestin in healthy postmenopausal women. JAMA. 2002;288(3):321-333. https://jamanetwork.com/journals/jama/fullarticle/195120
- The 2022 Hormone Therapy Position Statement of The North American Menopause Society. Menopause. 2022;29(7):767-794. https://pubmed.ncbi.nlm.nih.gov/35797481/
- Pinkerton JV, Zion AS. A critical review of the Women's Health Initiative. Menopause. 2023. https://pmc.ncbi.nlm.nih.gov/articles/PMC10758198/
- Canonico M, et al. Hormone therapy and venous thromboembolism among postmenopausal women. Circulation. 2007;115(7):840-845. https://www.ahajournals.org/doi/10.1161/circulationaha.106.642280
- Canonico M. Risk of venous thrombosis with oral versus transdermal estrogen. Current Opinion in Obstetrics and Gynecology. 2010. https://pubmed.ncbi.nlm.nih.gov/20601871/
- Endocrine Society Scientific Statement: Compounded Bioidentical Hormones in Endocrinology Practice. 2019. https://www.endocrine.org/advancing-research/scientific-statements/health-policy/compounded-bioidentical-hormones-in-endocrinology-practice
- Endocrine Society Position Statement: Compounded Bioidentical Hormone Therapy. https://www.endocrine.org/advocacy/position-statements/compounded-bioidentical-hormone-therapy
- Society Position Statements on Bio-Identical Hormones. Healthcare. 2021. https://pmc.ncbi.nlm.nih.gov/articles/PMC8306643/
- Manson JE, et al. Menopausal hormone therapy and long-term all-cause and cause-specific mortality: the Women's Health Initiative randomized trials. JAMA. 2017;318(10):927-938. https://pubmed.ncbi.nlm.nih.gov/28898378/
- NIH MedlinePlus: Hormone Replacement Therapy. https://medlineplus.gov/hormonereplacementtherapy.html
- Mayo Clinic: Hormone therapy: Is it right for you? https://www.mayoclinic.org/diseases-conditions/menopause/in-depth/hormone-therapy/art-20046372
- ACOG Committee Opinion: Postmenopausal Estrogen Therapy Route of Administration and Risk of Venous Thromboembolism. 2013. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2013/04/postmenopausal-estrogen-therapy-route-of-administration-and-risk-of-venous-thromboembolism