Formulations & Routes

The WHI tested one molecule delivered one way: conjugated equine estrogens plus medroxyprogesterone acetate, swallowed as a pill. Modern hormone therapy is a menu — estradiol or conjugated estrogens, micronized progesterone or synthetic progestins, oral, patch, gel, or vaginal — and the route is not a preference, it is biology. A pill that passes through the liver meets a different body than a patch that does not. This page maps the differences that have evidence behind them, and is explicit about the comparisons no trial has made.

Why Route Is Biology

Swallowed estrogen is absorbed into the portal vein and delivered straight to the liver before it ever reaches the rest of the body. The liver treats it as a signal and responds by cranking up production of specific proteins: clotting factors, sex-hormone-binding globulin, C-reactive protein, and triglycerides. Most of the dose is metabolized on that first pass — bioavailability runs around a few percent — so to get a therapeutic blood level through the gut you must dose high enough that the liver sees a large load. A transdermal patch, gel, or spray instead diffuses estradiol through the skin into the systemic circulation, bypassing the portal vein, and the liver sees roughly what every other tissue sees. The two routes deliver the same hormone to the body through two different chemistries — which is why "the estrogen" is not one drug but several, and why the WHI's oral-pill findings do not automatically transfer to a patch.

The Molecules: What the Options Actually Are

• 🧬 Estradiol (17β-estradiol): the estrogen a woman's ovaries made before menopause. Available orally, transdermally, and vaginally. The body-identical option.
• 🐴 Conjugated equine estrogens (CEE): a mixture of estrogens purified from pregnant mares' urine, including forms the human body does not make. The WHI formulation; still prescribed, and the subject of most of the long-term safety data on this topic.
• 🌸 Micronized progesterone: oral progesterone chemically identical to the ovarian hormone. The body-identical progestogen, with a neutral-to-favorable metabolic profile in the available studies.
• ⚗️ Synthetic progestins (e.g., medroxyprogesterone acetate): progesterone-like molecules with additional activities, including the WHI's medroxyprogesterone acetate. Effective at protecting the uterine lining; the subject of the breast-risk debate on the arms page.

Clots: The Sharpest Route Difference

The one risk where route matters most is venous thromboembolism. Because oral estrogen drives the liver's clotting-protein production, oral users form clots more readily — and the observational record agrees. The French ESTHER case-control study found venous thromboembolism risk roughly three and a half times higher in oral estrogen users than in nonusers, while transdermal users showed no measurable increase (Canonico et al., Circulation, 2007). A pooled analysis of the world's observational data landed in the same place: roughly a 2.5-fold increase for oral estrogens, and a small, statistically unclear association for transdermal (Canonico et al., BMJ, 2008). The chart below is the honest rendering of that contrast — and of its limits, since observational route comparisons can be confounded by who gets prescribed what.

Stroke, Heart, and Metabolism: The Softer Differences

Beyond clots, the route differences soften. A nested case-control analysis using the United Kingdom's primary care records found stroke risk increased with oral estrogen but not with low-dose transdermal estradiol (Renoux et al., BMJ, 2010) — a signal that fits the clotting story, but observational, and the WHI's stroke increases occurred with an oral regimen, so the transdermal case rests on that one pattern plus mechanism. On metabolic markers, the differences are consistent but of unclear clinical consequence: oral estrogen raises triglycerides, CRP, and SHBG, while transdermal estradiol leaves them roughly where they were — the same first-pass story, visible in bloodwork. What no trial has shown is whether any of this changes heart attacks or strokes in practice. The defensible summary: route plausibly matters most for women with existing clotting risk, and the data's confidence runs in that order — clots first, stroke second, everything else inference.

The Progesterone Question

The progestogen choice is the second formulation axis, and it matters for two outcomes. First, breast cancer: in the French E3N cohort of over 80,000 postmenopausal women, estrogen combined with micronized progesterone showed little or no breast cancer excess, while estrogen combined with synthetic progestins did (Fournier et al., Breast Cancer Research and Treatment, 2008). The finding matches the WHI arm difference and the pooled 2019 Collaborative Group review, but it is observational — women prescribed different progestogens differ in other ways too, and no randomized trial has settled the question. Second, cardiovascular markers: in the PEPI trial, medroxyprogesterone acetate blunted the HDL-cholesterol benefit of estrogen, while micronized progesterone did not (PEPI Writing Group, JAMA, 1995). The pattern is coherent: the body-identical progesterone has the friendlier profile in the available data, with the standing caveat that the head-to-head event trials do not exist.

The Vaginal Route: Local Therapy, Different Rules

Genitourinary syndrome of menopause — vaginal dryness, irritation, painful sex, recurrent urinary tract infections — is a localized estrogen-deficiency problem, and its first-line hormonal treatment is local. Ultra-low-dose estradiol as a tablet, cream, or ring sits in the vagina and treats the tissue it touches, with systemic absorption so small that blood estradiol levels stay in the postmenopausal range. The practical consequences are exactly what the biology predicts: no measurable effect on clotting or breast tissue signals in the available data, and — because the systemic dose is minimal — no routine need to add a progestogen for uterine protection, a decision each woman's clinician makes based on her history. The evidence for symptom relief is strong; the long-term endometrial safety data, while reassuring to date, is thinner than for systemic therapy, which is why duration and follow-up are conversations, not defaults.

The Forms at a Glance

Read the table as a map of questions, not answers: it says which comparisons have evidence and which do not. The choice among forms is downstream of the individual weighing on the decision framework page — and the person who maps a specific history onto these columns is a clinician.

Practical Questions to Bring Up

• 🧭 Which route, given my history? If clotting risk is the concern, the transdermal question is worth raising directly; a clinician weighs it against everything else in the file.
• 🌿 Which progestogen, if I need one? The micronized-progesterone question follows from the breast and metabolic data — with the honest caveat that the evidence is observational.
• 🌸 Is the problem local? For dryness, irritation, or recurrent UTIs with no other symptoms, vaginal low-dose estrogen is the evidence-backed first stop, and the systemic-risk conversation mostly does not apply.
• 🏷️ Is the product regulated? Unregulated compounded hormone products vary in dose and purity; if the pharmacy cannot show regulation and testing, that is a reason for caution, not a reason for a lower price.

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