A tier · strong
A long-approved estrogen with a defined FDA label, characterised depot pharmacokinetics and a large class-level outcome literature, held below the top tier because the modern controlled evidence belongs to the estrogen class rather than to this ester specifically and carries a boxed warning.
estradiol cypionate
Estradiol cypionate is a long-acting injectable ester of bioidentical 17-beta-estradiol, approved for vasomotor menopausal symptoms and hypoestrogenism due to hypogonadism. Its efficacy for those indications is established, but it carries the full estrogen class boxed warning and most of the hard outcome data comes from oral estrogen trials rather than from this ester.
the explanation
This is a slow-release injectable form of the body's main oestrogen, approved for hot flushes at menopause and for people whose ovaries do not produce enough oestrogen. It works, but it carries serious warnings about cancer, stroke and clot risk, and most of the long-term risk data comes from studies of other oestrogen products.
regulatory status
Approved (prescription)
FDA-approved as Depo-Estradiol, an oil-based intramuscular injection, for moderate-to-severe vasomotor symptoms associated with the menopause and for hypoestrogenism due to hypogonadism. The label carries a boxed warning covering endometrial cancer, cardiovascular disorders, breast cancer and probable dementia, and states that estrogens should not be used for the prevention of cardiovascular disease.
how it works · proposed mechanism
A pro-drug ester whose released estradiol acts through the canonical estrogen receptor system.
Ester depot and hydrolysis
The cyclopentylpropionate ester at C17-beta increases lipophilicity so the drug remains in an intramuscular oil depot and diffuses out gradually. Tissue and plasma esterases cleave the ester to release identical endogenous 17-beta-estradiol.
Nuclear estrogen receptor signalling
Released estradiol binds ERalpha and ERbeta, which dimerise and act at estrogen response elements to regulate transcription in endometrium, breast, bone, vasculature and brain. Rapid non-genomic signalling also occurs through membrane-associated GPER.
Hypothalamic-pituitary gonadotropin feedback suppression
Sustained estradiol exposure exerts negative feedback on GnRH pulsatility and pituitary LH and FSH secretion. In a 2025 retrospective cohort of transgender and nonbinary adults on injectable estradiol, this suppressed testosterone below 50 ng/dL in 100% of patients.
what’s reported
evidence shape
5 sourcesThe composition of the cited literature by study type. This describes the shape of the evidence, not its quality — and it does not by itself set the grade.
⚠ the catch
The approval and the boxed warning both sit on class-level estrogen data, and the pivotal outcome trials behind that warning studied oral conjugated equine estrogens, not this injectable ester. That means the risk estimates on the label are the best available but are an imperfect fit for a parenteral bioidentical formulation, and the gap has never been closed by a dedicated randomised outcome trial.
key published findings
- Official: the FDA label approves Depo-Estradiol for moderate-to-severe vasomotor symptoms associated with the menopause and for hypoestrogenism due to hypogonadism, and states that estrogens with or without progestins should not be used for prevention of cardiovascular disease or dementia.
- Official (pharmacokinetics): the label reports absorption over several weeks after intramuscular injection, with average estrogenic effect measured by vaginal smear lasting approximately 3 to 4 weeks and relief of vasomotor symptoms typically maintained for around five weeks.
- Official (safety, class-level): the boxed warning cites the Women's Health Initiative finding of increased risks of myocardial infarction, stroke, invasive breast cancer, pulmonary embolism and deep vein thrombosis in postmenopausal women during five years of combined estrogen-plus-progestin therapy.
- Human observational: a 2025 retrospective cohort in the Journal of the Endocrine Society (n=29 treatment-naive transgender and nonbinary adults assigned male at birth, 28 on estradiol valerate and 1 on cypionate) found 100% achieved testosterone below 50 ng/dL, with mean on-treatment estradiol of 347 pg/mL initially and 248 pg/mL at final measurement; concurrent spironolactone was associated with lower estradiol (285 vs 427 pg/dL, P=0.017) without additional testosterone suppression.
- Human observational: an Endocrine Practice (2024) scoping review of injectable estradiol in transgender and gender-diverse adults catalogued the published dose and serum-level literature and found it dominated by small retrospective cohorts, with estradiol valerate far better represented than cypionate.
limitations of the evidence
- No modern randomised controlled trial has evaluated estradiol cypionate specifically against placebo or an active comparator for symptom or outcome endpoints; approval rests on decades-old data and class evidence.
- The gender-affirming care literature, which accounts for much contemporary use, consists almost entirely of small retrospective cohorts with heterogeneous regimens and no randomised comparisons.
- Serum estradiol after depot injection varies widely between individuals with body composition and injection technique, complicating comparison across published cohorts.
documented safety signals
- Boxed warning for endometrial carcinoma risk with unopposed estrogen in people with a uterus.
- Boxed warning for cardiovascular disorders including myocardial infarction, stroke, deep vein thrombosis and pulmonary embolism, and for probable dementia in older postmenopausal women.
- Boxed warning for invasive breast cancer risk with combined estrogen-progestin therapy.
- Contraindicated in undiagnosed abnormal genital bleeding, known or suspected breast cancer, estrogen-dependent neoplasia, active or recent arterial thromboembolic disease, active DVT/PE, hepatic impairment, known hypersensitivity, and known or suspected pregnancy.
- The depot cannot be withdrawn once administered, so adverse effects persist for the duration of the release profile.
- The formulation is an oil-based injection containing cottonseed oil and chlorobutanol, relevant to hypersensitivity.
identity
| full name | Estradiol 17-beta-cyclopentylpropionate |
| category | Hormonal |
| modality | steroid |
| formula | C26H36O3 |
| molar mass | 396.6 g/mol |
| cas | 313-06-4 |
| half-life | Approximately 8-10 days after intramuscular injection of an aqueous suspension; the oil-based formulation produces an estrogenic effect lasting roughly 3-4 weeks by vaginal smear. |
laboratory handling
| storage | Store at controlled room temperature, 20-25 C, per the approved label; the product is an oil solution and should be protected from freezing and from light in its carton. |
| solubility | Practically insoluble in water and freely soluble in oils and organic solvents; the cypionate ester is markedly more lipophilic than unesterified estradiol, which is what allows the cottonseed-oil depot to release drug over weeks. |
| co-studied with | In people with an intact uterus, the label's endometrial cancer warning is the basis for concurrent progestogen use in clinical practice; in gender-affirming regimens, published cohorts commonly report concurrent antiandrogens, and the 2025 cohort found spironolactone co-use associated with lower serum estradiol without added testosterone suppression. |
Handling information describes laboratory practice for a research reagent. It is not a preparation guide for use in humans.
the receipts
5 citedPfizer / U.S. Food and Drug Administration · 2021 · official
U.S. Food and Drug Administration, Drugs@FDA · 2005 · official
Journal of the Endocrine Society · 2025 · observational
Endocrine Practice · 2024 · reviews
Wikipedia (secondary compilation of primary PK literature) · 2026 · reviews
others in Hormonal