Estriol
The weakest of the three principal human estrogens, dominant in pregnancy. Used mainly as low-dose vaginal therapy for urogenital atrophy, where its short action and low potency give a favourable safety profile with minimal systemic effect.
01 Overview
Estriol is a short-acting, low-potency estrogen produced in large amounts by the placenta during pregnancy. Therapeutically it is used chiefly as a vaginal cream or pessary for genitourinary syndrome of menopause (vaginal dryness, atrophy, recurrent UTIs), where local delivery restores vaginal tissue with negligible systemic absorption.
Because it binds estrogen receptors briefly and weakly, systemic and endometrial stimulation is minimal at vaginal doses, so unopposed use is generally considered low-risk for the endometrium. Oral estriol is used in some countries for menopausal symptoms.
02 Mechanism
Weak, short-acting estrogen-receptor agonist; brief receptor occupancy limits systemic estrogenic and endometrial effects while restoring local urogenital tissue.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Vaginal atrophy (intravaginal) | 0.03–0.5 mg/day | Rectal | 0.5 mg cream/pessary intravaginally, tapering to 2x weekly; ultra-low-dose 0.03 mg options exist. |
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Relief of vaginal atrophyRestores vaginal epithelium, reducing dryness, dyspareunia and recurrent UTIs. | Clinical | ||
| Low systemic exposureLocal vaginal use produces minimal systemic estrogen levels. | Clinical |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| Local irritationTransient vaginal irritation, itching or discharge when starting. | Mild | Common early | Clinical | |
| Endometrial stimulation (high oral dose)Higher systemic doses could theoretically stimulate endometrium. | Mild | Rare at vaginal doses | Observational |