PharmaceuticalSERMAnti-estrogen

Lasofoxifene

Also known as Fablyn · Oporia · CP-336156

Written by enwyReviewed 15 Jul 20261 sources0 comments

A potent third-generation SERM studied for osteoporosis and, more recently, ER-positive metastatic breast cancer with ESR1 mutations. It has high oral bioavailability and strong bone-agonist, breast-antagonist activity.

01 Overview

Lasofoxifene (Fablyn/Oporia) is a tetrahydronaphthalene SERM with notably high oral bioavailability. It was studied for postmenopausal osteoporosis, where it reduced vertebral and non-vertebral fractures, and has been revived in trials for ESR1-mutant ER-positive breast cancer.

It is not a common bodybuilding ancillary but appears in anti-estrogen discussions because of its high potency and favourable bone profile relative to aromatase inhibitors.

02 Mechanism

High-affinity SERM: agonist at bone and lipid oestrogen receptors, antagonist in breast and uterus; active against certain constitutively active ESR1 mutants.

03 Dosing

Tier	Dose	Route	Notes
Osteoporosis	0.5 mg/day	Oral	Dose studied in the PEARL fracture trial.

04 Effects

Effect	Magnitude	Evidence
Fracture risk reductionBone-agonist activity lowered fracture rates in large trials.	Reduced vertebral & non-vertebral fractures	Clinical
Breast cancer risk reductionAntagonist activity in breast tissue in the PEARL population.	Fewer ER-positive breast cancers	Clinical

05 Side effects

Effect	Severity	Frequency	Evidence	Countermeasures
Leg crampsCommon SERM-class muscular symptom.	Mild	Common	Clinical
Venous thromboembolismSERM class thrombotic risk.	Severe	Rare	Clinical

06 Legal status

Historically approved (Fablyn)

as of Jan 2026

United States

Not currently marketed

Approved in EU historically; investigational in the US for ESR1-mutant breast cancer. · as of Jan 2026

07 References

Lasofoxifene in postmenopausal women with osteoporosis (PEARL)N Engl J Med, 2010

08 Discussion0 comments

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This page is reference information, not medical advice. Doses and protocols are documented as they appear in the clinical literature and in practice — describing them is not a recommendation to use them. Countermeasures listed here are not a substitute for a physician. Legal status varies by jurisdiction and changes.