Endoxifen

guinea pig

Also seen as: Zonalta, Z-endoxifen, 4-hydroxy-N-desmethyltamoxifen, N-desmethyl-4-hydroxytamoxifen

The Dose Guy’s verdict

Sep 1, 2026
3 sources

Endoxifen is the most potent metabolite of tamoxifen, and the CYP2D6 enzyme that makes it is the reason some people suspect tamoxifen works better for them than others, though the largest prospective study found no link between endoxifen levels and outcomes. As a standalone compound it is in pharma development for breast cancer and bipolar disorder, with genuine phase 1 and phase 3 data behind it. But nobody in the physique community runs standalone endoxifen for PCT or gyno, and there is nothing you could buy and trust. This page is for understanding, not for a protocol.

Would I take it?Caution, research compound, not a tool yet.

What it is

When you take tamoxifen, your liver converts it through several steps into endoxifen, the metabolite that does most of the heavy lifting at the estrogen receptor. Endoxifen targets ER-alpha for degradation, not just blockade, which makes it substantially more potent than tamoxifen itself. The conversion depends on CYP2D6, and about 7 to 10% of people are poor metabolizers who produce very little endoxifen. That genetic gap is widely believed to explain poor tamoxifen response, though the prospective CYPTAM study found no association between endoxifen levels and relapse-free survival. The clinical question is unresolved, which is part of why endoxifen has been developed as a direct drug.

In pharma, endoxifen is being studied as a breast cancer treatment for patients who do not respond to tamoxifen, as a low-dose breast cancer prevention agent, and as an anti-manic drug in bipolar disorder through its protein kinase C inhibition. On the physique shelf it sits as context, the molecule that explains why PCT with tamoxifen works or fails, rather than a tool you would run directly.

What the research shows

The data is real but belongs to oncology and psychiatry, not to the physique or PCT space:

In 41 patients with endocrine-refractory metastatic breast cancer, oral Z-endoxifen produced a 26.3% clinical benefit rate, with 3 partial responses and 7 cases of stable disease lasting more than 6 months. CYP2D6 genotype did not affect clearance.JCO 2017 · phase 1 · n=41

In a phase 3 bipolar I trial, endoxifen 8 mg daily reduced Young Mania Rating Scale scores from 33.1 to 17.8 by day 21.Bipolar Disord 2021 · phase 3 · n=116

In 240 healthy premenopausal women, oral Z-endoxifen at 1 mg daily reduced mammographic breast density by 19.3% versus placebo, and 2 mg daily reduced it by 26.5%. At 2 mg, adverse-event discontinuations were 11 versus 4 on placebo, and vasomotor symptoms were significantly higher.JNCI 2026 · RCT · n=240 · 6 months

No trial has tested standalone endoxifen for PCT, hypogonadism, or gynecomastia. The compound’s relevance to this shelf is as the active metabolite that explains tamoxifen’s action, not as a direct tool.as of Aug 2026

Realistic protocols

There is no physique protocol for standalone endoxifen because there is no accessible product and no reason to use it over tamoxifen for PCT or gyno. What endoxifen tells you that matters: if tamoxifen is not working for you, the most likely explanation is that your CYP2D6 is not converting it efficiently. Get your CYP2D6 genotyped, or switch to enclomiphene, which does not depend on the same metabolic pathway. If you are on an SSRI that blocks CYP2D6, like paroxetine or fluoxetine, you are effectively taking a partial tamoxifen dose. Either switch the antidepressant or switch the SERM.

Side effects

Endoxifen’s side profile is known mostly from oncology trials at higher doses, with some data at 1 to 8 mg:

commonHot flashes and vasomotor symptoms at clinical doses. Dose-dependent, similar to tamoxifen.

monitorA dose-limiting pulmonary embolus occurred at 60 mg daily in the breast cancer phase 1. The same class-wide clot signal as tamoxifen.

unknownNo safety data exists for endoxifen in men for hormonal or physique endpoints. The compound’s risk profile in this population is entirely unmapped.

Where to buy

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