S4 (Andarine) research monograph — an early SARM and its dose-limiting visual effect

S4 — Andarine — is one of the earliest SARMs and is most useful to a researcher as a case study
in dose-limiting selectivity: it has a distinctive, well-documented visual side effect that
shaped how the class was understood.

Chemical identity & structure.
S4 (Andarine) is a first-generation nonsteroidal selective androgen receptor modulator (SARM)
developed by GTx as part of the early SARM effort. It is a small molecule structurally related to
the antiandrogen bicalutamide.

Mechanism of action.
Andarine binds the androgen receptor as a partial agonist with muscle/bone selectivity. Its
partial-agonist character is relevant: it can act as a weaker stimulus than full agonists and can
compete at the receptor, which is part of why it did not advance as far as later SARMs.

Key research findings.
Preclinical studies reported preservation of muscle and bone in androgen-deficient animal models.
The most-cited human-relevant observation is a reversible visual disturbance — users and early
testing report a yellowish tint to vision and difficulty adapting to darkness, attributed to the
compound's interaction with retinal tissue. This dose-related effect is the compound's signature
and a key reason it was largely abandoned for development.

The research / citation base.
S4 is not an approved drug, has no completed human clinical-trial program of note, and is
on the WADA Prohibited List. Its evidence base is largely preclinical plus the well-known
visual-side-effect reports.

Research protocols in the literature.
No approved human dosing protocol exists. The visual effect is reported as dose- and
exposure-related, which dominates any discussion of how it was studied. The compound is orally
active and supplied as a research-grade powder or solution.

Stacking considerations.
Community discussion sometimes positions S4 within SARM combinations, but its partial-agonist
behaviour and the visual effect make it an outlier; controlled combination data are absent.

Quality & sourcing notes.
As with all SARMs, a batch-specific COA confirming identity (mass spectrometry) and purity (HPLC)
is essential, and mislabelling is common. The visual effect is a property of the molecule, not a
contamination artefact, and no COA addresses it.

*Research-use note: This monograph is an educational summary of the published research literature.
S4 (Andarine) is not an approved drug and is prohibited in sport; it is described here for research
context only. Nothing here is medical advice or a usage recommendation.*