What Are SARMs? RAD-140, Ligandrol, S-23, S-4 and Cardarine in Research

Published 2026-08-22 · Updated 2026-09-12 · Waifu Labs Research Library

SARMs — selective androgen receptor modulators — are among the most searched compounds in the research-chemical space, and among the most misunderstood. This primer covers what a SARM actually is, how tissue selectivity is studied, the five compounds that dominate the literature, how they differ from classical anabolic steroids, and what purity standards apply when sourcing them for laboratory use.

Definition: selective androgen receptor modulators

SARMs are small molecules designed to bind androgen receptors with tissue-selective activity. The research hypothesis behind them is the “tissue selectivity” profile: in animal and in-vitro models, a well-designed SARM is intended to show anabolic-type activity in skeletal muscle and bone tissue while minimizing prostate-type activity. That selectivity is exactly what preclinical studies measure — it is the entire point of the molecule class.

How tissue selectivity is studied

In research settings, selectivity is assessed by comparing receptor-binding affinity across tissue types and measuring downstream gene-expression changes in muscle, bone and prostate models. Ligand chemistry matters enormously: small structural differences between compounds produce meaningfully different binding profiles, which is why the literature treats each SARM as a distinct research object rather than interchangeable “SARMs”. Purity directly affects those experiments — an under-purified sample contaminates every downstream measurement.

The most-studied research compounds

• RAD-140 (Testolone) — a non-steroidal androgen receptor ligand, one of the most published SARMs in muscle- and bone-selectivity studies.
• LGD-4033 (Ligandrol) — heavily studied non-steroidal selective androgen receptor modulator appearing throughout the preclinical literature.
• S-23 — a research SARM studied for its high binding affinity and its role in tissue-selectivity and contraceptive-research models.
• S-4 (Andarine) — an early-generation SARM widely used in selectivity and ligand-binding research.
• GW-501516 (Cardarine) — not technically a SARM: it is a PPARδ (peroxisome proliferator-activated receptor delta) agonist studied for metabolic and endurance-related pathways. It is catalogued alongside SARMs because researchers commonly stack the categories.

SARMs vs anabolic steroids in research

Classical anabolic steroids (testosterone esters, boldenone, trenbolone) are steroidal molecules that activate androgen receptors broadly across tissues. SARMs are non-steroidal ligands engineered for selective receptor activity. The research distinction is mechanistic, not cosmetic: steroidal compounds cannot be made tissue-selective, while the entire SARM research program exists to engineer selectivity in. Both categories require the same sourcing discipline — verified identity, verified purity, and sterile handling in the lab.

Purity and verification standards for SARMs

Because SARMs are synthesized multi-step small molecules, synthesis by-products and stereoisomers are the main contamination risks. Third-party HPLC at 99%+ with mass spectrometry confirmation is the working standard: HPLC quantifies impurity peaks, MS confirms the molecular weight matches the target compound. Suppliers who cannot produce batch-specific certificates are, functionally, selling unlabeled powders.

Where Waifu Labs fits in

Waifu Labs specializes in the research categories that pair with SARM studies: US-domestic healing and recovery peptides (BPC-157, TB-500, blends), GLP metabolic research compounds (GLP3/Retatrutide, Tirzepatide), US-domestic testosterone oils and ancillaries, and a recovery-support range. Our guides cover those lines in depth — see the testosterone esters and PCT primer below for the steroidal side of the research spectrum.

Frequently asked questions