S-40503
S-40503 is an experimental selective androgen receptor modulator developed as a candidate for osteoporosis, characterised in rodents by a strong bone-anabolic effect with comparatively little effect on the prostate. It has no human clinical trials; all data are preclinical, and its presence in the grey-market 'research chemical' supply is driven by anecdote rather than evidence.
01 Overview
S-40503 emerged from Japanese pharmaceutical research (Kaken) in the mid-2000s as a bone-selective SARM. In ovariectomised and orchidectomised rats it increased bone mineral density and cortical bone strength at doses that produced only modest prostate growth, giving it a favourable tissue-selectivity profile on paper.
Despite occasional appearance in the underground SARM market, S-40503 never entered human clinical development, and there are no published human pharmacokinetic, efficacy, or safety data. Any dosing seen in circulation is extrapolated from animal work or copied from other SARMs, and should be treated as unvalidated.
02 Mechanism
Binds the androgen receptor as a partial/selective agonist, producing anabolic signalling preferentially in bone and muscle tissue while exerting weaker agonism in the prostate and other reproductive tissues in animal models.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Anecdotal | 10–30 mg/day | Oral | No human data; range reflects grey-market anecdote only, not any validated protocol. |
Ester comparison
| Ester | Half-life | Injection freq. | Testosterone by mass |
|---|
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Increased bone mineral densityIncreased cortical bone density and mechanical strength in ovariectomised and orchidectomised rats. | n/a (rodent) | Preclinical | |
| Lean mass gainUsers report modest lean tissue gains, unverified by any controlled study. | Anecdotal |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| HPTA suppressionSuppression of endogenous testosterone is plausible for any AR agonist but has not been quantified in humans. | Moderate | Reported | Anecdotal | |
| Unknown long-term toxicityNo human toxicology exists; organ and cardiovascular effects are entirely uncharacterised. | Moderate | Uncharacterised | Unconfirmed |