S4, also known as Andarine, is a selective androgen receptor modulator (SARM) originally developed for conditions like muscle wasting and osteoporosis. It was never approved for medical use and remains an investigational compound, but it has gained attention in fitness communities for its ability to promote lean muscle retention and bone strength in animal studies. It is not approved by the FDA for any purpose and is banned in competitive sports.
How S4 Works in the Body
SARMs are designed to activate androgen receptors, the same receptors that testosterone binds to, but with a key difference: they’re selective about where they work. Traditional anabolic steroids activate androgen receptors everywhere, including in the prostate, skin, and scalp, which is what drives side effects like prostate enlargement, acne, and hair loss. S4 was engineered to preferentially target receptors in muscle and bone tissue while having a much weaker effect on reproductive organs.
This selectivity has been demonstrated in animal research. In a 2005 study published in the Journal of Pharmacology and Experimental Therapeutics, S4 fully restored muscle mass and strength in castrated rats to levels seen in healthy animals. At the same time, it only partially stimulated the prostate and seminal vesicles to less than 20% of normal levels, and less than 10% of what the potent hormone DHT produced. That gap between muscle effects and prostate effects is the core promise of SARMs as a drug class.
Effects on Muscle and Body Composition
The existing research on S4 comes entirely from animal models. In castrated rats (which lose muscle rapidly due to the absence of testosterone), S4 treatment at moderate doses increased lean body mass by roughly 15 grams over eight weeks, restoring it to levels comparable to healthy, intact animals. It also fully restored the mass and contractile strength of the soleus muscle, a key weight-bearing muscle.
One important distinction: S4 did not produce the same fat-burning effects as DHT in those same studies. DHT treatment reduced fat mass by more than 10 grams, while S4 did not significantly decrease fat mass on its own. Instead, it restored overall body composition to something closer to normal. This suggests S4’s primary role is preserving or building lean tissue rather than directly driving fat loss, though the shift in muscle-to-fat ratio can indirectly change how lean someone looks.
No controlled human clinical trials have been published for S4. The claims you’ll find online about its effects in people are based on anecdotal user reports, not verified data.
Bone Density Research
S4 was originally explored as a potential treatment for osteoporosis. The same animal studies that measured muscle effects also assessed bone outcomes. Alongside its muscle-preserving properties, S4 prevented the bone loss that typically follows testosterone deprivation in animal models. This dual action on both muscle and bone made it an appealing candidate for age-related conditions where both tissues deteriorate simultaneously. However, the compound never advanced far enough in clinical development to produce human bone density data.
Hormone Suppression
Because S4 activates androgen receptors, the body’s hormonal feedback system can interpret it as a signal that enough androgens are circulating. This causes the brain to reduce its signals for natural testosterone production, a process called suppression. The degree of suppression depends on dose and duration.
Human clinical data on S4’s suppressive effects is extremely limited. Anecdotal reports from users suggest that at lower doses (around 25 mg per day for six to eight weeks), suppression tends to be mild, with many users reporting no noticeable symptoms. At higher doses of 50 mg per day or cycles lasting longer than eight weeks, suppression may become more significant, with some users reporting fatigue, reduced libido, and mood changes. These reports, while consistent across online communities, are not backed by controlled studies.
Vision Side Effects
S4 is unusual among SARMs for one distinctive side effect: it can cause temporary vision disturbances. Users commonly describe a yellow tint to their vision, particularly in low-light conditions, and sometimes difficulty adjusting between light and dark environments. This appears to be related to S4 binding to receptors in the eye. The effect is dose-dependent and typically reverses after stopping the compound, but it is one of the most frequently cited reasons users either lower their dose or discontinue use.
Half-Life and Dosing
The half-life of S4 is a point of debate. Early preclinical data suggested a half-life of only about four hours in humans, which led to recommendations to split doses throughout the day. However, some researchers and experienced users have argued that the effective half-life is considerably longer, possibly closer to 36 to 48 hours, based on how long visual side effects and other physiological markers persist after dosing.
In practice, most users split their daily amount into a morning and evening dose. Commonly reported doses range from 25 to 50 mg per day, with many users starting at the lower end and adjusting based on how their body responds, particularly regarding vision changes. These are not medically validated dosing protocols.
Legal and Regulatory Status
S4 is not approved by the FDA for human consumption. It has never completed the clinical trial process required for approval as a pharmaceutical drug. In the United States, SARMs occupy a gray area: they cannot legally be sold as dietary supplements, but they are sometimes marketed as “research chemicals” and sold online. The FDA has issued warning letters to companies selling SARMs with claims about muscle building or fat loss.
For athletes, S4 is explicitly prohibited. The World Anti-Doping Agency (WADA) lists selective androgen receptor modulators, including andarine by name, under its “Other Anabolic Agents” category on the Prohibited List. This ban applies both in and out of competition. Athletes who test positive for S4 face sanctions regardless of whether they intended to gain a performance advantage.
Products sold as S4 also carry a contamination risk. Independent lab testing of SARMs purchased online has repeatedly found products that contain incorrect doses, different compounds than labeled, or additional unlisted ingredients. Without pharmaceutical-grade manufacturing standards, there is no guarantee that what’s on the label matches what’s in the bottle.