Andarine (S4) Half-Life: What the Pharmacokinetic Evidence Actually Shows

Key takeaways

What the Animal Pharmacokinetic Data Show

Andarine (S4, chemically S-3-(4-acetylamino-phenoxy)-2-hydroxy-2-methyl-N-(4-nitro-3-trifluoromethyl-phenyl)-propionamide) was developed by researchers at Ohio State University and studied in preclinical models as a nonsteroidal selective androgen receptor modulator. The only controlled pharmacokinetic studies of the compound that have been published were conducted in rats and dogs, not humans.

The primary source for the elimination half-life figure repeated across secondary sources is a 2004 study in Xenobiotica by Kearbey and colleagues, which examined S4 in rats after intravenous and oral dosing. That study reported that the half-life of S-4 over the dose range tested was between 2.6 and 5.3 hours, along with clearances of 1.0 to 2.1 mL/min/kg and a volume of distribution of approximately 0.448 L/kg. The same study found that oral bioavailability was dose-dependent, with the lower doses showing complete oral bioavailability.

A separate 2006 report in Drug Metabolism and Disposition, drawing on the same underlying rat dataset, specified that oral bioavailability was 120%, 100%, and 57% at dose levels of 1, 10, and 30 mg/kg, respectively, indicating that absorption became proportionally less complete as dose increased.

A related study by Perera and colleagues, published in Drug Metabolism and Disposition in 2006, examined S4 pharmacokinetics and metabolism in dogs. That study found that S-4 showed linear pharmacokinetics after both intravenous and oral administration, with a mean clearance of 4.6 mL/min/kg and a mean half-life of about 200 minutes (roughly 3.3 hours), and it noted that dose-dependent oral bioavailability was also observed in this species.

Taken together, the rat and dog data are broadly consistent with one another: both suggest an elimination half-life in the range of roughly 2.6 to 5.3 hours depending on species, dose, and study conditions. This is almost certainly the actual scientific basis for the approximate 4-hour half-life figure that appears across bodybuilding and supplement websites, even though it is often presented without qualification as an established human value.

No Validated Human Pharmacokinetic Data Exist

No SARM candidate aimed at conditions such as androgen deficiency or muscle wasting, including andarine, is known to have completed human clinical trials with published pharmacokinetic results. A 2012 in vitro metabolism study noted that although SARMs have been synthesized and investigated in humans, rats, and dogs, no SARM drug candidate aiming for testosterone replacement therapy had completed clinical trials, even as S4 was already circulating illicitly outside regulated channels.

A separate in vitro study using human, rat, and dog liver enzyme preparations examined the phase I metabolism of S4 and found that its major human metabolic pathways include deacetylation, amide bond hydrolysis, nitro-group reduction, and aromatic ring oxidation, with CYP3A4 implicated as a major contributing enzyme. This work characterizes likely human metabolism at the enzyme level, but it is an in vitro liver-preparation study, not a clinical pharmacokinetic trial in living human subjects, and it does not establish a human plasma half-life.

Because of this gap, any statement of andarine's half-life in humans is an extrapolation from animal data rather than a directly measured value. Species differences in clearance, metabolism, and protein binding mean such extrapolations carry meaningful uncertainty, and the true human elimination half-life has not been confirmed in the literature.

Detection in Doping Control

Anti-doping laboratories do not rely on tracking the parent compound's plasma half-life to detect andarine use. Mass spectrometry-based methods developed for routine doping control purposes have characterized urinary metabolites of andarine, including glucuronide and sulfate conjugates, which can remain detectable in urine well after the parent drug itself has been eliminated from plasma. Andarine has also been identified through mass spectrometric methods in black-market bodybuilding products, independent of pharmacokinetic data in the people who used them.

Because detection windows in doping control depend on assay sensitivity, individual metabolism, dose, and duration of use, the published literature does not specify one universal detection window that applies to all users, and no peer-reviewed source ties a specific number of detection days directly to the animal-derived half-life figures.

A Distinct Visual Side Effect Unrelated to Elimination Kinetics

Andarine is associated in case literature and forensic summaries with a distinctive, dose-related visual disturbance, most often described as a yellowish tinge to vision and difficulty adapting to darkness. This effect appears to reflect off-target interaction with structures in the eye rather than the compound's plasma elimination rate, and it is reported anecdotally and in limited case material rather than in a large, controlled human dataset. The evidence base here is thin, and no rigorous dose-response or mechanistic human study has been published to define how quickly this effect appears or resolves relative to plasma clearance.

Regulatory Status and Safety Context

Andarine is not approved by the FDA for any medical indication, and no SARM has been approved for human use. The FDA has issued a series of warning letters to companies marketing SARM-containing products as dietary supplements, including recent letters that continue into 2025, stating that these products are unapproved new drugs. In one such letter, to Elite Supplement Center LLC and Elite Training Facility LLC in July 2022, the FDA described SARMs, including compounds sold alongside andarine, as steroid-like substances that "may cause liver toxicity, increased risk of heart attack and stroke, and life-threatening reactions."

The FDA has also issued a specific consumer update warning about SARM use among teens and young adults, citing reports of serious health problems associated with this drug class, including cardiovascular events, liver injury, and reproductive effects, and cautioning that because these products are not approved drugs, the true rate of adverse events is likely underreported.

Published case-report literature reinforces these concerns with respect to liver injury specifically. A systematic review of SARM-related adverse events, searching the literature through October 2023, identified 20 published records relevant to suspected SARM-associated harms in humans, with drug-induced liver injury identified among the serious adverse effects discussed alongside cardiovascular risk. A separate systematic review of SARM safety in healthy adults, covering 33 studies including case reports, case series, and clinical trials, similarly flagged drug-induced liver injury and tendon rupture as concerns warranting attention in recreational users.

Because andarine products sold outside regulated pharmaceutical channels are not verified for identity, purity, or dose, any pharmacokinetic estimate applied to real-world use is further complicated by uncertainty about what is actually being ingested and in what amount.

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