Andarine

Andarine (S-4) is a non-steroidal, orally active selective androgen receptor modulator from the aryl-propionamide class — the same chemical lineage as bicalutamide, repurposed from antagonist to tissue-selective partial agonist. The practical payoff: it hits the androgen receptor hard in muscle and bone while barely touching prostate and seminal vesicles, producing visible hardening and recomp without aromatising to estrogen.

Tissue-Selective AR Partial Agonism#

S-4 binds the androgen receptor with moderate-to-high affinity (Ki ≈ 4 nM in vitro) and behaves as a near-full agonist in skeletal muscle and bone, but only a weak partial agonist in androgen-sensitive accessory tissues like prostate. This is the core of the "SARM" concept and the reason S-4 can drive lean-mass and hardness gains without the prostate hypertrophy, oily skin, and DHT-mediated scalp damage you'd expect from exogenous testosterone or DHT.

"S-4 increased the weight of the levator ani muscle to the level of the intact control while stimulating the prostate and seminal vesicles to only 16–17% of the stimulation observed with dihydrotestosterone." — Gao W. et al., Endocrinology, 2003

For the reader: this is why S-4 is a recomp and cutting compound, not a mass-builder. Muscle AR activation is real but ceiling-limited compared to a full agonist like testosterone — you get quality over quantity.

Downstream Anabolic Transcription#

Once AR is bound and translocated to the nucleus, S-4 drives the standard androgen-response transcriptional program — upregulation of MyoD and myogenin, enhanced IGF-1 signalling in muscle, and Akt/mTOR-mediated increases in muscle protein synthesis. No aromatisation to estrogen, no 5-alpha reduction to DHT. The result in practice: dryer-looking tissue, sharper vascularity within 3–4 weeks, and strength retention in a caloric deficit without the water retention profile of testosterone.

Bone and Fat Partitioning#

S-4's AR activity extends to osteoblasts and adipocytes, giving it a recomp profile that's more than just muscle. In ovariectomized rat models, S-4 preserved bone mineral density and reduced visceral fat without uterine stimulation — a partitioning effect that translates, anecdotally, to why users report visibly tighter skin and better fat distribution on S-4 cuts than on equivalent caloric deficits alone.

"S-4 significantly prevented bone loss and reduced body fat in ovariectomized rats without significant stimulation of uterine tissue." — Kearbey JD. et al., Pharm Res, 2007

This is also the rationale for running S-4 through an injury-phase deload: you hold composition and protect bone density while training volume is capped.

HPTA Suppression#

Despite the tissue selectivity, AR activation in the hypothalamus and pituitary is enough to downregulate GnRH, and LH/FSH fall at anabolic doses. Endogenous testosterone production drops during a cycle — not to the zero floor a trenbolone cycle produces, but meaningfully, and enough that a SERM-based PCT is standard at 50 mg/day or above.

"Andarine has been observed to produce significant reductions in luteinizing hormone and follicle-stimulating hormone, indicating suppression of the hypothalamic-pituitary-gonadal axis." — Machek SB. et al., Steroids, 2020

Plan for nolvadex 20 mg/day × 3–4 weeks post-cycle. Skipping PCT on the assumption that "SARMs don't suppress" is the most common rookie mistake with S-4.

Retinal Binding and the Vision Effect#

The signature S-4 side effect — yellow-green tint, reduced night vision, slowed dark adaptation — is mechanistically distinct from its muscle activity and is the reason GTx shelved clinical development. The working hypothesis is that S-4 binds AR or AR-like targets in retinal tissue and interferes with phototransduction or opsin recycling. The exact target isn't confirmed, but the effect is dose-dependent, onset typically 2–4 weeks at 50 mg+, and fully reversible within days to a couple of weeks after discontinuation.

"A signature adverse effect of S-4 (Andarine) is a yellow-tinted vision disturbance and slowed adaptation to darkness, which is reversible after discontinuation." — Hall E, Vrolijk MF, Nutrients, 2023

Practical implication: cap at 50 mg/day for most users, split the dose to avoid peak-concentration spikes, and if you drive at night for work, pick a different compound.

Common (most users)#

• Yellow/green tint to vision — the signature S-4 effect. Starts 2–4 weeks in at 50 mg/day, more pronounced in low light. Mitigation: cap at 50 mg/day, run 5-days-on / 2-days-off, keep cycles ≤8 weeks. Fully reversible within days to ~2 weeks of cessation.

"A signature adverse effect of S-4 (Andarine) is a yellow-tinted vision disturbance and slowed adaptation to darkness, which is reversible after discontinuation." — Hall & Vrolijk, Nutrients 2023

• Slowed dark adaptation / night-driving impairment — pairs with the tint. Walking from a lit room into a dark hallway takes noticeably longer to resolve. Mitigation: don't drive at night during weeks 4–8, especially at ≥50 mg. If your job depends on night vision, pick a different SARM.
• Reduced libido / softer erections late-cycle — LH/FSH drop kicks in by week 4–6. Mitigation: low-dose tadalafil (2.5–5 mg daily) handles the vascular side; libido rebounds post-PCT.
• Testicular volume loss — visible shrinkage by week 6 at 50 mg+. Cosmetic, fully reversible with proper PCT.
• Mild lethargy / flat mood — usually tracks suppression. Resolves as HPTA recovers.
• Appetite suppression — useful on a cut, annoying if you're trying to eat at maintenance. Front-load calories earlier in the day.

Uncommon (dose-dependent or individual)#

• HDL drop / LDL creep — expected with any oral SARM. Check a lipid panel at baseline and week 4. If HDL drops below ~35 mg/dL or LDL climbs meaningfully, add 2–4 g EPA/DHA daily and reconsider stacking with other orals.
• ALT/AST elevation — less common than with LGD-4033 or RAD-140, but real. Pull an LFT panel at week 4. Mild elevations (up to ~2× ULN) typically normalise post-cycle; higher warrants stopping.
• Blood pressure creep — watch if you're borderline going in. Cuff weekly.
• Meaningful suppression of total testosterone — confirmed by LH/FSH reduction in the literature. Dose-dependent; 75 mg/day hits harder than 25 mg/day.

"Andarine has been observed to produce significant reductions in luteinizing hormone and follicle-stimulating hormone, indicating suppression of the hypothalamic-pituitary-gonadal axis." — Machek et al., Steroids 2020

• Flickering / photopsia at 75 mg+ — distinct from the tint, occasional light flashes in peripheral vision. Back off to 50 mg immediately; if it persists >48 h after dose reduction, stop the cycle.
• Hair shedding — reported but uncommon given the low androgenic profile (rating ~5). If you're predisposed, run topical finasteride or minoxidil alongside rather than dropping the compound.

Rare but serious#

• Persistent vision changes post-cycle — the vast majority of users report full reversal within 2 weeks of stopping. A small minority report lingering tint or reduced night vision for months. Warning signs: no improvement by 4 weeks off, worsening after cessation, new floaters or visual field loss. Stop immediately and get an ophthalmology workup.
• Clinically significant hepatotoxicity — rare but documented across the SARM class. Warning signs: jaundice, dark urine, right-upper-quadrant pain, ALT/AST >3× ULN. Stop the cycle.
• Cardiovascular events in predisposed users — the Nutrients 2023 review flags SARM-class cardiovascular risk as underappreciated, particularly when stacked or extended beyond 8 weeks.

"A signature adverse effect of S-4 (Andarine) is a yellow-tinted vision disturbance and slowed adaptation to darkness, which is reversible after discontinuation." — Hall & Vrolijk, Nutrients 2023

• Prolonged HPTA shutdown — rare at standard protocols, but users who skip PCT, run past 8 weeks, or stack S-4 with other suppressive compounds can take months rather than weeks to recover. Bloodwork at 4 weeks post-PCT confirms.

Hard contraindications#

• Pre-existing retinal disease, macular degeneration, or retinitis pigmentosa — do not run S-4. The visual mechanism is not fully characterised and you are stacking risk on already-compromised tissue.
• Glaucoma — same logic.
• Occupations requiring intact night vision — pilots, truckers, ER/night-shift surgeons, anyone driving at night as a job. The impairment is real and dose-dependent; your livelihood is not worth a cutting stack.
• Untreated hypertension or dyslipidemia — get these controlled first. S-4 will shift lipids further.
• Planning conception within 3–6 months — HPTA suppression plus possible semen-parameter effects. Wait until post-PCT bloodwork confirms recovery.
• Pregnancy or lactation — absolute. No SARM has a safety profile in pregnancy, and tissue selectivity in females is uncharacterised.
• Concurrent use of other hepatotoxic orals (Winstrol, Anadrol, high-dose Anavar) — the lipid and liver hit compounds badly.

Women and PCT considerations#

S-4 is not recommended for women. Tissue selectivity data in females is limited to ovariectomised rat models (Kearbey et al., 2007) and does not translate cleanly to intact human females at anabolic doses. Virilisation risk — clitoral enlargement, voice deepening, menstrual disruption — is real and at least partially irreversible. Women seeking a mild SARM overwhelmingly run Ostarine 10 mg instead.

PCT is required for any protocol ≥50 mg/day run for 6–8 weeks. The community standard is nolvadex (tamoxifen) 20 mg/day for 3–4 weeks starting the day after the last S-4 dose (no washout needed given the 4 h half-life). Clomid is overkill at these suppression levels. Pull bloodwork (total T, free T, LH, FSH, estradiol, lipids, LFTs) at 4 weeks post-PCT to confirm recovery before considering another cycle — and give yourself at least time-on / time-off between cycles.