Fluoxymesterone (Halotestin)
Fluoxymesterone ('Halotestin', 'Halo') is a 17α-methylated, 9α-fluoro, 11β-hydroxy derivative of testosterone with very high androgenic potency and negligible mass-building effect. It is prized among strength and combat athletes for sharp increases in strength, aggression and hardness at low body-weight impact, but is strongly hepatotoxic and harsh on lipids.
01 Overview
An old pharmaceutical androgen (approved for hypogonadism, delayed puberty and inoperable breast cancer), fluoxymesterone has a strongly androgenic, non-anabolic profile. It does not aromatise and does not add appreciable mass or water.
Recreationally it is used pre-competition by powerlifters, strongmen and fighters for a rapid boost in aggression, drive and strength without weight gain. Those benefits come with a very poor safety profile: pronounced hepatotoxicity, severe lipid deterioration and heightened aggression, so it is typically used briefly and at low doses.
02 Mechanism
Highly androgenic AR agonist; the 9α-fluoro and 11β-hydroxy groups plus 17α-methylation increase potency and oral activity while preventing aromatisation.
03 Dosing
| Tier | Dose | Route | Notes |
|---|---|---|---|
| Therapeutic | 5–20 mg/day | Oral | Historical clinical dosing. |
| Common | 10–20 mg/day | Oral | Typical pre-competition strength range. |
| Heavy | 20–40 mg/day | Oral | High dose; hepatic and lipid risk escalates steeply. |
Ester comparison
| Ester | Half-life | Injection freq. | Testosterone by mass |
|---|
04 Effects
| Effect | Magnitude | Evidence | |
|---|---|---|---|
| Sharp strength and aggression increaseRapid boost in strength, drive and aggression, used before competition. | Observational | ||
| Increased hardness without weight gainNo water or appreciable mass; adds a dry, hard look. | Observational | ||
| Heightened aggression / irritabilityStrong androgenicity can produce irritability and mood disturbance. | Anecdotal |
05 Side effects
| Effect | Severity | Frequency | Evidence | Countermeasures |
|---|---|---|---|---|
| Severe hepatotoxicityOne of the more hepatotoxic orals; associated with cholestasis and, with prolonged use, peliosis hepatis. | Severe | Common | Clinical | |
| Severe adverse lipid changesMarked HDL suppression and cardiovascular strain. | Severe | Very common | Observational | |
| HPTA suppressionStrong suppression of endogenous testosterone. | Severe | Universal | Clinical |
06 Commonly used with
What fluoxymesterone is combined with, and why. This is about intent rather than safety — the interactions table below covers what is dangerous. Nothing here is listed without what it costs.
| Compound | Frequency | Purpose & trade-off |
|---|---|---|
| Stacked compounds | ||
| Trenbolone acetatepeak week / powerlifting | Combined briefly before contests or meets to maximise dry hardness, aggression and strength when peak performance is the goal.Trade-off: This is an extreme stack — Halo's severe liver toxicity and lipid damage compound trenbolone's cardiovascular and psychological side effects. | |
| Support & ancillaries | ||
| Testosteronebase dose | Halotestin is strongly suppressive and non-aromatising, so a testosterone base is required to preserve normal androgen levels during use.Trade-off: The testosterone base aromatises and adds water, offsetting some of the extreme dryness and aggression Halo is used for. | |
| TUDCAliver support | Run with fluoxymesterone to help protect the liver during use of this highly hepatotoxic C17-alkylated oral.Trade-off: Support only partly mitigates Halo's pronounced liver strain and does nothing for its aggressive suppression of HDL cholesterol. | |
| Post-cycle | ||
| ClomifenePCT | Used post-cycle to stimulate gonadotropin release and recover natural testosterone after the strong suppression from Halo and the base.Trade-off: Recovery can be slow and incomplete, and clomifene commonly causes mood and visual disturbances during PCT. | |