Half-life
The interval over which serum concentration of a compound falls by 50%. Drives injection frequency, cycle-tail kinetics, and PCT timing.
Half-life (t½) describes first-order elimination kinetics: the interval across which serum concentration drops by 50% via hepatic metabolism, renal clearance, or both. For exogenous androgens administered as esterified oils, the ester chain length — not the parent steroid — is the rate-limiting variable. Esterase cleavage in plasma releases free hormone; longer fatty-acid chains are cleaved more slowly.
A compound is considered to have reached steady-state serum concentration after 4–5 half-lives of consistent dosing. Until then, weekly bloodwork readings are misleadingly low.
Clinical half-life of injectable androgens:
– Testosterone Propionate (3-carbon): 0.8 days. EOD or 3x weekly mandatory.
– Testosterone Enanthate (7-carbon): 4.5 days. Twice-weekly splits produce tighter trough-to-peak than once-weekly.
– Testosterone Cypionate (8-carbon): 8 days. Weekly acceptable; bi-weekly misses trough.
– Nandrolone Decanoate (10-carbon): 15 days. Bi-weekly holds steady-state.
– Boldenone Undecylenate (11-carbon): 14 days. Weekly.
Trenbolone Acetate at 1-day half-life is a different conversation — EOD is the floor, daily is better. The short-ester profile explains the aggressive peak-to-trough swings behind user-reported sleep disturbance and night sweats.
Peptides obey different kinetics. GHRP-6 clears in 30 minutes. Somatropin serum half-life is 2–3 hours, though downstream IGF-1 signalling persists 12–16 hours — which is why once-daily dosing produces the clinical effect despite short serum residency.
Low-frequency dosing of short-ester compounds produces exaggerated peak-to-trough variance. That variance drives side-effect expression: mood lability, erythrocytosis acceleration at peaks, perceived flat-day troughs. Steady-state calculation is not optional when scheduling bloodwork — a trough draw at week 3 of enanthate is reading a concentration 70% below future steady-state.
PCT begins when exogenous compound has cleared sufficiently for hypothalamic-pituitary feedback to resume. Rule of thumb: 5x half-life of the longest-acting ester. Testosterone Enanthate: day 14 post-last-injection. Nandrolone Decanoate: day 21–28 at minimum. Attempting SERM-based recovery while exogenous androgen is still suppressing endogenous LH is dose wasted.
Detection window exceeds pharmacological activity by a wide margin. Nandrolone metabolites are detectable 16–18 months post-cessation via GC-MS/MS despite zero biological activity past week 8. Half-life is not a detection estimate.