Sermorelin (GHRH 1-29) has a plasma terminal half-life of approximately 10–20 minutes based on human pharmacokinetic data reviewed by Prakash & Goa (BioDrugs 1999).[1] The peptide is rapidly cleared from plasma by serum peptidases within 1.5–2.5 hours (5 half-lives), but the downstream pituitary GH pulse it triggers peaks at 20–60 minutes and persists for 3–4 hours — making once-nightly bedtime dosing the standard protocol. Also known as: GHRH(1-29)-NH₂, Geref (discontinued brand), Sermorelin acetate, GRF 1-29.
Sermorelin's plasma half-life of approximately 10–20 minutes places it among the shortest-lived therapeutic peptides in clinical use. The primary reference is the comprehensive pharmacokinetic review by Prakash and Goa published in BioDrugs (1999),[1] which synthesized human PK data from Geref clinical trials. Alba et al. (2006) confirmed the short-acting nature of sermorelin in a clinical trial demonstrating significant GH axis stimulation despite rapid plasma clearance.[3]
Plasma half-life is determined by measuring sermorelin concentrations at multiple time points following IV or SC administration, then fitting a pharmacokinetic model to the concentration-time curve. For sermorelin, the terminal elimination phase (after distribution) reflects peptidase-mediated degradation and renal filtration — not hepatic metabolism via cytochrome P450 enzymes. The short half-life means no accumulation occurs with daily dosing: each injection produces a discrete, self-terminating GH pulse.
A critical distinction with sermorelin — and with GHRH analogs generally — is that the biological effect outlasts plasma presence by a significant margin. Sermorelin is cleared from plasma within roughly 1.5–2.5 hours, but the GH pulse it stimulates at the pituitary peaks at 20–60 minutes post-injection and can persist for 3–4 hours as GH continues to be secreted and then cleared at its own rate (~2–4 hr half-life for GH itself). More importantly, with chronic daily use, the cumulative effect on IGF-1 — which has a plasma half-life of 12–20 hours and is the primary downstream anabolic mediator — accumulates over weeks. Practitioners and patients should understand that the "half-life of sermorelin" describes the peptide's plasma persistence, not the duration of its biological action.
Using a conservative midpoint half-life of 15 minutes, the following table illustrates plasma clearance. Standard pharmacokinetic convention holds that 97% clearance (5 × t½) effectively represents full elimination from the plasma compartment.
| Time Post-Injection | Half-Lives Elapsed | Sermorelin Remaining (%) | Practical Status |
|---|---|---|---|
| 15 min | 1 × | ~50% | Peak GH stimulation beginning |
| 30 min | 2 × | ~25% | GH pulse rising |
| 45 min | 3 × | ~12.5% | Plasma nearly cleared; GH peak |
| 60 min | 4 × | ~6% | Sermorelin essentially cleared |
| 75–90 min | 5 × | <3% | Full plasma clearance; GH still elevated |
| 3–4 hr | — | 0% | GH elevation also resolved; IGF-1 rising |
Note: The above table addresses plasma clearance of sermorelin. The biological GH elevation triggered by sermorelin persists well beyond plasma clearance, typically resolving 3–4 hours post-injection. With a daily protocol, IGF-1 levels begin to rise within the first 1–2 weeks and plateau after 4–6 weeks of consistent dosing.
The rationale for bedtime dosing of sermorelin is rooted in human GH physiology. Endogenous GH is secreted in discrete pulses — the largest of which occurs approximately 60–90 minutes after sleep onset, coinciding with slow-wave (deep) sleep. This peak pulse accounts for a disproportionate fraction of daily GH secretion.
Because sermorelin acts at the pituitary GHRH receptor to amplify and synchronize GH release, administering it 30–60 minutes before sleep allows the drug to reach the pituitary and initiate signaling just as the sleep-associated GH pulse is naturally cued. The result is an augmented physiological GH pulse that mimics — rather than overrides — the body's own rhythm. By contrast, daytime dosing tends to produce smaller, ectopic GH pulses at times when somatostatin tone is higher and pituitary responsiveness is lower.[2]
Standard dosing ranges from 200–500 mcg subcutaneously each evening. Because sermorelin acts through an intact hypothalamic-pituitary axis, efficacy is dependent on residual pituitary function; patients with severe pituitary damage may not respond.
Given sermorelin's short half-life and lack of accumulation, a missed dose has a simple outcome: no GH pulse stimulation occurs on that evening. Unlike long-acting depot formulations where a missed dose creates a gradual decline, sermorelin's activity is entirely dose-event-dependent. Patients can resume the following evening without concern for rebound or altered kinetics. The IGF-1 elevation from a chronic protocol will begin to decline gradually after 48–72 hours of consecutive missed doses, reflecting the downstream IGF-1 half-life rather than sermorelin's own kinetics.
The GHRH analog and GH secretagogue landscape includes several compounds with meaningfully different half-lives and dosing requirements:
| Compound | Class | Half-Life | Typical Frequency | Mechanism Note |
|---|---|---|---|---|
| Sermorelin | GHRH analog | ~10–20 min | Once nightly SC | Native GHRH(1-29); peptidase-sensitive |
| CJC-1295 (no DAC) | GHRH analog | ~30 min | Daily–2× weekly SC | Modified GHRH(1-29); better peptidase resistance[4] |
| CJC-1295 (with DAC) | GHRH analog | 6–8 days | Weekly SC | Albumin-binding Drug Affinity Complex[4] |
| Tesamorelin | GHRH analog | ~26–38 min | Once daily SC | FDA-approved for HIV-associated lipodystrophy |
| Ipamorelin | GH secretagogue (GHRP) | ~2 hours | 1–3× daily SC | GHS-R1a agonist; often combined with sermorelin |
Subcutaneous (SC) injection is the standard clinical route for sermorelin and the route used in virtually all off-label protocols. Following SC injection, bioavailability is high and Tmax occurs rapidly at 5–20 minutes, reflecting fast absorption from the subcutaneous depot into systemic circulation. The SC route allows self-administration and is appropriate for the once-nightly protocol.
Intravenous (IV) administration was used in clinical pharmacokinetic and diagnostic studies, particularly for the sermorelin stimulation test (a provocative test of pituitary GH reserve). IV dosing produces an immediate peak with a measured plasma half-life of ~10–12 minutes. The IV route is not used in standard therapeutic protocols.
Intranasal administration has been investigated experimentally for GHRH analogs but no published PK data for intranasal sermorelin in humans are available. Peptide bioavailability via the intranasal route is generally low and highly variable for molecules of sermorelin's size (~3.3 kDa). Intranasal sermorelin is not an established protocol.
Sermorelin is not screened for on standard employment or clinical urine drug panels, which target small molecules (opioids, cannabinoids, amphetamines, benzodiazepines, etc.) and not peptide hormones.
In the context of sports anti-doping (WADA, USADA), the relevant targets are typically GH itself and its downstream biomarkers (IGF-1, ALS, the GH-2000 and GH-biomarker assays), rather than GHRH analogs directly. Sermorelin peptide could theoretically be detected by liquid chromatography-mass spectrometry (LC-MS/MS) in plasma or urine for approximately 1–2 hours post-dose, given its ~10–20 minute half-life and rapid proteolytic degradation. However, WADA-accredited labs currently do not include sermorelin in their standard prohibited substance detection panels.
Indirect detection — via elevated IGF-1, suppressed endogenous GHRH, or altered GH pulsatility — is theoretically possible with sensitive assays and known baseline values, but is not currently part of established anti-doping protocols.
Sermorelin is the synthetic 29-amino-acid N-terminal fragment of endogenous growth hormone-releasing hormone (GHRH), the naturally occurring 44-amino-acid neuropeptide secreted by hypothalamic arcuate nucleus neurons. The first 29 amino acids of GHRH contain the complete biological activity needed for GHRH receptor (GHRHR) binding and activation, which is why the truncated peptide retains full agonist activity despite lacking residues 30–44.
The short half-life is a direct consequence of the peptide's primary structure. Sermorelin, like native GHRH, is rapidly cleaved by circulating serum dipeptidyl peptidase IV (DPP-IV) and other peptidases at specific sites along the N-terminus. Once the N-terminal residue is removed, the peptide loses biological activity. This is the same vulnerability that makes native GHRH itself unsuitable as a therapeutic — its plasma half-life in physiological conditions is only a few minutes.
Sermorelin binds pituitary GHRH receptors (Gs-coupled GPCRs) to activate adenylyl cyclase, increase intracellular cAMP, and trigger both GH synthesis and secretion from somatotroph cells. This mechanism is distinct from direct GH secretagogues like ipamorelin (which binds GHS-R1a/ghrelin receptor) or MK-677 (oral GHS-R1a agonist). Critically, sermorelin's action through the hypothalamic-pituitary axis preserves normal somatostatin feedback: as GH rises, somatostatin release from the hypothalamus increases, dampening further GH secretion and preventing runaway excess. This self-limiting mechanism is absent when exogenous recombinant HGH is administered directly.[2]
Because sermorelin does not interact with corticotroph or prolactin-secreting cells, it produces no cortisol or prolactin elevation — a significant advantage over GHRP-6 and GHRP-2, which carry these off-target effects.
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