Half-Life

Overview

In pharmacology, half-life (abbreviated t½ or t1/2) refers to the time required for the concentration of a substance in the body to decrease by exactly one-half (50%) from its peak level. It is one of the most important pharmacokinetic parameters for understanding how long a peptide remains active and how frequently it needs to be administered.

For example, if a peptide has a half-life of 4 hours and reaches a peak plasma concentration of 100 ng/mL, after 4 hours the concentration would be approximately 50 ng/mL, after 8 hours it would be 25 ng/mL, and so on.

Half-life is distinct from duration of action — a peptide may continue to exert biological effects even after its plasma concentration has dropped below detectable levels, particularly if it triggers downstream signaling cascades or gene expression changes.

Types of Half-Life

Several types of half-life are referenced in peptide research:

Plasma Half-Life (Elimination Half-Life)

The most commonly cited form. This is the time for the plasma (blood) concentration to decrease by 50%. It reflects the combined effects of metabolism, distribution, and excretion.

Distribution Half-Life (t½α)

The time for a substance to distribute from the blood into tissues. This initial rapid phase occurs before the slower elimination phase begins. Peptides with high tissue affinity may have a very short distribution half-life but a longer elimination half-life.

Biological Half-Life

A broader concept that accounts for both the compound's presence in the body and its biological effects. Some peptides initiate repair cascades or signaling pathways that continue long after the peptide itself has been cleared.

Terminal Half-Life (t½β)

The half-life measured during the terminal elimination phase, after distribution is complete. This is typically the half-life reported in pharmacokinetic studies and is the most relevant for dosing frequency calculations.

Half-Life Elimination Curve

The elimination of most peptides follows first-order kinetics, meaning a constant fraction (not a constant amount) is eliminated per unit of time:

Half-Lives Elapsed	% Remaining	% Eliminated
0	100%	0%
1	50%	50%
2	25%	75%
3	12.5%	87.5%
4	6.25%	93.75%
5	3.125%	96.875%

Key insight: After approximately 5 half-lives, a substance is considered effectively eliminated from the body (less than 3.2% remaining). This is the pharmacological "washout" period.

Why Half-Life Matters for Peptides

Dosing Frequency

Half-life directly determines how often a peptide needs to be administered to maintain effective levels:

• Short half-life (< 30 minutes): May require multiple daily administrations or continuous infusion
• Moderate half-life (1–6 hours): Typically administered 1–3 times daily
• Long half-life (> 12 hours): May allow for once-daily or less frequent dosing

Steady State

When a peptide is administered at regular intervals, the concentration in the body gradually builds up until the amount being administered equals the amount being eliminated. This equilibrium is called steady state and is typically reached after 4–5 half-lives of consistent dosing.

For a peptide with a 4-hour half-life dosed every 4 hours, steady state would be reached after approximately 20 hours (5 × 4 hours).

Timing Relative to Meals and Activity

Peptides with very short half-lives may need to be timed precisely relative to meals, sleep, or exercise to align with the compound's peak activity window.

Half-Lives of Common Research Peptides

The following values are approximations based on published pharmacokinetic data and should be interpreted with caution, as half-life can vary based on route of administration, individual metabolism, and study methodology:

Peptide	Approximate Half-Life	Typical Dosing Frequency
BPC-157	~4 hours (estimated)	1–2× daily
TB-500	~2–3 hours (Tβ4 in plasma)	1–2× weekly (loading), then weekly
GHK-Cu	~1 hour	1–2× daily
Ipamorelin	~2 hours	1–3× daily
CJC-1295 (DAC)	~6–8 days	1–2× weekly
CJC-1295 (no DAC) / Mod GRF 1-29	~30 minutes	1–3× daily
GHRP-6	~15–60 minutes	1–3× daily
GHRP-2	~15–60 minutes	1–3× daily
Sermorelin	~10–20 minutes	1× daily (before bed)
PT-141 (Bremelanotide)	~2.7 hours	As needed
AOD-9604	~30–60 minutes	1× daily
Selank	~Several minutes	2–3× daily

Note: Many peptide half-life values are extrapolated from animal studies or limited human pharmacokinetic data. Individual variation is expected.

Factors That Affect Peptide Half-Life

Several variables influence how quickly a peptide is cleared from the body:

Route of Administration

• Intravenous: Fastest onset, but often shortest effective duration (no absorption phase)
• Subcutaneous: Slower absorption creates a depot effect, effectively extending the functional half-life
• Oral: Most peptides have very poor oral bioavailability due to gastric degradation and first-pass metabolism. BPC-157 is a notable exception, with some studies investigating oral stability

Molecular Weight and Structure

• Larger peptides tend to have longer half-lives
• Peptides modified with polyethylene glycol (PEGylation) or fatty acid chains have dramatically extended half-lives — this is the principle behind CJC-1295 with DAC
• Cyclic peptides may be more resistant to enzymatic degradation than linear ones

Enzymatic Degradation

Peptides are broken down by proteases and peptidases throughout the body. Key sites of degradation include:

• Blood plasma — circulating proteases
• Liver — hepatic enzymes (first-pass metabolism)
• Kidneys — renal clearance and degradation
• Target tissues — local peptidases

Individual Factors

• Metabolic rate — faster metabolism = shorter effective half-life
• Body composition — affects distribution volume
• Liver and kidney function — primary elimination organs
• Age — elderly individuals may have altered clearance rates
• Hydration status — affects blood volume and concentration

Half-Life Extension Strategies

Researchers have developed several techniques to extend peptide half-lives for less frequent dosing:

• PEGylation — attaching polyethylene glycol chains to shield the peptide from enzymatic degradation. Example: CJC-1295 with DAC uses a Drug Affinity Complex to bind albumin, extending half-life from ~30 minutes to ~6–8 days
• Fatty acid conjugation — attaching lipid chains that bind to serum albumin, extending circulation time. This is the principle behind long-acting insulin analogs
• D-amino acid substitution — replacing L-amino acids with their D-enantiomers at vulnerable positions to resist protease cleavage
• Cyclization — forming the peptide into a ring structure to reduce vulnerability to exopeptidases
• Encapsulation — nanoparticle or liposomal delivery systems that create a sustained-release depot

Practical Implications

Understanding half-life helps inform several practical decisions:

1. Timing of administration — Knowing when peak concentration occurs and how long effects last helps optimize timing relative to sleep, meals, or training
2. Splitting doses — Short-half-life peptides may benefit from multiple smaller doses throughout the day rather than one large dose
3. Washout periods — Understanding clearance time is important when cycling on and off compounds, or when transitioning between different protocols
4. Drug interactions — Compounds administered concurrently should have their pharmacokinetic profiles considered to avoid unintended interactions at peak concentrations

Related Topics

For understanding how to prepare peptides for administration, see Reconstitution. For the most common administration route, see Subcutaneous Injection.