Understanding peptide half-lives — why timing and frequency matter

Half-life is one of the most important concepts in peptide research, and misunderstanding it leads to suboptimal dosing schedules.

What is half-life:
Half-life is the time it takes for the concentration of a substance in the body to drop to 50% of its peak level. After one half-life, 50% remains. After two half-lives, 25% remains. After five half-lives, less than 3% remains — this is generally considered "cleared."

Why it matters for dosing:
A peptide with a 30-minute half-life requires frequent dosing to maintain effective levels. A peptide with a 7-day half-life requires dosing once or twice a week. Getting the frequency wrong means the peptide is either at effective levels for too little of the day (underdosing by frequency) or building up to unnecessarily high levels (overdosing by frequency).

Common peptide half-lives:

Short half-life (minutes to hours):

• Mod GRF 1-29 — approximately 30 minutes
• Ipamorelin — approximately 2 hours
• GHRP-2 — approximately 30 minutes
• GHRP-6 — approximately 30 minutes
• BPC-157 — approximately 4 hours (estimated; exact human data limited)

These peptides require 2-3x daily dosing.

Medium half-life (hours to days):

• Semaglutide — approximately 7 days (dosed weekly)
• Tirzepatide — approximately 5 days (also dosed weekly)
• CJC-1295 with DAC — approximately 6-8 days
• Selank — approximately 2-4 hours
• Semax — approximately 2-4 hours

Weekly peptides should not be dosed more frequently than their half-life suggests.

Long half-life (days to weeks):

• Melanotan II — approximately 36 hours (supports every-other-day or less frequent maintenance dosing)
• TB-500 — approximately 48-72 hours (supports twice-weekly dosing)

Practical implications:

For GH secretagogues (short half-life): The short half-life is actually desirable — it creates a discrete GH pulse that mimics natural physiology. Dosing 2-3 times daily with gaps between doses allows each injection to create a separate pulse. This is better than one large daily dose.

For GLP-1 peptides (long half-life): The long half-life means steady-state is reached after approximately 4-5 weeks of weekly dosing. This is why the full effects of semaglutide take weeks to manifest — the drug is still accumulating. It also means effects persist for weeks after the last dose.

For healing peptides (medium half-life): BPC-157's estimated 4-hour half-life supports twice-daily dosing. TB-500's longer half-life supports twice-weekly dosing. This is why they are dosed on different schedules even when used in the same stack.

The bottom line:
Matching dosing frequency to the peptide's half-life is essential. More frequent is not always better. Less frequent is not always sufficient. Understanding this single concept significantly improves research protocol quality.