What Does a Peptide's Half-Life Mean for Dosing Frequency?

Half-life (t½) is simply the time it takes for the concentration of a compound in the bloodstream to fall to half of its peak value. It's the single biggest factor determining how often a peptide needs to be dosed to stay within an effective range.

Unmodified peptides tend to have short half-lives because they're rapidly broken down by proteolysis and cleared by the kidneys — GHRP-6, for example, has a half-life of roughly 30 minutes, which is why GH-secretagogue protocols often call for two to three doses a day. Compounds chemically modified to resist that breakdown last much longer: CJC-1295 with DAC (Drug Affinity Complex) has a half-life measured in days rather than minutes, permitting once- or twice-weekly dosing for a comparable effect.

As a general pharmacological rule of thumb, a compound is considered practically cleared after five half-lives have passed — at that point, less than 4% of the original dose remains in circulation. Subcutaneous injection also extends effective half-life compared to intravenous delivery, since the peptide absorbs gradually from the tissue depot rather than entering circulation all at once.