Peptide Half-Life Explained: How It Sets Your Dosing Schedule

Half-life is the most useful number in peptide pharmacology, and it is also the most commonly misread. It explains why semaglutide is a once-weekly injection while ipamorelin is typically split into two or three doses per day, why a weekly peptide takes a month to reach full effect, and why doubling your injection frequency is not the same as doubling your dose.

What Half-Life Actually Means

Elimination half-life (written as t½) is the time it takes for the concentration of a compound in your blood to fall by 50 percent. It is a property of the molecule and your physiology, not of the dose. A 250 mcg dose and a 500 mcg dose of the same peptide clear at the same relative rate: the larger dose simply starts from a higher peak.

Because each half-life removes half of what is left, clearance follows a predictable decay curve rather than a straight line:

• After 1 half-life: 50 percent of the dose remains
• After 2 half-lives: 25 percent remains
• After 3 half-lives: 12.5 percent remains
• After 4 half-lives: 6.25 percent remains
• After 5 half-lives: roughly 3 percent remains, which is generally treated as fully cleared

Half-Life Reference Table for Common Peptides

Published half-life values vary by study, route of administration, and assay method. The ranges below reflect the commonly cited figures for subcutaneous administration unless noted otherwise.

Incretin peptides (long half-life, weekly dosing):

• Semaglutide: roughly 165 hours (about 7 days), which is what makes once-weekly dosing possible
• Tirzepatide: roughly 5 days (about 120 hours), also dosed once weekly
• Retatrutide: roughly 6 days, supporting a once-weekly schedule in phase 2 trials

Growth hormone secretagogues (mostly short half-life, with one long-acting exception). The contrast between ipamorelin and CJC-1295 with DAC is the clearest example of how much half-life drives protocol design:

• Sermorelin: roughly 11 to 12 minutes, one of the shortest in common use
• Modified GRF 1-29 (CJC-1295 without DAC): roughly 30 minutes
• GHRP-6: roughly 15 to 60 minutes
• GHRP-2: roughly 55 minutes
• Hexarelin: roughly 55 to 70 minutes
• Ipamorelin: roughly 2 hours
• MK-677 (ibutamoren, taken orally): roughly 4 to 6 hours, which supports once-daily dosing
• CJC-1295 with DAC: roughly 6 to 8 days, because the drug affinity complex binds it to serum albumin

Repair, cosmetic, and other peptides (short half-life, effect often outlasts plasma levels):

• PT-141 (bremelanotide): roughly 2.7 hours, dosed on demand rather than on a schedule
• Melanotan-2: roughly 1 to 2 hours, though human pharmacokinetic data is limited
• TB-500 and thymosin beta-4: roughly 2 to 3 hours in plasma
• BPC-157: no validated human pharmacokinetic data, with rodent studies suggesting rapid plasma clearance
• GHK-Cu, epithalon, selank, and semax: all measured in minutes rather than hours
• Injected NAD+: degraded very rapidly, which is why infusion protocols run over 30 to 120 minutes rather than as a single push

Why Short Half-Life Peptides Need Multiple Daily Doses

A peptide with a 2 hour half-life is roughly 97 percent gone 10 hours after injection. If you inject once in the morning, the compound is effectively absent for the second half of your day. Splitting the same total daily amount into two or three doses keeps blood levels inside the useful window for far longer without increasing the total amount used.

This is why growth hormone secretagogue protocols are usually written as 200 to 300 mcg two or three times daily rather than 600 to 900 mcg once. The peak matters for pulsatile GH release, and a single large dose produces one oversized pulse instead of several physiologic ones.

The practical consequence is more injections, which makes site rotation matter. See the peptide injection sites guide for a rotation schedule that supports multiple daily doses without irritating one area.

Why Weekly Peptides Take Weeks to Reach Steady State

When the dosing interval is shorter than the time needed for full clearance, each dose lands on top of residual drug from previous doses. Levels climb until the amount cleared between doses equals the amount injected. That plateau is steady state, and it arrives after about 4 to 5 half-lives.

1. Semaglutide has a half-life near 7 days, so steady state takes roughly 4 to 5 weeks at any fixed dose.
2. Tirzepatide has a half-life near 5 days, so steady state takes roughly 3 to 4 weeks.
3. Retatrutide behaves similarly at roughly 4 weeks per dose step.
4. CJC-1295 with DAC has a half-life of 6 to 8 days, so its GH and IGF-1 elevation continues building for several weeks after the first injection.

This explains the standard 4 week titration interval on GLP-1 protocols. Increasing the dose before steady state means judging tolerability from an incomplete picture, and side effects that appear mild in week 2 can be considerably stronger in week 5 at the same weekly amount.

How Half-Life Changes Your Injection Timing

Once you know a peptide half-life, several timing questions answer themselves:

• Long half-life (days): pick any consistent day of the week. Being 12 to 24 hours late has minimal effect on levels.
• Medium half-life (4 to 8 hours): once daily works, and consistency of time of day matters more.
• Short half-life (under 2 hours): split into 2 or 3 doses and time them around the effect you want, such as fasted state or pre-sleep for GH secretagogues.
• Very short half-life (minutes): timing relative to food and sleep dominates, since the compound is gone long before the next dose.

Half-Life Is Not the Same as Duration of Effect

A peptide can be cleared from plasma long before its biological effect ends. Sermorelin has a half-life near 11 minutes, but the growth hormone pulse it triggers, and the downstream IGF-1 response, plays out over hours. Semax and selank are cleared from blood within minutes while their reported cognitive effects last considerably longer.

The reverse is also true. A long half-life guarantees drug presence but not continued response, since receptor downregulation and tolerance operate on their own timeline. Treat half-life as the answer to how often to inject, not as a prediction of how long you will feel something.

Practical Takeaways

1. Look up the half-life before designing a schedule. It determines dosing frequency more than any other single variable.
2. Allow 4 to 5 half-lives before judging a dose. On weekly peptides that means about 4 weeks per titration step.
3. Split short half-life peptides rather than increasing the single dose, since more frequent smaller doses hold levels better at the same total amount.
4. Expect a slow taper of effect after stopping a long half-life compound, on the order of 4 to 5 weeks for weekly incretins.
5. Reconstitute at a concentration that makes your split doses easy to measure. Very small draws below 5 units on a U100 syringe are difficult to hit accurately.