Half-life and dosing frequency: why some peptides are daily and others weekly

What a half-life is

A drug's half-life is the time it takes for its concentration in the blood to drop by half. After one half-life, half is left; after two, a quarter; after three, an eighth; after five, about 3%, which is why a drug is often treated as largely gone after four or five half-lives. The idea matters because it sets two practical things: how often a medicine has to be given to keep levels steady, and how long it lingers after the last dose. The Wegovy label shows both: semaglutide's elimination half-life is about one week, and the label says it will be present in the circulation for about 5 to 7 weeks after the last dose ¹.

Half-life is measured in people under specific conditions, by a specific route. The same molecule can show different figures when injected into a vein or under the skin, because absorption from under the skin is slower. The Genotropin label gives a clear example: somatropin's mean terminal half-life is 0.4 hours given intravenously but 3.0 hours given under the skin, a difference the label puts down to slow absorption from the injection site ².

Half-lives of common peptides

The table gathers half-lives from US drug labels, plus one research peptide with a human study. Our Half-Life Visualizer plots these curves over time.

From minutes to a week: how peptides are made to last

Natural peptide hormones are built to act briefly. The Saxenda label explains that the body's own GLP-1 has a half-life of only 1.5 to 2 minutes because enzymes called DPP-4 and neutral endopeptidases break it down quickly; liraglutide resists both and has a plasma half-life of 13 hours, suitable for once-daily dosing ³. Exenatide, cleared by the kidneys, lasts 2.4 hours and is given twice a day ⁴.

The weekly drugs go further by hitching a ride on albumin, the most abundant protein in blood. The Wegovy label says semaglutide's main protraction mechanism is albumin binding, made possible by a fatty acid chain attached to the peptide; it is more than 99% bound to albumin, which reduces clearance by the kidneys and protects it from degradation ¹. Tirzepatide uses the same trick: its label says it contains a C20 fatty diacid that enables albumin binding and prolongs the half-life, giving about 5 to 6 days ⁵.

Growth hormone shows another approach. Daily somatropin under the skin has a half-life of about 3 hours ²; Skytrofa, a weekly form, releases somatropin from a carrier so that the somatropin it releases has an apparent half-life of about 25 hours ⁶. CJC-1295 with DAC was designed to bind albumin too, and in healthy adults its half-life was estimated at 5.8 to 8.1 days (human study) ⁷.

Short-acting peptides and why they are used anyway

A short half-life is not a flaw for every purpose. Tesamorelin, approved for abdominal fat in people with HIV, has a mean half-life of 11 minutes after injection under the skin ⁸, yet it is given once daily, because its job is to trigger the body's own growth hormone release, an effect that outlasts the drug. Bremelanotide (Vyleesi), with a half-life of about 2.7 hours, is meant to be used on demand before sex, not to keep a steady level ⁹. Ipamorelin, a research peptide, had a half-life of about two hours in healthy men, and each dose produced a single pulse of growth hormone (human study) ¹⁰.

So half-life is only half the story. What matters is how long the effect lasts and whether the goal is a steady level or a pulse.

Steady state, build-up and wash-out

When a drug is given repeatedly at intervals shorter than the time it takes to clear, levels build up until intake and clearance balance, a point called steady state. For a long-acting drug, that takes weeks. It also means the drug lingers: the Wegovy label's 5 to 7 weeks after the last dose follows from a one-week half-life ¹. The Zepbound label, for the same reason, says observation after an overdose may need to account for tirzepatide's half-life of about 5 days ⁵.

That has practical consequences. Side effects from a weekly drug can persist for days after a dose, and stopping does not end its effects at once. It is one reason the labels escalate doses slowly over weeks rather than starting at full strength. Our Split-Dose Simulator shows how splitting the same weekly amount into smaller, more frequent doses changes the peaks and troughs; it illustrates the principle and is not dosing advice.

Terminal, apparent, elimination: reading the labels

Labels use slightly different terms, and they are worth knowing. An elimination or terminal half-life describes the final, slowest phase of clearance, after the drug has spread through the body; the Vyleesi label, for example, gives bremelanotide's mean terminal half-life as about 2.7 hours, with a range of 1.9 to 4.0 hours across people ⁹. An apparent half-life is what is observed when absorption or release is slower than elimination, so the slow step sets the pace: the Skytrofa label reports an observed half-life of 30.7 hours for the carrier-bound drug, and an apparent half-life of about 25 hours for the somatropin it releases ⁶. The Genotropin label's two figures, 0.4 hours intravenously and 3.0 hours under the skin, show the same effect from slow absorption ².

The ranges matter. A half-life is an average, and people differ. A single number on a chart is a guide, not a promise about any individual, and it says nothing about how strongly the drug acts while it is there.

How the body clears peptides

Small peptides are usually cleared by the kidneys and broken down into amino acids. The Byetta label says exenatide is predominantly eliminated by glomerular filtration, the kidneys' filtering, followed by breakdown ⁴. Albumin binding is what slows that down for semaglutide, by reducing clearance through the kidneys ¹. Whether kidney or liver problems change a peptide's levels has to be tested for each drug. For cagrilintide, single doses produced similar exposure in people with different degrees of kidney or liver function, with no consistent pattern by impairment (human study) ¹¹. Those findings do not transfer automatically to other peptides.

Half-life and missed doses

Long half-lives also explain the generous windows for missed doses on weekly drugs. The Zepbound label says a missed dose can be taken within 4 days; after that, it should be skipped and the next dose taken on schedule ⁵. With a half-life of 5 to 6 days, levels have not fallen far after a day or two's delay. A short-acting drug offers no such margin, which is why daily schedules are less forgiving. Our travel guide uses these windows to plan doses around time zones.

Research peptides: mostly unknown

For many peptides sold online, no human half-life has been measured. BPC-157 is a good example: a 2022 study in rats and beagle dogs found its elimination half-life was under 30 minutes after injection, with the intact peptide quickly broken down into small fragments, and bioavailability after intramuscular injection of about 14% to 19% in rats and 45% to 51% in dogs (animal study) ¹². There are no human pharmacokinetic data. Schedules sold with such peptides, daily or twice daily, are not based on human measurements.

Some research products also borrow a name from a studied molecule without its design. The CJC-1295 sold without DAC is a different, short-acting peptide; the 5.8-to-8.1-day half-life applies only to the DAC version that was studied ⁷. Our CJC-1295 and ipamorelin page explains the difference.

Natural hormones are short for a reason

The body keeps many peptide hormones short-lived so it can switch them on and off quickly. Oxyntomodulin, a gut hormone, had a half-life of about 12 minutes in plasma during infusion in six volunteers (human study) ¹³. Making a drug last a week changes the biology: instead of brief signals after meals, the receptor is stimulated around the clock. That is part of why long-acting GLP-1 drugs work so well for weight loss, and also why their side effects can be persistent. The contrast with liraglutide is instructive: the same receptor, a 13-hour half-life and a daily injection ³, against semaglutide's week ¹. Both are effective drugs; the difference is how continuously the receptor is stimulated and how often a person has to inject.

Questions to ask about any peptide's schedule

• Is the half-life measured in people, and by the route I would use?
• Is the goal a steady level or a short pulse?
• How long would the drug and its side effects linger after stopping?
• For a research peptide, is there any human pharmacokinetic study at all?

The bottom line

Half-life explains why semaglutide is weekly and exenatide twice daily: the weekly drugs are engineered to bind albumin and resist breakdown. It also explains why a weekly drug lingers for weeks after the last dose. For approved peptides, the label gives the number; for most research peptides, nobody has measured it in people.

Frequently asked questions

What is the half-life of semaglutide?

About one week, according to the Wegovy label, which says semaglutide stays in the circulation for about 5 to 7 weeks after the last dose.

What is the half-life of tirzepatide?

About 5 to 6 days in people with overweight or obesity, according to the Zepbound label.

Why are some peptides taken weekly?

Drugs like semaglutide and tirzepatide carry a fatty acid chain that binds them to albumin in the blood, slowing their clearance and breakdown so that one injection lasts about a week.

How long does BPC-157 stay in your system?

No human data exist. In rats and dogs, its half-life was under 30 minutes, and it was quickly broken into small fragments.

Does a short half-life mean a peptide doesn't work?

No. Tesamorelin has an 11-minute half-life but is effective given daily, because it triggers the body's own growth hormone release.

Related on Grey Peptides

• Half-Life Visualizer
• Split-Dose Simulator
• Semaglutide

Sources

1. Novo Nordisk. WEGOVY (semaglutide) US prescribing information, sections 10, 11 and 12.3. DailyMed version 19, effective June 18, 2026; read October 3, 2026.
2. Pfizer. GENOTROPIN (somatropin) US prescribing information, section 12.3. DailyMed version 37; read October 3, 2026.
3. Novo Nordisk. SAXENDA (liraglutide) US prescribing information, sections 12.1 and 12.3. DailyMed version 22; read October 3, 2026.
4. AstraZeneca. BYETTA (exenatide) US prescribing information, section 12.3. DailyMed version 26; read October 3, 2026.
5. Eli Lilly. ZEPBOUND (tirzepatide) US prescribing information, sections 10, 12.1 and 12.3. DailyMed version 40, effective August 28, 2026; read October 3, 2026.
6. Ascendis Pharma. SKYTROFA (lonapegsomatropin-tcgd) US prescribing information, section 12.3. DailyMed version 13; read October 3, 2026.
7. Teichman, S. L., et al. (2006). Prolonged stimulation of growth hormone (GH) and insulin-like growth factor I secretion by CJC-1295, a long-acting analog of GH-releasing hormone, in healthy adults. J Clin Endocrinol Metab, 91(3), 799-805. PMID: 16352683
8. Theratechnologies. EGRIFTA WR (tesamorelin) US prescribing information, section 12.3. DailyMed version 2; read October 3, 2026.
9. VYLEESI (bremelanotide injection) US prescribing information, section 12.3. DailyMed version 1; read October 3, 2026.
10. Gobburu, J. V., et al. (1999). Pharmacokinetic-pharmacodynamic modeling of ipamorelin, a growth hormone releasing peptide, in human volunteers. Pharm Res, 16(9), 1412-6. PMID: 10496658
11. Nielsen, M. J. F., et al. (2026). Renal or Hepatic Impairment Does Not Affect Pharmacokinetics, Safety, or Tolerability of Subcutaneous Cagrilintide. Clin Pharmacokinet, 65(7), 1087-1099. PMID: 42228334
12. He, L., et al. (2022). Pharmacokinetics, distribution, metabolism, and excretion of body-protective compound 157, a potential drug for treating various wounds, in rats and dogs. Front Pharmacol, 13, 1026182. PMID: 36588717
13. Schjoldager, B. T., et al. (1988). Oxyntomodulin: a potential hormone from the distal gut. Pharmacokinetics and effects on gastric acid and insulin secretion in man. Eur J Clin Invest, 18(5), 499-503. PMID: 3147901

Educational information, not medical advice. Each dose in this guide names its source, an approved label or a published study. None is a recommendation for you. An unapproved compound has no established safe or effective human dose, and products sold for “research use only” are not made or tested for people. Talk to a doctor before acting on anything on this site, including before you start, stop or change any medicine or dose.