A GLP-1 receptor agonist with an exceptionally long half-life of approximately 7 days, enabling once-weekly subcutaneous dosing. Albumin binding extends its duration far beyond native GLP-1.
Semaglutide is a glucagon-like peptide-1 (GLP-1) receptor agonist — a synthetic analog of the naturally occurring incretin hormone GLP-1. Native GLP-1 has a half-life of only 1-2 minutes due to rapid enzymatic degradation by DPP-4. Semaglutide's molecular modifications extend this to approximately 7 days, a pharmacokinetic transformation that enables once-weekly dosing.
The compound achieves its extended duration through two key structural modifications: a C-18 fatty acid chain that promotes non-covalent albumin binding (human serum albumin acts as a circulating reservoir, slowly releasing the peptide), and strategic amino acid substitutions at positions 8 and 34 that confer resistance to DPP-4 degradation.
Semaglutide's pharmacokinetic profile makes it one of the longest-acting peptides in common use. The ~7-day half-life means it takes approximately 4-5 weeks of consistent weekly dosing to reach steady-state concentration — the point at which each weekly dose is maintaining stable levels rather than building them up. This extended approach to steady state is why titration protocols start low and escalate monthly.
Semaglutide mimics native GLP-1, binding to GLP-1 receptors distributed across multiple organ systems. Its mechanism is well-characterized in published clinical literature.
Semaglutide binds to GLP-1 receptors on pancreatic beta cells, potentiating glucose-dependent insulin secretion. This incretin effect is glucose-dependent — it is most active when blood glucose is elevated and diminishes as glucose normalizes.
GLP-1 receptor activation slows gastric emptying, extending the time food remains in the stomach. This pharmacological effect on gastric motility is one of the mechanisms behind the satiety effects observed with GLP-1 agonists.
Semaglutide's C-18 fatty acid sidechain binds reversibly to circulating albumin. This albumin reservoir effect is the primary pharmacokinetic mechanism responsible for the ~7-day half-life, as the bound peptide is shielded from renal clearance and enzymatic degradation.
Semaglutide has one of the most well-characterized pharmacokinetic profiles of any peptide in current use, supported by extensive clinical trial data.
| Parameter | Value |
|---|---|
| Half-Life | ~161 hours (~7 days) |
| Bioavailability (SubQ) | ~89% |
| Time to Peak (Tmax) | ~48 hours (1–3 days) |
| Duration of Activity | ~504 hours (~21 days) |
| Time to Steady State | 4–5 weeks |
| Route | Subcutaneous |
| Clearance | ~0.05 L/h |
The 161-hour half-life means that after a single injection, approximately 50% of the dose remains in circulation after one week. With consistent weekly dosing, concentrations accumulate over 4-5 weeks until the amount eliminated per week equals the amount injected — this is steady state.
Peak concentration occurs approximately 48 hours post-injection, with a gradual decline over the following 5 days. The peak-to-trough ratio at steady state is relatively narrow due to the long half-life, meaning blood levels remain relatively stable throughout the week once steady state is achieved.
At steady state, semaglutide's trough level (just before the next dose) is approximately 70-80% of the peak level. This narrow peak-to-trough range is pharmacokinetically favorable — it means the compound maintains consistent receptor occupancy throughout the dosing interval.
Semaglutide protocols are characterized by a gradual dose titration schedule. The following represents the most commonly observed pattern, with each step lasting approximately 4 weeks.
The gradual dose escalation is pharmacokinetically strategic. Starting at the full dose would cause rapid receptor saturation before the body has adapted to GLP-1 agonism. The 4-week steps roughly correspond to the time needed to reach steady state at each dose level, allowing physiological adaptation before escalating.
Pre-filled pens (branded semaglutide) do not require reconstitution. Compounded lyophilized semaglutide requires the following preparation.
Compounded semaglutide vials vary widely — common sizes include 2 mg, 5 mg, and 10 mg. Note the total mg on the vial label before calculating water volume.
Clean the rubber stoppers on both the semaglutide vial and bacteriostatic water vial with alcohol swabs. Allow to air dry.
Use the Milligram calculator to determine the exact bacteriostatic water volume for your vial size and desired dose. For example: a 5 mg vial + 2.5 mL bac water = 2 mg/mL concentration.
Draw the calculated volume of bacteriostatic water and inject it slowly along the inside wall of the semaglutide vial. Do not spray directly onto the powder.
Swirl the vial gently — do not shake. Semaglutide should dissolve into a clear, colorless solution. If particles remain, continue gentle swirling. Do not use if the solution is cloudy.
Store reconstituted semaglutide at 2–8°C (refrigerator). Reconstituted with bacteriostatic water, the solution is generally stable for up to 28 days. Mark the reconstitution date on the vial.