How is semaglutide's long half-life achieved?

Semaglutide is a modified GLP-1 analog. A C18 fatty-acid chain with a hydrophilic spacer is attached at position 26 (Lys), which allows reversible binding to serum albumin and slows renal clearance. A second modification at position 8 protects the peptide from degradation by dipeptidyl peptidase-4 (DPP-4). Together these give an absolute subcutaneous bioavailability of about 89% and a half-life near one week.

How many units of semaglutide equal 0.25 mg?

It depends on the concentration. For a compounded 5 mg vial with 2 mL bacteriostatic water (2.5 mg/mL), 0.25 mg equals 10 units on a 100-unit insulin syringe. For a 5 mg vial with 5 mL bac water (1 mg/mL), 0.25 mg equals 25 units. For a 10 mg vial with 2 mL (5 mg/mL), 0.25 mg equals 5 units. The Ozempic pen delivers pre-measured doses that are labeled directly (0.25 mg, 0.5 mg, 1 mg, 2 mg) without requiring unit math.

How long does semaglutide stay in your system after stopping?

With a half-life of about 1 week, semaglutide is typically cleared to negligible plasma concentrations in 5 to 6 weeks after the last dose (5–7 half-lives). Pharmacological effects such as appetite suppression often taper over this window; individual experience varies.

GLP-1 receptor agonist · FDA-approved

Semaglutide

Q: What is the half-life of semaglutide?

Semaglutide has an elimination half-life of approximately one week (roughly 165 hours). This is long enough to support once-weekly subcutaneous dosing in the injectable formulations (Ozempic, Wegovy). Steady-state concentrations are typically reached after 4 to 5 weeks of consistent once-weekly dosing.

Q: How do I reconstitute a compounded semaglutide vial?

For a compounded 5 mg vial reconstituted with 2 mL of bacteriostatic water, the concentration is 2.5 mg/mL. On a 100-unit insulin syringe (1 mL = 100 units), 1 unit equals 25 mcg of semaglutide. A 250 mcg dose equals 10 units; a 500 mcg dose equals 20 units; a 1,000 mcg (1 mg) dose equals 40 units. For a 10 mg vial reconstituted with 2 mL, 1 unit equals 50 mcg, so 250 mcg = 5 units. Commercial Ozempic and Wegovy pens come pre-filled and are not reconstituted by the user.

Q: Is semaglutide FDA-approved?

Yes. Semaglutide is approved by the FDA as three brand-name medications: Ozempic for type 2 diabetes (2017), Rybelsus for type 2 diabetes as an oral tablet (2019), and Wegovy for chronic weight management (2021). Compounded semaglutide sold outside these FDA-approved products is not itself FDA-approved.

Used for: chronic weight management (Wegovy) · type 2 diabetes (Ozempic, Rybelsus)

Semaglutide is a long-acting GLP-1 receptor agonist approved by the FDA as Ozempic (2017, type 2 diabetes), Rybelsus (2019, oral tablet for type 2 diabetes), and Wegovy (2021, chronic weight management). Its defining characteristic is an approximately one-week elimination half-life that supports once-weekly subcutaneous dosing. This page is an educational reference and does not substitute for a clinician or the FDA prescribing information.

Class: GLP-1 receptor agonist (long-acting)
Elimination half-life: ~1 week (~165 h)^[1][2]
Absolute bioavailability (SC): ~89%^[3]
Tmax: 1–3 days after SC injection^[3]
Steady state reached: 4–5 weeks of weekly dosing^[3]
Dosing route: Once-weekly SC (injectable) · daily oral (Rybelsus)
Brand names: Ozempic · Wegovy · Rybelsus
FDA status: Approved (multiple indications)

Educational reference only. Semaglutide is a prescription medication. Do not initiate, modify, or discontinue treatment without a qualified healthcare professional. The FDA prescribing information for your product (Ozempic, Wegovy, or Rybelsus) is the authoritative source for dosing and safety; the citations below link to it.

Last reviewed April 22, 2026 by the Enhanced Health Editorial Team. Reviewed for source alignment, FDA-status language, and medical-claim boundaries. See the editorial policy.

What semaglutide is

Semaglutide is a structural analog of the native incretin hormone glucagon-like peptide-1 (GLP-1), modified to resist rapid enzymatic degradation and to bind reversibly to serum albumin. Two specific modifications give semaglutide its pharmacological profile:^[1][4]

Position 26 (Lys) carries a hydrophilic spacer (γ-Glu–2×OEG) linked to a C18 fatty acid. This fatty-acid tail binds reversibly to albumin, markedly slowing renal clearance.
Position 8 (Aib substitution) protects the peptide backbone from cleavage by dipeptidyl peptidase-4 (DPP-4), the enzyme that degrades native GLP-1 within minutes.

These two modifications are the reason semaglutide has a half-life measured in days rather than minutes.

Mechanism

Semaglutide selectively binds and activates the GLP-1 receptor. Downstream effects that underlie its glucose-lowering and weight-loss activity include:^[2]

Glucose-dependent insulin secretion from pancreatic β-cells (the incretin effect). Because stimulation is glucose-dependent, semaglutide rarely causes hypoglycemia on its own.
Suppression of glucagon secretion from pancreatic α-cells, also glucose-dependent.
Delayed gastric emptying, contributing to lower post-prandial glucose excursions and greater satiety.
Central appetite modulation through GLP-1 receptors in hypothalamic and brainstem regions involved in food intake and reward.

Pharmacokinetics

Key parameters are summarized in the FDA prescribing information and reviewed in the peer-reviewed literature:^[1][2][3]

Absolute subcutaneous bioavailability: approximately 89%.
Elimination half-life: approximately 1 week (roughly 165 hours), consistent across subcutaneous and oral formulations.
Time to maximum plasma concentration (Tmax): 1 to 3 days after a subcutaneous dose.
Steady state: reached after approximately 4 to 5 weeks of once-weekly dosing; this is why titration schedules in the FDA label step up the dose at 4-week intervals.
Metabolism: proteolytic cleavage of the peptide backbone and sequential β-oxidation of the fatty-acid sidechain.
Excretion: via both urine and feces; renal impairment does not require dose adjustment in the approved labels.

Storage and handling

Ozempic / Wegovy pens. Store refrigerated at 2–8 °C before first use. After first use, the pen may be kept refrigerated or at room temperature up to 30 °C for up to the period specified on the FDA label for your product. Do not freeze; discard if frozen.
Compounded vials. Handling depends on the preparing pharmacy's instructions. Compounded semaglutide commonly uses bacteriostatic water (0.9% benzyl alcohol) or sterile water. Reconstituted peptide drug products are typically stored refrigerated at 2–8 °C; specific beyond-use dates are set by the pharmacy.
General. Avoid vigorous shaking and freeze–thaw cycles. Discard solutions that are cloudy, discolored, or contain particulates.

Reconstitution math (compounded vials)

Ozempic and Wegovy pens deliver pre-measured doses labeled directly in milligrams — users of those products do not perform unit math. The math below applies to compounded semaglutide dispensed as a lyophilized powder in a vial, which must be reconstituted before first use. Math does not recommend a dose.

Example 1 · 5 mg vial + 2 mL bac water concentration = 5 mg / 2 mL = 2.5 mg/mL = 2500 mcg/mL
1 unit = 0.01 mL = 25 mcg
→ 0.25 mg (250 mcg) = 10 units
→ 0.5 mg (500 mcg) = 20 units
→ 1 mg (1000 mcg) = 40 units
→ 1.7 mg (1700 mcg) = 68 units
→ 2.4 mg (2400 mcg) = 96 units

Example 2 · 10 mg vial + 2 mL bac water concentration = 10 mg / 2 mL = 5 mg/mL = 5000 mcg/mL
1 unit = 0.01 mL = 50 mcg
→ 0.25 mg = 5 units
→ 0.5 mg = 10 units
→ 1 mg = 20 units
→ 2 mg = 40 units
→ 2.4 mg = 48 units

Example 3 · 5 mg vial + 5 mL bac water concentration = 5 mg / 5 mL = 1 mg/mL = 1000 mcg/mL
1 unit = 0.01 mL = 10 mcg
→ 0.25 mg = 25 units
→ 0.5 mg = 50 units
→ 1 mg = 100 units (full syringe)

Doses on the Ozempic titration schedule (0.25 mg, 0.5 mg, 1 mg, 2 mg) and the Wegovy titration schedule (0.25 mg → 0.5 mg → 1 mg → 1.7 mg → 2.4 mg) are drawn from the same math when working with a compounded vial — only the concentration you chose at reconstitution changes the unit count.

Frequently asked questions

What is the half-life of semaglutide?

About one week (roughly 165 hours). Steady state is reached in 4–5 weeks of once-weekly dosing.^[1][3]

How is the long half-life achieved?

A C18 fatty acid at position 26 binds reversibly to serum albumin, slowing clearance; a position-8 modification protects against DPP-4 cleavage.^[4]

How do I reconstitute a compounded semaglutide vial?

Typical: 5 mg vial + 2 mL bacteriostatic water = 2.5 mg/mL → 1 unit = 25 mcg, 250 mcg = 10 units, 1 mg = 40 units. Commercial pens are pre-filled and are not user-reconstituted.

How many units equal 0.25 mg?

At 2.5 mg/mL (5 mg in 2 mL), 0.25 mg = 10 units. At 5 mg/mL (10 mg in 2 mL), 0.25 mg = 5 units. At 1 mg/mL (5 mg in 5 mL), 0.25 mg = 25 units.

Is semaglutide FDA-approved?

Yes — as Ozempic, Wegovy, and Rybelsus. Compounded semaglutide sold outside those approved products is not itself FDA-approved.^[1][2]

How long does it stay in your system after stopping?

Typically 5–6 weeks to clear to negligible plasma concentrations (5–7 half-lives). Appetite effects taper across that window; individual experience varies.

Primary references

U.S. Food and Drug Administration. OZEMPIC (semaglutide) injection — full prescribing information. Updated label, 2023. accessdata.fda.gov — Ozempic label
U.S. Food and Drug Administration. WEGOVY (semaglutide) injection — full prescribing information. Updated label, 2023. accessdata.fda.gov — Wegovy label
Hall S, Isaacs D, Clements JN. Pharmacokinetics and clinical implications of semaglutide: a new glucagon-like peptide (GLP)-1 receptor agonist. Clin Pharmacokinet, 2018 (PubMed 29915923). PubMed
Lau J, Bloch P, Schäffer L, et al. Discovery of the once-weekly glucagon-like peptide-1 (GLP-1) analogue semaglutide. J Med Chem, 2015. (Structure and albumin-binding modification.)
Ahmann AJ, et al. Semaglutide — StatPearls. NCBI Bookshelf (NBK603723). ncbi.nlm.nih.gov — StatPearls

Log semaglutide and lab context in one private workspace

Enhanced Health keeps injection history, vial concentrations, lab trends, and Apple Health context together on iPhone. Local-first. No ads. Research/educational framing throughout.

About the app