Reconstituted semaglutide is generally stable for about 28 days when stored at 2 to 8 degrees Celsius in bacteriostatic water, kept upright, and shielded from light. That 28 day window is not a chemistry estimate of when the molecule degrades. It is the in-use shelf life that matches how long the preservative in bacteriostatic water can be relied on to suppress microbial growth in a punctured multi-dose vial. Past day 28 the molecule is usually still present, but the antimicrobial margin is gone and the vial should be discarded.
For practical purposes treat 28 days as the in-use shelf life of any peptide reconstituted with standard 0.9 percent bacteriostatic water and stored refrigerated. This matches the in-use shelf life Novo Nordisk publishes for Ozempic and Wegovy pens, both of which use semaglutide. Ozempic pens are labeled for use within 56 days after first use when kept refrigerated, and Wegovy single-dose injectors are single-use. The 28 day figure is the conservative number used across compounded and research-grade reconstitutions because the actual concentration of benzyl alcohol in the user-prepared vial is not certified.
For unreconstituted, lyophilized semaglutide, the shelf life is much longer. A sealed vial kept in the freezer is typically stable for 18 to 24 months or longer when stored according to the supplier specification. The clock only starts the moment you add water.
Lyophilized semaglutide is a dry, freeze-dried powder sealed under vacuum. In that state it is extremely stable. Water is what changes the math. The moment you reconstitute, two things happen at the same time. First, the peptide goes into solution and is now exposed to hydrolysis, temperature, light, and physical shear every time you draw a dose. Second, the vial gets punctured by a needle every time you draw, which creates a path for environmental microbes.
The first issue is slow at refrigerator temperatures. The second issue is fast at any temperature unless the diluent suppresses microbial growth. The diluent of choice for multi-dose reconstitution is bacteriostatic water, which is sterile water mixed with 0.9 percent benzyl alcohol. That 28 day window comes from the antimicrobial duration of benzyl alcohol at 0.9 percent in a typical 2 to 5 mL volume, not from the chemistry of the peptide.
If a vial was instead reconstituted with plain sterile water or saline, the in-use shelf life drops to roughly 24 hours and the vial should be treated as single use. That is the trade. The preservative is what makes a multi-dose vial work.
Benzyl alcohol is a small molecule that disrupts bacterial membranes at the concentrations used in pharmacy-grade bacteriostatic water. It does not sterilize. It does not kill every microbe on contact. What it does is keep low-level contamination from amplifying inside the vial over the days and weeks the vial is in use. The 0.9 percent concentration is well below toxicity thresholds for adult subcutaneous use and above the minimum effective bacteriostatic concentration for the common environmental contaminants that show up in clinical and research literature.
Benzyl alcohol has known issues in neonates and is contraindicated for infants, which is why pediatric and obstetric formulations avoid it. For adult subcutaneous peptide reconstitution at sub-mL draw volumes the total exposure is tiny. The reason to know any of this is that it explains why you cannot just substitute saline and assume the vial is still a multi-dose vial. The chemistry of the preservative is the whole point.
Properly reconstituted semaglutide should be a clear, colorless to faintly opalescent solution. The signs that something has gone wrong are visual and you should check before every draw:
At the 28 day mark the cleanest behavior is to discard the remaining solution and reconstitute a fresh vial. The half-life of semaglutide is about 165 hours, so even if you skipped a dose during the transition between vials, you would still be inside the effective window from the prior dose. If you are running a titration schedule, this is the easiest time to step up or hold the dose, because you are starting fresh and the concentration in the new vial can be calibrated to the next step.
For volume planning, log how many doses you actually drew from the prior vial and compare it to what the math predicted. If your vial ran out before day 28, your reconstitution volume was too low for your dose, and the next vial should be reconstituted with more bacteriostatic water. If the vial had a lot of solution remaining at day 28, the next vial can be reconstituted with less water to produce a higher concentration and longer-lasting vial. The reconstitution calculator handles this math for you.
The 28 day figure assumes a typical 0.9 percent benzyl alcohol concentration. Some research-context users use 2 percent or 3 percent benzyl alcohol bacteriostatic water, which extends the antimicrobial duration but raises the total benzyl alcohol exposure. The published literature on this is thin and the formulations are not standardized. Most people running protocols do not need to optimize this variable. Buying pharmacy-grade 0.9 percent bacteriostatic water from a reputable source and respecting the 28 day window is the right default.
Freezing a reconstituted peptide does not extend the window. Freeze-thaw cycles physically damage peptides through ice crystal formation and the local concentration changes at the freezing front. Reconstituted vials should be refrigerated, not frozen. Lyophilized (still-dry) vials are the ones that can go in the freezer for long-term storage. See peptide storage: fridge, freezer, room temp for the full storage matrix.
Adding more bacteriostatic water mid-vial does not reset the clock. The clock started at first puncture. Topping up the diluent does not change the cumulative microbial exposure and does not restore preservative concentration in any meaningful way once the vial has been in use.
The cleanest way to think about this is that the 28 day in-use shelf life is the constraint and you optimize concentration and dose around it. If you finish vials too early, decrease concentration. If vials sit unused past day 28, decrease vial size or get fewer vials per shipment.
Reconstituted semaglutide lasts about 28 days at 2 to 8 degrees Celsius when reconstituted with 0.9 percent bacteriostatic water. The constraint is the preservative duration, not the peptide chemistry. Check every draw for cloudiness, color change, or particulates. Discard at day 28. Do not freeze reconstituted solution. Plan vial concentration and dose so you finish near day 28, not before and not long after.
For the rest of the protocol logistics see the Peptide AI FAQ, the reconstitution calculator, and the semaglutide reference page. For deeper background on stability of GLP-1 peptides in solution, the published literature is searchable at PubMed.
Peptide AI Stack Intelligence logs every dose, runs the reconstitution math, and tracks days remaining on each vial. Free on iOS and Android.