"Lyophilized" is on almost every research-peptide vial, and it does most of its work as reassurance: it sounds pharmaceutical. It is a real pharmaceutical process with a precise meaning, a known set of failure points and a regulator's inspection guide behind it. This page sets out what the word means, what the process does and does not do to a peptide, and how the handful of approved peptide drugs that are actually sold as lyophilized powders are labelled — which shows, by contrast, what a research vial leaves unstated. Storage temperatures from the peptide manufacturers' own handling guides are on our peptide storage page; the vial, stopper and seal are on our peptide vials page. How brands appear on this site is on our disclosure page.
What the word means
FDA's Guide to Inspections of Lyophilization of Parenterals (July 1993), written as reference material for its investigators, gives the definition: "Lyophilization or freeze drying is a process in which water is removed from a product after it is frozen and placed under a vacuum, allowing the ice to change directly from solid to vapor without passing through a liquid phase." It describes "three separate, unique, and interdependent processes":
| Stage | What happens | What can go wrong |
|---|---|---|
| Freezing | The filled solution is frozen solid in the vial | If the product is not held below its eutectic (or collapse) temperature, it is not fully solid when drying starts |
| Primary drying (sublimation) | Under vacuum, ice turns directly to vapour; most of the water leaves | Too warm, and the frozen structure softens — the cake collapses or "melts back" |
| Secondary drying (desorption) | Temperature is raised to remove water still bound to the solid | Stopped too early, residual moisture stays high and shortens shelf life |
The guide says it is "desirable after freezing and during primary drying to hold the drying temperature (in the product) at least 4-5° below the eutectic point", and that a manufacturer cannot evaluate an abnormal cycle "without knowing the eutectic point". Every one of those parameters is specific to a formulation, which is why the same peptide can be freeze-dried well by one manufacturer and badly by another.
Why peptides are made this way
An older FDA technical guide on the same subject (Lyophilization of Parenterals, ITG 43, 1986) puts the reason in one line: "Products are manufactured in the lyophilized form due to their instability when in solution." Water is the medium for the reactions that degrade peptides — hydrolysis of the backbone, deamidation of asparagine and glutamine, oxidation of methionine and cysteine. Take the water away and those reactions slow to a crawl, which is why a dry cake can be kept for years while the same peptide dissolved lasts days to weeks.
The trade-off is that lyophilization moves the stability question rather than removing it. The 1986 guide says "generally lyophilized products have short expiration dates", and that expiry should be justified "based on batches with the higher moisture content". The 1993 guide calls moisture "an obvious concern" and expects manufacturers to set "moisture specifications for both product release and stability". A research vial carries no moisture figure on its label, and a certificate of analysis reports one only if the laboratory was asked to measure it — see what certificates do and do not cover on our COA page.
Not a sterilising step
A dry powder feels cleaner than a liquid, and that intuition is wrong. In pharmaceutical manufacturing the solution is sterile-filtered and filled before it is dried, and the dryer is loaded under the same barriers used for filling — because the vials go into the chamber with their stoppers only partly seated so vapour can escape. The 1993 guide is direct: "the contents of the vial are subject to contamination until they are actually sealed." The stoppers are pushed home inside the chamber at the end of the cycle.
Freeze-drying kills nothing reliably. Bacteria and spores can survive it — freeze-drying is also how laboratories preserve bacterial cultures — and endotoxin, the fragment of dead bacteria that causes fever, is not removed by drying at all. So a lyophilized vial is exactly as sterile and endotoxin-free as the process that filled it. That is the part of the manufacturing a buyer cannot see and a cake's appearance cannot reveal; why endotoxin testing matters is covered on our peptide testing page.
The 1986 guide adds a detail that bears on testing rather than use: when FDA's analysts reconstitute a product to check it, they are told to use Sterile Water for Injection even if the label names bacteriostatic water, because the preservative "may kill some of the vegetative cells if present as contaminants" — it could hide the contamination the test is looking for.
What the powder can and cannot show
FDA's investigators look at "the presence of correct volume of cake and the cake appearance". A well-dried cake is usually a uniform, intact plug. The failure the guide names is meltback, "a form of cake collapse … caused by the change from the solid to liquid state" when sublimation is incomplete; a shrunken, glassy or partly liquefied deposit points to a drying problem or to moisture getting in afterwards.
What the cake cannot show is everything a buyer actually needs to know: which peptide it is, how much of it there is, how pure it is, whether it is sterile, and whether it carries endotoxin. A vial with very little peptide and no bulking agent can look nearly empty — a few milligrams leave a thin film — which says nothing about whether the stated quantity is present, as our vials page explains. A collapsed cake is a reason for concern; a perfect one is not evidence of anything beyond the drying.
What four approved lyophilized peptides show
Most approved peptide drugs are not lyophilized at all. The semaglutide, tirzepatide and liraglutide pens, teriparatide, bremelanotide and elamipretide are labelled as ready-to-use solutions — their formulations are stable enough in water. The approved peptides that are sold as freeze-dried powder are few, and their labels answer the questions a research vial leaves open. Read from the current labels via the openFDA label endpoint on 2026-09-27:
| Product | Peptide | Diluent the label supplies | After mixing | Unopened vial |
|---|---|---|---|---|
| Egrifta SV (2 mg) | Tesamorelin | 0.5 mL Sterile Water for Injection | Use immediately; discard unused solution; do not refrigerate | Room temperature, protected from light |
| Egrifta WR (11.6 mg) | Tesamorelin | 1.3 mL Bacteriostatic Water for Injection (to 8 mg/mL) | Doses for 7 days; discard 7 days after mixing | Room temperature, protected from light |
| Pregnyl (10,000 units) | hCG | Solvent with 0.9% benzyl alcohol | Refrigerate; discard after 60 days | Controlled room temperature |
| Novarel (5,000 or 10,000 units) | hCG | Bacteriostatic Water for Injection | Refrigerate; use within 30 days | Room temperature |
Three things in that table matter for reading research vials. The in-use life belongs to the product, not the molecule: the same tesamorelin is thrown away at once in one presentation and kept a week in the other, and the label states the two "are not substitutable". The diluent is part of the approval: the preservative in bacteriostatic water is what lets a multi-dose vial be re-entered, and the labels pair each powder with a named diluent. The formulation is stated: Egrifta SV lists sucrose and histidine among its inactive ingredients; Novarel lists 100 mg of mannitol and phosphate buffers in each 5,000-unit vial — excipients chosen to protect the peptide through drying and to form a stable cake. A research vial labelled only with a peptide name and milligram figure states none of this, which is why no in-use figure from an approved label transfers to it. How the diluent choice plays out, and what the bacteriostatic-water label itself allows, is on our bacteriostatic water page; the procedure and arithmetic of dissolving a cake are on our reconstitution page.
Also worth noticing: all four approved powders are stored unopened at room temperature, not in a freezer. That is a finding from their own stability studies, not a general rule — the peptide manufacturers' handling guides for research material recommend −20 °C, as our storage page records — but it shows that "keep it frozen" is a property of a particular product's data, not of lyophilization itself.
What this page does not do
It does not say how any particular research vial was made, whether it was freeze-dried in the vial or filled as bulk powder, or what it contains; none of that can be read off a label or a cake. It does not give storage or in-use times for research material, which have no stability data behind them. And it does not describe how to prepare an injection. Questions about using any of these compounds belong with a clinician.
Sources and dates
Read 2026-09-27: FDA, Guide to Inspections of Lyophilization of Parenterals (7/93) and Inspection Technical Guide 43, Lyophilization of Parenterals (1986); current US labels for Egrifta SV and Egrifta WR (effective 2026-07-29), Pregnyl (2025-12-02) and Novarel (2025-08-27), dosage forms, description, dosage and administration and how-supplied sections; the dosage-forms sections of the Wegovy, Mounjaro, Victoza, Saxenda, Forteo, Vyleesi and Forzinity labels, all via the openFDA drug label endpoint. Corrections go to the contact page.