How Long Does Wegovy Last Once Opened — and What Happens If You Keep It Too Long?

The myth: 'opened' means partially used

Most medicines with an 'in-use' storage period can be drawn from multiple times, insulin vials are a good example. Wegovy works differently. Each pre-filled pen contains a single dose, used in one injection and then discarded. There is no second draw, no reservoir to return to the fridge, no question of how many days a vial remains viable after the first use. When manufacturers and pharmacists talk about Wegovy once 'opened', they mean once removed from refrigerated conditions and exposed to room temperature.

That framing matters because it changes the practical question. You are not managing a multi-use container. You are managing a window of time between taking the pen out of cold storage and actually injecting. If you take a pen out on Monday morning, decide to delay until Wednesday, and then forget about it until the following week, the relevant question is how many days at room temperature the pen has spent in total, not whether the needle cap has been removed.

The NHS patient information for semaglutide sets out the specific in-use storage period and temperature limits; those are the figures that govern your pen, and they are stated precisely in the leaflet that comes in the box. You can also read the NHS overview of semaglutide for a plain-English summary of storage guidance.

What temperature actually does to semaglutide

Semaglutide is a peptide, a protein-based molecule. Like most proteins, it is stable within a defined temperature range and begins to degrade outside it. Refrigeration at 2–8 °C slows that degradation almost to a standstill, which is why unopened pens are stored in the fridge. Above that range, degradation accelerates; the warmer the environment, the faster the process.

Freezing causes a different problem. Ice crystal formation can disrupt the solution's structure. A pen that has been frozen, even briefly, should not be used, the Patient Information Leaflet is explicit on this point. If you are unsure whether a pen was exposed to freezing temperatures during delivery or storage, contact your pharmacy before injecting.

Direct sunlight adds another variable. UV exposure can degrade active ingredients independently of temperature, which is why the outer carton and the pen cap are opaque. Keep the pen in its carton until you are ready to use it, and store it away from windowsills and car dashboards, both get warmer than most people assume. For a full breakdown of how shelf life compares across opened and unopened states, our guide on Wegovy after opening walks through both scenarios side by side.

A one-minute check before every injection

Before attaching the needle, hold the pen up to the light and look at the solution through the viewing window. It should be clear and colourless to slightly yellow, with no particles or cloudiness. That check takes about ten seconds and it matters: a degraded or contaminated solution will not necessarily look dramatically different, but any visible change is a hard stop. Do not inject it. Contact your pharmacy for a replacement.

Also check the expiry date printed on the pen and carton. An in-date pen kept consistently refrigerated is not the same as an in-date pen that sat in a warm bag for a fortnight. Both the expiry date and the in-use storage period apply simultaneously, whichever limit is reached first is the binding one.

If you are travelling and need to take your pen outside cold storage for longer than the permitted room-temperature window, that is a conversation worth having with your prescriber before the trip, not after. People thinking about how long each dose works in the body sometimes conflate pharmacological duration with storage duration, and if you want a clear answer to how long Wegovy lasts in both senses, the two topics are worth reading about separately.

When in doubt, ask rather than guess

Storage questions are among the most common things people ask after starting weekly injections. There is no shame in ringing your pharmacy about a pen you left out overnight: it is exactly the right thing to do. Using a compromised pen means a week without effective treatment at best, and an unpredictable injection at worst.

At nume, aftercare is available seven days a week, so if a storage question comes up on a Sunday evening before your scheduled injection day, there is someone to ask. Our prescribers also review your case before every repeat order, which means any pattern of storage difficulty or side effects gets picked up as part of routine clinical oversight, not only when something goes wrong.

For the complementary question (how long an unopened pen remains viable in the fridge) this page on unopened Wegovy storage covers the refrigerated shelf life in full. And if you are still early in treatment and want to understand how long semaglutide stays active after each dose, that is a separate timeline worth reading alongside the storage basics, particularly if you are curious about how long Wegovy takes to work from the point of your first injection.

Wegovy is a prescription-only medicine. A prescriber assesses your suitability before treatment begins, and clinical review continues at every repeat. If you are considering starting a consultation to check your eligibility, that process is free and reviewed the same day by a GPhC-registered prescriber.