Can You Use Semaglutide After 28 Days? What the Evidence Says

The pen in the fridge, four weeks later, what actually happens

Picture this: you injected your first dose on a Monday three weeks ago, life got busy, you missed last week's injection, and you're now looking at the pen wondering whether the remaining doses are still usable. It's a genuinely common situation, and the instinct to avoid waste is understandable. The honest answer, though, is that the pen's clock started the moment you pressed the plunger for the first time.

Semaglutide in its pre-filled pen format is stable when stored and used within the conditions described in the Patient Information Leaflet. Once the pen has been opened and used, the manufacturer's data supports a 28-day in-use period, after that, the medicine's integrity can no longer be guaranteed, even if it looks unchanged. You cannot tell from appearance whether the active ingredient has degraded. The NHS medicines page for semaglutide makes clear that you should follow the storage and expiry instructions in the leaflet carefully, and those instructions are specific: discard after 28 days of first use.

A practical habit worth building: when you give your first injection from a new pen, write the date directly on the pen's label with a fine marker. That takes under a minute and removes all ambiguity four weeks later. It sounds obvious, but it is easy to lose track, particularly during a dose-escalation phase when pens are changing. If you are also unsure about whether to inject before or after a meal, that is worth getting clear at the same time so your routine is consistent from the start.

If you missed a dose within the 28-day window, the question of timing is covered separately in guidance on injection intervals, but crossing the 28-day boundary is a different issue entirely, and the answer there is clear: the pen should not be used.

Why the limit exists, and why it matters more than you might think

The 28-day in-use expiry is not arbitrary caution. It reflects the stability data submitted to the MHRA as part of the authorisation process for Wegovy. Biologics like semaglutide are sensitive to conditions that are difficult to control once a pen is in regular use: repeated removal from the fridge, minor temperature fluctuations, light exposure. The peptide structure that makes semaglutide effective can degrade in ways that are invisible to the naked eye.

A degraded dose is not simply a less effective dose. Injecting a compromised medicine can, in some circumstances, trigger injection-site reactions or other adverse effects. More practically, if the active ingredient has broken down, you are not receiving the treatment your prescriber intended, which matters when your clinical review and any dose adjustment is based on the assumption that you have been taking a consistent, effective amount.

The Wegovy Summary of Product Characteristics on the eMC is the authoritative source for the exact storage instructions, including the permitted room-temperature period during those 28 days. It is worth reading that section once, not because it is complicated, but because the specific figures there supersede any general guidance you may have read elsewhere. Storage conditions interact with the in-use limit; a pen left at room temperature for longer than permitted may reach the end of its safe life before day 28.

If you have any doubt about whether a dose falls inside or outside the safe window, speaking to a pharmacist or your prescriber is the right move. That is not an overreaction, it is exactly what clinical support is for.

What happens to unused doses, and what this means for your schedule

Wegovy pens are designed so that each pen contains four weekly doses. In a straightforward four-week cycle, day 28 and dose four coincide neatly. But treatment rarely runs in a straight line: illness, travel, a missed injection, or a brief pause recommended by a prescriber can all create a gap between reality and the ideal schedule.

The key point is that the pen does not carry a "remaining doses" counter that pauses the clock. Four doses, used on days 1, 8, 15 and 22, leave the pen empty on approximately day 22, no conflict with the limit. But if dose two slipped to day 11, dose three to day 18, and dose four has still not been given by day 28, that fourth dose should not be used, and a new pen opened instead. That means a wasted dose, which is frustrating, and it is worth raising with your prescriber to understand why the schedule drifted and whether anything needs adjusting.

People sometimes ask whether this affects how often they can inject, a question that touches on the broader rules around injection frequency. The short answer is no: the once-weekly schedule and the 28-day in-use limit are separate considerations, and neither creates a loophole in the other.

Eligibility for semaglutide treatment (including BMI thresholds and the weight-related conditions that qualify) is assessed at consultation, before any pen is dispensed. If you are exploring whether Wegovy might be right for you, the criteria are explained clearly so you can approach a consultation with a realistic picture.

When to contact your prescriber rather than making the call yourself

The 28-day rule has one unambiguous answer. A few related situations are less clear-cut, and those are worth flagging so you know when to reach out rather than assume.

If you are mid-titration and a pen expires before you have completed the dose level your prescriber planned, let them know. Depending on how far through the schedule you were, they may want to keep you at the current dose level for another cycle before escalating, or they may advise something different, either way, that is a clinical call, not one to make on your own.

Similarly, if a pen was stored outside the recommended conditions (left in a hot car, for instance, or frozen accidentally) the 28-day limit becomes academic; that pen should not be used at all, and your prescriber should know so your supply can be reviewed.

Questions about timing, storage and what to do in unusual situations are exactly the kind of thing our prescribers at nume deal with regularly. Aftercare is part of the service, available seven days a week, and there is no such thing as a question too small to ask. If you are at the stage of considering whether to start treatment, check your eligibility through a free consultation and a prescriber will go through your situation with you properly.