How Long Does a Wegovy Pen Last Once Opened?

What does the 28-day rule actually mean in practice?

Each Wegovy pen holds enough medicine for four weekly injections. The moment you give yourself that first dose, a 28-day clock starts. It does not reset when you move on to the next dose within the same pen. So if week one goes smoothly but week two is delayed by a few days, that delay still counts against the same 28-day total. The full breakdown of how long a Wegovy pen lasts across its four doses is worth reading if you are planning ahead around travel or holidays.

The 28-day figure is not arbitrary. Semaglutide, the active ingredient in Wegovy, is a protein-based molecule that gradually loses stability once it is no longer kept continuously cold. Manufacturers run stability studies to establish the longest period at which the medicine remains safe and effective at room temperature, and 28 days is where the data draws that line. Going a day or two over is not something to guess at; the right answer is to discard the pen and speak to your prescriber before opening a new one. You can read more about Wegovy storage once opened for a fuller look at the temperature and handling rules.

One practical question our prescribers hear regularly: does it matter which order I store the pen in the fridge door versus the main shelf? The short answer is that the fridge door is fine for an unopened pen, but the door tends to see more temperature fluctuation each time it is opened. Once you have started a pen, keeping it away from the door and away from the freezer compartment is the tidier habit. The rules for an unopened pen in the fridge are slightly different, so check those separately.

Can a Wegovy pen go back into the fridge after it has been used?

Yes. Unlike some injectables, Wegovy does not have a strict one-way rule where refrigeration after opening is forbidden. You can move the pen between fridge and room temperature freely, as long as neither condition exceeds its limit: up to 30°C at room temperature, 2–8°C in the fridge. What does not change is the 28-day ceiling from the date of first use. Moving the pen back into the fridge does not reset or pause that clock.

This flexibility matters because many people keep the pen in the fridge most of the week and take it out an hour or so before injecting, letting it reach room temperature to reduce injection discomfort. That is a sensible approach. Just be consistent about tracking the date you first opened it, whether that means writing it on the label with a marker or setting a reminder on your phone.

If you are unsure whether your pen has been stored correctly, for instance after a power cut or an accidental freezer placement, the NHS patient information for semaglutide sets out what to look for and the NHS semaglutide medicines page gives clear guidance on when a pen should not be used. A pen that has been frozen or left in a hot car should be discarded rather than injected. If that leaves you without your next dose, contact your prescriber or pharmacy rather than skipping the injection without advice.

What happens if you inject from a pen that is past 28 days?

The honest answer is that the data on what happens beyond 28 days is limited, which is exactly why the limit exists. Protein-based medicines like semaglutide can degrade in ways that are not visible to the naked eye: the liquid may look clear and unchanged while the active molecule has broken down enough to be less effective, or to carry a small risk of an immune reaction to degraded protein.

If you realise you have injected from an out-of-date pen, the most likely outcome is a reduced or absent therapeutic effect for that week rather than an acute harm, but that is not a reason to take the risk routinely. Report any unexpected reaction via the MHRA's Yellow Card scheme, which allows patients and healthcare professionals to flag suspected problems with medicines.

It is also worth thinking about what led to the pen going past its date. If doses are being missed or delayed regularly, that is a conversation worth having with your prescriber. Difficulties with the injection itself are another common reason pens get used out of sequence, and there are practical fixes for most of them. For a broader look at the treatment before committing, the Wegovy overview covers the licensed uses, the evidence and what the treatment journey looks like.

What should you do with an expired or unused pen?

Do not put pens in household rubbish or flush them. Used injection pens are clinical waste. The right disposal route is a sharps container, which your local pharmacy or GP surgery can advise on returning once full. If you find yourself with several unused pens because your dose has been changed or treatment paused, speak to your prescriber first. Stopping or pausing Wegovy is a clinical decision, not just a storage problem.

If cost is a factor in how you are managing your pens, it is worth understanding what you are paying for in full. The Wegovy price comparison guide sets out the market context clearly. A private prescription through a properly registered pharmacy includes clinical oversight at every step, which is the thing that keeps your treatment safe and your pens arriving when they should.

When you are ready to explore treatment properly, starting a free consultation with our prescribers is the first step. A named clinician, not an automated system, reviews your answers the same day.