How Long Does a Wegovy Pen Last After Opening?

The 28-day limit: where it comes from and why it is fixed

Semaglutide is a biological medicine, a protein-based molecule that degrades when exposed to fluctuating temperatures and light over time. The 28-day post-opening window is not a manufacturer's estimate of convenience; it is a regulatory stability figure derived from the product's clinical and pharmaceutical testing, and it is stated in the Mounjaro and Wegovy SmPCs on the eMC as well as in the leaflet that comes inside the box. The specific figure for Wegovy is: once out of the fridge, keep below 30°C and use within 28 days, then discard.

The practical consequence is simple. If you take your first dose on a Monday, the pen must be gone (either used up or disposed of) no later than the Monday four weeks later. It does not matter that the pen contains further doses; the clock started when you opened it. A pen left unused for 29 days is out of specification, regardless of how carefully it was stored.

There is also a decision embedded in this: if you are titrating slowly or your prescriber has paused your dose, plan ahead so pens align with your schedule. That is a conversation worth having at your next clinical review, not something to work around independently. For a broader look at how long Wegovy lasts in different scenarios, our full guide covers both opened and sealed pens.

Temperature, light and the one check that takes under a minute

Two environmental factors degrade semaglutide faster than the 28-day window assumes: heat above 30°C and direct sunlight. A pen left in a hot car, on a sunny windowsill, or in a bag placed against a laptop for several hours may be compromised before the 28 days are up. The solution is not complicated, but it does require a habit.

Before each weekly injection, hold the pen up to the light and look at the solution in the cartridge window. It should be clear, colourless to slightly yellow, and free from visible particles or cloudiness. If it looks different from when you first opened it (darker, cloudy or containing floating specks) do not inject it. Contact your prescriber or call 111 for guidance on getting a replacement.

That single visual check, done in the 30 seconds before you inject, is the most reliable early warning you have. The NHS medicines page for semaglutide reinforces this: inspect the solution before every use and do not use a pen that appears discoloured or contains particles.

Unopened pens, meanwhile, should stay in their original packaging in the fridge door, not the freezer, not pressed against the back wall where ice can form. If you want to read about how long unopened Wegovy pens last, that guide covers fridge life, travel exceptions and what to do if a pen has been left out overnight.

What an out-of-window pen means for your results

This is the decision point most people are quietly weighing when they search this question. Using a degraded pen is unlikely to cause obvious harm in most cases, but it may mean your dose is lower than intended, and an unexpectedly reduced dose can feel like the medicine has stopped working, leading to unnecessary dose increases or treatment changes. The timeline for when Wegovy starts working already extends to several weeks; you do not want to extend that further by injecting a pen that is outside its stability window.

There is a cost consideration too. Discarding a pen with doses remaining feels wasteful, and for anyone thinking carefully about the cost of private treatment, that is understandable. Wegovy's private cost in the UK varies by dose and provider, and losing even part of a pen is frustrating. The honest answer is that the 28-day rule exists because the alternative (using compromised medicine) costs more in the end, both to your health outcomes and potentially to your treatment timeline.

If you are losing doses regularly because your schedule does not align well with the four-weekly pen cadence, raise it at your next clinical review. Adjusting the exact day of the week you inject, or the timing of repeat orders, can close that gap. Our prescribers at nume review these practicalities as part of ongoing aftercare, available seven days a week.

Disposal, travel and edge cases

A used or out-of-date Wegovy pen should not go in the household bin. Return it to a pharmacy for safe disposal, or use a sharps container if you have one. Do not recap the needle and leave it loose in a bag.

Travelling with Wegovy adds a layer of planning. Most airlines allow temperature-sensitive medicines in hand luggage with a pharmacist's letter; checked baggage holds can freeze. A compact cool wallet (not a block of ice) keeps the pen below 30°C without risking freezing. The 28-day room-temperature window actually makes short holidays straightforward: if you have opened the pen, it can travel unrefrigerated for the duration of a typical trip, provided temperatures stay below 30°C. For anything longer, or if you are unsure, check the guidance in your Patient Information Leaflet and speak to your prescriber before you fly. Questions about how each Wegovy shot works and how long its effects last are also worth reviewing if you are planning time away from your normal routine.

If you are considering starting treatment or have questions about whether Wegovy is the right option for your circumstances, start your free consultation with a GPhC-registered prescriber at nume, every review is carried out by a real clinician, the same day.