What are the side effects of expired semaglutide — and is it safe to use?

Step 1: understand what expiry does to the medicine

Semaglutide is a biological molecule (a GLP-1 receptor agonist) and biological molecules break down over time. The expiry date on your Wegovy pen is the manufacturer's guarantee that the medicine inside meets its licensed potency and purity standards up to that day. Beyond it, Novo Nordisk makes no such guarantee.

In practical terms, degradation may mean a reduced dose reaching your system even though you inject the full volume. That matters in two ways. First, you may experience little or no appetite reduction, which some people mistake for the medicine stopping work rather than the pen being past its date. Second, degraded proteins can occasionally trigger an immune or inflammatory response, a reaction your body would not have had to intact medicine. The NHS patient information for semaglutide is clear that the medicine should be used before the expiry printed on the label and pen.

Heat, light and improper storage accelerate degradation. A pen left in a gym bag on a warm day, rather than in the fridge door at home, may already be compromised before the printed date is reached. That is not a hypothetical, it is a pattern prescribers see. The rules around what to do with an expired Wegovy pen cover disposal too, because an expired pen should not simply be re-used.

Step 2: recognise which side effects are more likely with a degraded pen

The standard side-effect profile of semaglutide is well-documented and predominantly gastrointestinal: nausea, vomiting, diarrhoea, constipation, indigestion and burping are the most common, usually most noticeable in the first weeks of treatment or after a dose increase. You can read the full pattern of semaglutide injection side effects in detail.

With an expired pen, that picture shifts in a few ways. Reduced potency may mean milder GI effects, which sounds like a benefit but is really a sign the medicine is not doing its job. Conversely, degradation products (the fragments the molecule breaks into) could irritate the injection site more than intact semaglutide would, producing redness, swelling or a lump that persists longer than usual. Injection-site reactions that seem disproportionate to a normal dose are a specific flag worth noting.

There is also the question of unpredictability. With in-date medicine in proper storage, the side-effect timeline is reasonably predictable, most people want to know how long after a semaglutide injection side effects start, and the answer is consistent enough to give. With degraded medicine, that consistency goes. Reactions may be earlier, later or different in character. That unpredictability itself is a reason not to use an expired pen.

Step 3: know when to get medical help immediately

Certain symptoms warrant urgent attention regardless of whether the pen was in date, but if you have used an expired pen and experience any of the following, do not wait to see if things settle.

Severe, persistent abdominal pain that reaches into the back, with or without vomiting, could indicate acute pancreatitis. In January 2026, the MHRA issued a Drug Safety Update confirming that acute pancreatitis is a recognised, if infrequent, side effect of GLP-1 medicines; the regulator's advice is to seek urgent medical help for this symptom pattern. Signs of a serious allergic reaction (rapid swelling of the face, lips or throat, difficulty breathing, a widespread rash) also need emergency care. Severe dehydration from vomiting or diarrhoea that you cannot manage at home is a third reason to call 111 or go to A&E rather than waiting.

If you think the pen you used was counterfeit rather than simply expired, tell the medical team that when you call, it affects how they assess you. The MHRA has documented cases where fake pens contained insulin rather than semaglutide, causing dangerous hypoglycaemia. Report any suspect medicine via the Yellow Card scheme, which accepts reports from patients as well as clinicians.

Step 4: what to do next if you have an expired pen

Do not use it. That is the short answer. Dispose of used and expired pens safely, a sharps container is the correct method for pen needles; check with your local council or pharmacy for collection points.

If you are on treatment and a pen has expired before you could use it (because of a prescription delay, a gap in supply or a missed collection) contact your prescribing service before injecting. At nume, prescribers are available for aftercare queries seven days a week, and the right call is always to check rather than assume an expired pen is fine.

For anyone not yet on treatment, Wegovy for weight management requires a private prescription following a clinical assessment. Our prescribers talk through storage, timing and what to expect from treatment as a standard part of that process. You can also look at the difference between Ozempic and Wegovy if you are unsure which semaglutide product applies to your situation, the two are licensed for different conditions and are not interchangeable. If you are thinking about what Wegovy costs privately, that page sets out what a legitimate prescription service includes. And if you want to compare the broader side-effect picture for weight-loss semaglutide, that resource covers the full licensed context.

When you are ready, check your eligibility with our prescribers, the consultation is free, and the clinical assessment covers everything relevant to your situation, including any questions about storage, missed doses or pen condition.