Semaglutide subcutaneous injection: what the evidence says and how it works in practice

What the STEP 1 trial established about the weekly subcutaneous dose

The most cited evidence for semaglutide as a weight-management injection comes from the STEP 1 trial, a 68-week study in adults with obesity or overweight plus at least one weight-related condition, published in the New England Journal of Medicine. Participants receiving 2.4mg semaglutide weekly lost an average of roughly 15% of their body weight, compared with about 2.4% in the placebo group. That gap is large enough that NICE concluded the medicine represents a cost-effective option for eligible adults, you can read the full recommendations in NICE's appraisal of semaglutide for weight management (TA875).

What the trial also illustrated is that the subcutaneous route is central to how semaglutide achieves these results. Injecting beneath the skin allows the medicine to absorb gradually into the bloodstream, producing a relatively stable plasma concentration that tracks the weekly dosing rhythm. This sustained exposure is what sustains appetite suppression between doses, a quite different pharmacological picture from a tablet taken and cleared the same day.

The titration schedule matters too. Starting low and stepping up slowly isn't bureaucracy; it's how the gastrointestinal side-effect burden is kept manageable enough for people to stay on treatment long enough to see the weight-loss results the trials recorded.

Where on the body, and how the injection actually works

Three injection sites are licensed: the abdomen (at least a couple of centimetres from the navel), the front of the thigh, or the outer upper arm. Rotating between sites (and between spots within each site) reduces the chance of local skin reactions such as redness or firmness building up in one area.

The injection itself is subcutaneous, meaning the needle goes into the fatty layer just beneath the skin, not the muscle below it. Wegovy pens are pre-filled and auto-injecting: you hold the pen at a 90-degree angle to the skin, press to activate, and hold for a few seconds until the injection is complete. Our detailed step-by-step guide to injecting semaglutide walks through the full process, including what to do if the injection site bleeds slightly or if you notice the window hasn't turned.

A question our prescribers hear most weeks is whether it matters which site you choose. The short answer is: the absorption difference between sites is clinically small, so the best site is whichever one you can reach comfortably and rotate reliably. Consistency in your rotation pattern matters more than the starting site.

Used needles should go directly into a sharps container rather than a household bin. Most councils offer sharps collection; your pharmacist can advise on local arrangements.

Managing the side-effect profile specific to the injection route

The side-effect profile of the Wegovy subcutaneous injection is dominated by gastrointestinal symptoms: nausea is the most commonly reported, followed by vomiting, diarrhoea, constipation and indigestion. These are not unique to the injection route (they reflect semaglutide's mechanism of action on the gut) but the titration schedule is specifically designed to give the body time to adjust at each dose level before moving higher.

For most people, nausea peaks in the days after starting or after a dose step and then settles. Eating smaller portions, avoiding rich or fatty meals around injection day, and staying well hydrated all help. If symptoms remain severe or you're unable to keep fluids down, contact your prescriber or seek medical advice promptly, dehydration can affect kidney function.

One serious but uncommon risk is acute pancreatitis. In January 2026, the MHRA issued a Drug Safety Update highlighting this for GLP-1 medicines as a class: severe stomach pain that spreads to the back, with or without vomiting, needs urgent medical attention. You can report any suspected side effects directly through the MHRA Yellow Card scheme. The NHS patient information for semaglutide covers the full list of side effects and what action to take for each, it's worth reading before you start.

Storage and what 'subcutaneous' means for how you travel with Wegovy

Refrigeration is the baseline requirement: Wegovy pens should be kept at 2–8°C, which in most homes means the fridge body rather than the door (where temperature can fluctuate). Once a pen is in use it may be kept at room temperature for a limited period, the precise window is specified in the patient information leaflet that comes with your pen, and it's the document to follow rather than any general figure online.

Travelling with a subcutaneous medicine does need a little planning. A cooling wallet or small travel fridge keeps the pen within the required temperature range on longer journeys. Airlines will ask for a letter confirming the prescription; our guide to storing semaglutide safely covers both fridge and travel scenarios in detail.

People sometimes ask about the non-injection options for semaglutide, the oral Wegovy tablet, approved by the MHRA in June 2026, is now available for those who prefer not to inject, though the clinical profile and dosing schedule are different. If cost is a factor in your thinking, the cost of Wegovy in the UK page sets out what private treatment typically involves and what an honest price includes. If you're prescribed a higher-dose pen and want to understand your options before the next tier, our guide to how to split Wegovy pens across doses explains what's practical and what to discuss with your prescriber. When you're ready to talk through which approach suits you, speak to our prescribers through a free consultation, a real clinician reads every assessment the same day.