Subcutaneous injections for weight loss: what actually happens under the skin

The moment before you inject: what the pen is actually doing

Picture Sunday evening. The pen comes out of the fridge door, where it's been sitting next to the salad dressing. You let it reach room temperature for around 15–30 minutes, cold medicine can sting. The Mounjaro KwikPen and the Wegovy pen are both pre-filled and dialled to your current dose; there's nothing to draw up, no syringe to fill. You clean the chosen site with an alcohol swab, pinch a fold of skin, press the pen firmly against it, and press the button. The needle (typically 4–5 mm) passes into the subcutaneous layer, the fatty cushion that sits between your skin and the muscle beneath. That layer is rich in small blood vessels, which absorb the medicine gradually over the following days.

The GLP-1 and, in tirzepatide's case, GIP pathways are then activated: appetite signals to the brain are dampened, gastric emptying slows, and fullness arrives earlier at meals. This isn't a quick metabolic hit, the effect builds over weeks of consistent weekly dosing. That's by design, not a flaw. The NHS patient information for tirzepatide covers the mechanism and common reactions in plain language.

Which sites work, and why rotation matters

Three areas are approved: the abdomen (avoiding the two-inch zone around the navel), the front and outer thigh, and the outer upper arm. All three offer adequate subcutaneous depth in most adults. Rotating between sites each week (or even between different spots within the same site) prevents the small lumps of lipohypertrophy that can develop if fat cells in one patch take repeated needle trauma. Lipohypertrophy doesn't just look and feel uneven; it slows absorption, which can make your weekly dose less consistent.

If you're self-injecting into the upper arm, the outer, slightly fleshy portion works best. Some people find a friend or partner easier for that site, though it's entirely manageable alone. For anyone curious about how a weight loss injection is actually administered, site guidance and storage requirements vary slightly by product, always defer to the leaflet that came in your box.

Side effects that are worth knowing before your first pen

The most common reactions are gastrointestinal: nausea, loose stools, constipation, indigestion, and burping tend to cluster in the days after a dose, particularly when you're new to treatment or have just moved up a step. These usually settle within a week or two. Mild soreness, redness or itching at the injection site is also normal and typically brief.

A smaller number of people experience dizziness, fatigue or headache, especially early on. These rarely require stopping treatment, but they're worth flagging to your prescriber. The MHRA Yellow Card scheme exists for anyone who wants to report a suspected side effect formally, it's a straightforward online form and genuinely useful for national safety monitoring.

Two things do warrant prompt medical attention: severe stomach pain that reaches into the back (a possible sign of pancreatitis), and symptoms of a serious allergic reaction such as swelling of the face, throat or tongue. These are uncommon, but knowing the signs matters more than any reassurance that they're rare. If you're uncertain about a symptom, our aftercare team is available seven days a week.

Eligibility and how the first prescription happens

Both Mounjaro and Wegovy are licensed for adults with a BMI of 30 or above, or 27 and above when a weight-related condition such as high blood pressure, high cholesterol or obstructive sleep apnoea is present. Lower BMI thresholds apply for some ethnic backgrounds under UK clinical guidance. If you are unsure whether your current weight qualifies, our page on what weight you need to be for weight loss injections walks through the criteria in detail. BMI alone doesn't determine suitability, a prescriber also looks at your medical history, current medicines, and any contraindications before issuing a prescription.

At nume, that assessment is done by a GPhC-registered Independent Prescriber, not an automated system. Consultations are reviewed the same day. If you are based locally and want to learn more about weight loss injections in Harrow, that page covers what to expect from an in-person consultation. For anyone researching the M injection for weight loss, it's worth reading through the full criteria and typical outcomes before you start comparing quotes.

If you're considering starting, check your eligibility with our prescribers, the consultation is free, and there's no obligation.