What actually happens when you start a weight loss med injection

The moment the pen arrives — and what to do before you use it

Picture this: a plain DPD parcel lands on your doorstep the morning after your consultation was approved. Inside is a pre-filled pen, cold from transit. Before you do anything else, the pen needs to reach room temperature (typically around 30 minutes out of the fridge) and you'll want to read the Patient Information Leaflet in full, because it covers the exact injection steps, storage windows and missed-dose advice your prescriber relies on you following. The NHS tirzepatide medicines page is also a readable starting point if you want a plain-English overview alongside the leaflet.

Mounjaro (tirzepatide) and Wegovy (semaglutide) both come as pre-filled, once-weekly pens. You inject into the fatty tissue just under the skin, abdomen, front of the thigh, or the back of the upper arm are the approved sites. Rotating between spots each week matters: using the same site repeatedly can cause changes in the skin that affect how consistently the medicine absorbs. Clean the area first with a fresh swab, let it dry for a few seconds, then follow the pen's activation steps exactly as described in the leaflet. That's the whole practical process.

Storage is refrigerated at 2–8°C, but each pen has a limited window in which it can be kept at room temperature if you need it. The precise duration differs between products and should be confirmed in the SmPC or leaflet, not assumed from memory or online forums.

Why the first few weeks feel underwhelming, and why that's intentional

Both medicines begin at a dose designed for tolerance rather than maximum effect. The starter dose for tirzepatide is 2.5 mg; semaglutide's schedule begins at 0.25 mg. Neither figure represents the dose at which most of the clinical-trial weight loss was observed. Your prescriber increases the dose in steps, typically at four-week intervals, watching for how you respond.

This phased approach exists because the most common side effects, nausea, loose stools, constipation, indigestion and fatigue, are dose-dependent and front-loaded. They tend to be most noticeable in the days after a new pen or a dose increase, and for most people they ease significantly within a couple of weeks as the body adjusts. The titration schedule is not just a formality; it is the mechanism that keeps those side effects manageable. Pushing through a higher dose faster, without prescriber oversight, is the kind of decision that leads to avoidable drop-outs.

SURMOUNT-1, the pivotal trial for tirzepatide published in the New England Journal of Medicine, reported average body-weight reductions of around 20–21% at 15 mg over 72 weeks. That result was achieved at the highest maintained dose after a full titration sequence, not at week four. Patience, and a prescriber monitoring your progress, are part of what produces those numbers.

Side effects that need a GP, and those that just need time

Most GI side effects resolve on their own and don't require urgent action beyond staying hydrated and eating smaller, lower-fat meals to ease nausea. A few signs do warrant prompt medical attention. Severe, persistent stomach pain that radiates towards the back, with or without vomiting, can be a symptom of acute pancreatitis; the MHRA highlighted this in a Drug Safety Update in January 2026 and advises seeking urgent care if it occurs. Gallbladder symptoms, signs of a serious allergic reaction, and significant dehydration from persistent vomiting or diarrhoea are also reasons to contact a healthcare professional promptly rather than waiting to see if things settle.

If you're using the weight loss medication injection and notice side effects that feel unusual or are worsening rather than improving, your prescriber should know. For questions about what's normal versus what warrants a call, the aftercare team can help you make that call quickly.

One thing our prescribers are asked most weeks: can you just skip the dose to avoid side effects? The answer, per the SmPC, is that missed doses have specific guidance that depends on timing and how long until the next scheduled injection. The leaflet walks through exactly what to do; it is not a decision to improvise.

Eligibility, private access and how the clinical review works

Both Mounjaro and Wegovy are licensed for adults with a BMI of 30 or above, or 27 or above with at least one weight-related condition such as high blood pressure, raised cholesterol or obstructive sleep apnoea. Lower BMI thresholds apply for some ethnic backgrounds under UK guidance. BMI alone doesn't determine approval; the prescriber considers your full health picture, current medicines and any contraindications before issuing a prescription.

On the NHS, access follows NICE's phased rollout and often involves long waiting lists; the NHS England guidance on weight-management injections explains the current criteria clearly. Private treatment through a regulated pharmacy is the route for people who don't meet NHS criteria yet, or who'd rather not wait. If you'd like to understand your options, the treatment overview sets out what's available.

At nume, every consultation is read the same day by a GPhC-registered Independent Prescriber, not automated software. ID and weight are verified before anything is dispensed. If you order before midday on a working day and your consultation is clinically approved, your pen ships that afternoon with DPD, arriving tracked to your door the next working day, in plain, unbranded packaging, so there's nothing obvious about what's inside when it turns up before the school run. The process for getting started is straightforward once you know what to expect.

If you'd like to explore whether a weight loss injection might be appropriate for you, check your eligibility with our prescribers, the consultation is free, and there's no obligation. People often ask us about the weight loss injections we offer and how they compare, and our prescribers are happy to talk through the differences, including questions about weight loss injection r options and, for those who've seen it mentioned elsewhere, what sets medi weight loss injections apart from the licensed treatments we prescribe.