What a weight loss injection course actually involves

The biggest myth: that the first dose is the one that does the work

Many people arrive expecting the starter dose to deliver results immediately. It is an understandable assumption, but it is not how the medicines work. The opening phase of a Mounjaro course, for example, uses 2.5mg, a dose whose job is settling your system rather than shifting weight. The GI side effects most associated with these medicines (nausea, loose stools, indigestion) are most likely to appear in the early weeks, and starting low keeps them manageable for the vast majority of people.

After roughly four weeks, the prescriber reviews progress and, if you are tolerating treatment well, moves the dose up. This pattern repeats at 5mg, 7.5mg, 10mg, 12.5mg and up to 15mg, though not everyone needs or reaches the highest strength. How each dose step works in practice is worth reading before you begin. The clinical trial evidence, published in the New England Journal of Medicine, found average body-weight reduction of around 20–21% at the 15mg maintenance dose over 72 weeks, but those participants completed the full titration schedule, not a shortened course.

The implication is practical: if you are three weeks in and feeling little change, that is expected. The course has not failed; it has barely started its first chapter.

What actually happens week to week on a course

Once your prescription is confirmed, you inject once a week, on the same day each week where possible, into the abdomen, thigh or upper arm, rotating the site to keep skin healthy. The pen is kept in the fridge (2–8°C), and you take it out a little before use; the exact room-temperature window is in the Patient Information Leaflet and worth reading before your first injection. A good guide to injection technique can settle nerves before that first use.

Appetite changes tend to arrive gradually rather than overnight. Most people describe eating less without consciously trying to, feeling full sooner, and finding that cravings for high-calorie foods ease. That is the medicine doing what the trial data predicted, slowing gastric emptying and acting on the appetite centres in the brain. If you are researching your options, our page covering how weight loss injections work and what to expect is a helpful place to start. Each pen of Mounjaro covers four weekly injections at one dose level, so you are typically re-ordering once a month. At a GPhC-registered pharmacy like ours every repeat order involves a clinical review, not an automatic refill. That check exists to catch problems early and to confirm whether a dose change is appropriate.

Side effects across the course: when they peak and when they ease

The most common side effects are gastrointestinal. Nausea sits at the top of the list, followed by changes in bowel habit, indigestion and occasional burping or reflux. For most people these are at their worst in the first days after starting or after a dose increase, then settle within a week or two as the body adjusts. Fatigue and mild headaches appear for some people, particularly early on.

Eating smaller meals, staying well hydrated and avoiding very fatty foods during the adjustment period all help. One practical thing many people find useful: using an alcohol swab before each injection keeps the site clean and makes the routine feel more settled.

Rare but serious symptoms need prompt attention. Severe, persistent stomach pain that spreads toward the back (with or without vomiting) can be a sign of pancreatitis. The MHRA issued a Drug Safety Update on this in January 2026, reinforcing that patients and clinicians should treat it as urgent. Any symptom that worries you should go straight to your GP or, if severe, to 111 or A&E. You can also report suspected side effects via the MHRA's Yellow Card scheme.

How long a course runs, and what happens at the end

There is no fixed end date built into the licence the way there is with, say, a course of antibiotics. Weight regain is common when these medicines are stopped, because the underlying biology that drove the original weight gain does not disappear. Many people continue treatment for as long as it remains clinically appropriate and they are tolerating it well, and that conversation belongs with your prescriber rather than a general guide.

NICE's appraisal of tirzepatide, published in December 2024, sets out the NHS criteria under which the medicine can be used, including a review point if less than 5% of body weight has been lost after six months at the highest tolerated dose. Private prescribers follow clinical best practice around equivalent review points. For context on what different providers charge and what those prices typically do and do not include, the treatment overview is a sensible place to look.

If you want to understand whether a weight loss injection course is appropriate for your situation, our prescribers review consultations the same day, a real clinician reads your answers, not software. You can read more about what the weight loss injection review process involves before you begin, find out about using Mounjaro as an injection for weight loss if that is the treatment you are considering, or explore how to buy weight loss injections through a regulated pharmacy. You can start your free consultation when you are ready.