How long do weight loss injections last — and what happens when you stop?

How long does each injection stay active in your body?

The short answer: about a week. Both Mounjaro and Wegovy work by mimicking gut hormones (tirzepatide activates two receptors (GIP and GLP-1), while semaglutide targets one) and both are formulated to keep those receptors engaged across a seven-day window. That is why dosing is once weekly rather than daily. Miss a dose by a day or two and the medicine is still present; miss it by more than a few days and you may notice appetite returning before your next injection is due.

The four-weekly KwikPen format for Mounjaro reflects this rhythm: one pen, four injections, one month of treatment. Storage matters too, both medicines need refrigeration, and the Patient Information Leaflet sets out the exact window during which a pen can be kept at room temperature. Our guide to weight loss injections covers the practical side of handling and storage in more detail.

One misconception worth clearing up: some people assume the injection is simply flushed out within 24 hours, like a painkiller tablet. If you want a fuller picture of how long a weight loss injection actually lasts in your system, the half-life of semaglutide is around a week and tirzepatide is similar. The medicine accumulates over the first few weeks, which is one reason appetite suppression often grows noticeably stronger after the first month or two of treatment.

How long does a course of treatment usually last?

The landmark clinical trials (SURMOUNT-1 for tirzepatide and STEP 1 for semaglutide, both published in the New England Journal of Medicine) ran for 72 and 68 weeks respectively. Those timeframes were chosen to capture meaningful, sustained weight change, not because treatment stops at that point.

In practice, how long someone stays on treatment is a prescriber decision made at each review. NICE's recommendation for semaglutide (TA875) sets a maximum of two years within NHS specialist services; tirzepatide (TA1026) carries no fixed maximum in its current NICE guidance. Private treatment sits outside NHS commissioning rules, but a responsible prescriber still reviews progress regularly, asking whether the medicine is working, whether side effects are manageable, and whether continuing is appropriate. Questions about how long you can stay on weight loss injections are best answered at that ongoing review, not upfront.

At nume, there are no subscriptions. Every repeat requires a fresh clinical assessment. A real prescriber reads your update (not software) and approves continuation only when it is clinically appropriate to do so.

What happens when you stop taking weight loss injections?

This is the question that matters most, and the honest answer is that stopping treatment without sustainable lifestyle changes in place tends to lead to weight regain. A follow-up analysis of STEP 1 participants who stopped semaglutide showed that average weight returned toward pre-treatment levels within a year of stopping. Tirzepatide data points in the same direction.

That is not a reason to avoid treatment, it is a reason to use it as a window. Appetite suppression creates space to build habits that last: adjusting portion sizes, eating more protein, becoming more active. When those habits are established, some people do stop injections successfully. Others continue longer-term because their medical picture warrants it. The question of how long you can use weight loss injections is therefore inseparable from the question of what you are building while you use them.

If you're looking at costs over a longer period, the treatment page explains what is included in a single transparent price (consultation, prescription, delivery and aftercare) with no hidden extras.

How quickly do the injections start working, and how long before you notice a difference?

Most people notice some reduction in appetite within the first one to two weeks, though early doses are calibrated for tolerability rather than maximum effect. Mounjaro starts at 2.5mg; Wegovy starts at 0.25mg. These lower doses help your digestive system adjust and reduce the likelihood of nausea, the medicine is easing you in, not holding back. Measurable weight change typically becomes visible from around weeks four to eight, once the dose has been increased and the medicine has reached steadier levels.

The full effect builds over months. In SURMOUNT-1, the greatest average weight reductions were seen at 72 weeks, not at week 12. If you are wondering how long weight loss injections take to work, that trajectory is longer than many people expect going in, and our page on how long weight loss injections take to produce results goes into more detail about what to expect at each stage of treatment. Impatience in the early weeks is one of the most common reasons people stop too soon. Our clinical team are available seven days a week if you have questions about how treatment is going, that aftercare is built in, not an add-on. You can find out more about the team at our clinical team page.

These are prescription-only medicines assessed individually. If you'd like to understand whether you might be suitable, start your free consultation and a GPhC-registered prescriber will review your details the same day.