Weight loss injections: how long do you take them for?

How long did the clinical trials run, and why does that matter?

The landmark trials give a useful anchor. SURMOUNT-1, which studied tirzepatide (the active ingredient in Mounjaro), ran for 72 weeks and enrolled thousands of adults living with obesity. At the 15mg dose, participants saw average body-weight reductions of around 20 to 21 percent over that period. The STEP 1 trial for semaglutide (the active ingredient in Wegovy) ran for 68 weeks, with an average reduction of around 15 percent at the 2.4mg maintenance dose. Both figures come from people taking the medicine consistently throughout the full trial period, alongside reduced-calorie eating and increased physical activity.

That context matters for duration. These results were not achieved in eight weeks. The titration phase alone (where the dose is gradually increased to let your body adjust) takes several months. For Mounjaro, that process starts at 2.5mg and can progress through six dose steps; meaningful weight loss tends to build across the full titration arc. You can read more about how the early weeks feel and when results typically become noticeable in our overview of how long weight loss injections take to work.

So if you are asking how long you need to take them for, the honest starting answer is: at least the length of a proper trial of treatment, which clinical evidence places at over a year. If you want a fuller picture of what that commitment looks like in practice, our guide to how long you can take weight loss injections walks through the key factors that shape that decision. Shorter stints tell you less and deliver less.

What happens when you stop, is there a natural end point?

This is the question our prescribers are asked most weeks, and it deserves a direct answer. For most people, stopping weight loss injections leads to some weight regain. The medicines work partly by suppressing appetite and slowing gastric emptying, effects that reverse when the medicine leaves your system. Studies following STEP 1 participants after they stopped semaglutide found substantial weight regain within a year of discontinuation, with participants recovering on average around two thirds of the weight lost.

That does not mean you must stay on these medicines indefinitely. It means stopping is a clinical decision that benefits from a plan: tapering, lifestyle reinforcement, and honest monitoring. NICE guidance for Wegovy (TA875) sets a maximum recommended duration of two years within NHS specialist services, partly reflecting the evidence base and partly the cost-effectiveness threshold, but private prescribing is not capped in the same way, and the clinical picture for each person varies. If you are weighing up longer-term use, our page on how long you can be on weight loss injections covers the safety and practical considerations in more depth.

The short version: there is no automatic stop date built into the medicine. There is, however, a formal review point.

What is the six-month review point, and what triggers a stop?

Both NICE appraisals (TA1026 for tirzepatide and TA875 for semaglutide) build in a structured review at six months on the maintenance dose. If you have not achieved at least five percent body-weight reduction by that point, guidance supports stopping treatment. That threshold exists because the medicine is not working as hoped for that person, and continuing carries cost and side-effect exposure without a commensurate benefit.

If you are responding well (which most people in trials did) there is no equivalent rule forcing a stop at six months. You and your prescriber continue to weigh the ongoing benefit against any side effects and your evolving goals. The question of how long you need to take weight loss injections often resolves into a more specific one: what would a planned, supported stop look like for you?

It is also worth knowing that NICE's recommendation on tirzepatide, published in December 2024 and updated in September 2025, applies specific BMI thresholds and comorbidity criteria for NHS access, different rules from private prescribing. The weight loss injections overview explains how NHS and private routes differ in practice.

Does the type of injection affect how long you take it?

Mounjaro and Wegovy share the same broad logic (both are once-weekly subcutaneous injections, both are titrated upward over months, both involve a minimum effective period measured in over a year) but there are differences worth knowing. Mounjaro is a dual GIP and GLP-1 receptor agonist, which means it activates two gut-hormone pathways simultaneously; Wegovy activates one. The higher average weight loss seen with tirzepatide in the SURMOUNT-5 head-to-head trial, published in the New England Journal of Medicine in 2025, may influence how long a prescriber feels treatment is needed before a useful plateau is reached, or whether switching makes sense. That said, the individual response matters far more than the average trial result.

Whichever medicine you are on, safe disposal of used pens matters throughout. A sharps container is the correct way to discard used pens, not household bins. It is a small practical detail that matters from week one right through to the last injection, whenever that comes.

Questions about private treatment costs over time are reasonable, and our weight loss injections cost page lays out what a transparent, all-included price covers. Treatment duration affects the overall outlay, which is another reason the prescriber review structure at nume's consultation page is worth understanding before you start.