How long do you have to take weight loss injections?

What clinical trials actually tell us about treatment length

The major phase-3 trials for both licensed weight management injections ran for well over a year. The STEP 1 trial for semaglutide 2.4mg (Wegovy) followed participants for 68 weeks and reported an average body-weight reduction of around 15%; the full paper is published in the New England Journal of Medicine. SURMOUNT-1 for tirzepatide (Mounjaro) ran to 72 weeks, with average losses reaching around 20–21% at the highest dose. Neither trial was designed to answer "how long should treatment last after the trial ends", they were designed to establish that meaningful weight reduction happens and is sustainable over that window.

What those timelines do confirm is that the body's response to GLP-1 and dual GIP/GLP-1 medicines is gradual. Weight loss tends to be most rapid in the first few months, then continues at a slower rate as the dose increases. Expecting a short course (a few weeks, say, like an antibiotic) misreads how these medicines work. You can read more about the early timeline on our page covering how long weight loss injections take to work.

The NICE guidance position on stopping and continuing

NICE, in its appraisals of both medicines, sets a practical checkpoint: if a patient has not lost at least 5% of their body weight after six months at the highest dose they can tolerate, the prescriber should review whether continuing is appropriate. That isn't a rule that everyone stops at six months, it's a minimum-efficacy checkpoint. For people who are responding well, the guidance supports continued treatment.

For semaglutide specifically, NICE's appraisal (TA875) recommends use for a maximum of two years within a specialist weight management setting on the NHS. That two-year limit applies to NHS commissioning, not to private prescribing, where duration is governed by clinical judgement at each review. Tirzepatide's appraisal (TA1026) does not carry an equivalent fixed upper limit, though the same six-month checkpoint applies.

The question of exactly how long makes sense for you specifically is one our prescribers cover as part of each repeat review, and if you want a detailed breakdown of how treatment periods tend to unfold, our guide on how long you take weight loss injections for walks through the key considerations. That's worth knowing before you start: duration isn't a decision made once and filed away.

What happens when the injection is stopped

This is probably the thing people most want a straight answer on. The evidence is clear that weight regain is common (often substantial) when treatment ends without sustained lifestyle changes already in place. It's not a personal failing. The medicines work partly by reducing appetite signals that are physiologically driven; when the medicine is removed, those signals return. That's biology, not willpower.

A 2022 withdrawal study for semaglutide showed participants regained most of the lost weight within a year of stopping. The same pattern is observed with tirzepatide. Some people do maintain a meaningful proportion of their weight loss after stopping, particularly those who have used the treatment window to embed different eating and activity habits. But the honest position is that the data doesn't support treating these as short courses with permanent results after a few pens. Our page on whether you have to take weight loss injections for life goes into this in more detail.

If you're considering the cost over an extended period, our weight loss injections cost page covers what's typically included in a private treatment price and how to read what you're actually buying.

How duration is managed in practice at a regulated service

At a properly run private prescribing service, duration isn't a commercial decision, it's a clinical one, made at every repeat. There's no responsible way to pre-authorise six months of treatment on day one without any further clinical contact. Equally, there's no clinical basis for stopping treatment at an arbitrary point if someone is responding well, tolerating it and has a clear health rationale to continue. It's also worth being aware that some people ask about amino acids injections for weight loss as an alternative approach, though the evidence base and regulatory status differ considerably from licensed GLP-1 medicines.

At nume, a GPhC-registered Independent Prescriber reviews every repeat request, not a form-routing system. They look at progress, tolerance, any reported symptoms and whether continuation is appropriate. That's what clinical oversight of duration actually looks like. If you're unsure whether your current weight or health picture would meet the eligibility criteria, our free consultation is the right starting point; a prescriber can give you a far more useful answer than any general guide can. You can also explore the broader range of weight loss injections available in the UK if you're still deciding which medicine might suit you. Our clinical team page sets out who's behind those reviews.