What are the effects of weight loss injections on your body?

What the large trials actually measured

The clearest picture of weight loss injection effects comes from two long Phase 3 programmes. SURMOUNT-1, published in the New England Journal of Medicine, followed 2,539 adults with obesity over 72 weeks. At the highest dose of tirzepatide (15mg), participants lost an average of around 20–21% of their starting body weight. STEP 1, which tested semaglutide 2.4mg over 68 weeks, showed average reductions of approximately 15%. Both trials used a placebo arm alongside a reduced-calorie diet and activity programme, so the medicine's specific contribution is well-characterised.

Weight loss was not the only measured outcome. Participants in both programmes also showed improvements in waist circumference, fasting blood sugar, blood pressure and lipid profiles. These metabolic shifts matter clinically, because excess weight carries cardiovascular and metabolic risk that goes well beyond appearance. NICE's appraisal of tirzepatide, TA1026, explicitly factored these broader benefits into its recommendation.

It is worth being clear about what the trials were not. Participants were enrolled under controlled conditions, with dietary support and regular follow-up. Real-world effects vary. Some people lose considerably more than the trial average; others less. The figures are a reliable guide to what is possible, not a personal guarantee.

How these medicines produce their effects, and why the side-effect profile follows from that

Tirzepatide and semaglutide both act on receptors in the gut and brain that regulate appetite and digestion. Semaglutide activates GLP-1 receptors; tirzepatide activates both GLP-1 and GIP receptors, making it the only dual-agonist weight management medicine licensed in the UK. Both slow gastric emptying, food moves through your stomach more gradually, which extends the sensation of fullness after meals.

That mechanism is also why the most common weight loss injection effects are gastrointestinal. Nausea, indigestion, looser stools or constipation, and burping are reported frequently, particularly in the first weeks of treatment and after each dose increase. For most people these settle as the body adjusts. If you want to understand what side effects of weight loss injections tend to look like, how often they occur, and what helps, our dedicated guide covers this in detail.

Less common but more serious effects include acute pancreatitis. In January 2026 the MHRA issued a Drug Safety Update for GLP-1 medicines specifically highlighting this risk: if you develop severe stomach pain that spreads to your back, with or without vomiting, you should seek urgent medical help. Report any suspected side effects via the MHRA Yellow Card scheme. Our guide to more serious side effects covers these in full.

Effects on appetite, eating habits and day-to-day life

One thing people often say they were not prepared for is how differently food feels. It is not just that portions shrink, appetite itself changes. Many people find that foods they previously craved feel less compelling, and that they feel full after amounts that would previously have left them hungry. This is the appetite-suppression effect of GLP-1 activity operating at a neurological level, not just a slower stomach.

That said, reduced appetite brings its own practical considerations. Protein intake can drop alongside overall calories, which matters for preserving muscle mass during weight loss. Staying well hydrated becomes more important, particularly if nausea has put you off fluids. The NHS patient information for tirzepatide includes practical advice on eating and drinking during treatment, and the same guidance applies broadly to weight loss injections as a category.

If you are thinking about whether these effects suit your situation (whether you have a condition that might interact, or you are on other medicines) it is a question worth raising at consultation rather than trying to work out alone. If you want to understand how the Waygo weight loss injection fits into the treatment options available through our pharmacy, that page covers the specifics in full. If you want to understand how much these treatments typically cost privately, the cost and pricing guide lays that out honestly.

What happens when treatment stops

This is, honestly, one of the questions that gives people pause before they start. The trial evidence is consistent: when GLP-1 and dual-agonist medicines are stopped, a significant proportion of the lost weight is typically regained over the following 12 months. The STEP 1 extension study and post-SURMOUNT data both point in the same direction. Appetite returns toward its pre-treatment baseline and the metabolic effects reverse.

This does not mean the medicines are not worth taking. It does mean they are more like ongoing management than a one-time fix. UK guidance from the NHS England weight management injections page acknowledges this and emphasises building diet and activity habits alongside treatment. NICE also sets a review point at six months, if less than 5% of starting weight has been lost by then at the highest tolerated dose, continuing is reassessed. That clinical review process is part of responsible prescribing, not a hurdle.

Understanding both the positive effects and their limits is exactly what a consultation with a prescriber is for. If you would like to explore whether you meet the clinical criteria for treatment, you can check your eligibility with our prescribing team, the consultation is free, there is no obligation, and it is reviewed by a GPhC-registered prescriber the same day.