How effective are weight loss injections?

Weight loss injections have been shown to be safe and effective in clinical trials. That’s why they’re licensed for use in Ireland, and why so many people now use them alongside healthy lifestyle changes and clinical support. 

But what does “effective” actually mean in the research, and what kind of results did people see in clinical studies? 

Important: This is an informational guide. For medical advice, please speak to our IMC registered team.

Mounjaro: How effective is it? 

Mounjaro (tirzepatide) has been studied in large clinical trials in adults living with obesity or overweight with weight-related health complications (excluding diabetes). In the SURMOUNT-1 trial, people took a weekly injection of Mounjaro alongside lifestyle support, including guidance on food and physical activity and their weight loss results were recorded. 

Average weight loss in the trial 

By week 72, average weight loss depended on the dose: 

• 5 mg: 15.0% average weight loss 
• 10 mg: 19.5% average weight loss 
• 15 mg: 20.9% average weight loss 
• Placebo: 3.1% average weight loss 

In another analysis from the same study, weight loss at the highest dose (tirzepatide 15 mg) averaged 22.5%. 

How many people saw meaningful results? 

The study also looked at how many people hit specific milestones by week 72: 

• Losing at least 5% of body weight: 85% to 91% on tirzepatide vs 35% on placebo 
• Losing at least 20% of body weight: 50% to 57% (on 10 mg and 15 mg) vs 3% on placebo 

In other words, many people lost a clinically meaningful amount of weight, and a significant number lost 20% or more. 

What about side effects? 

The most common side effects reported were gastrointestinal, including nausea, diarrhoea, and constipation. In the study, these were usually mild to moderate and were more likely to happen during dose increases. No long-term side effects were observed in the trial. 

Wegovy: how effective is it? 

Wegovy (semaglutide) has also been studied extensively in adults living with overweight or obesity who do not have diabetes. One of the largest and most widely cited studies is the STEP 1 trial. 

In this study, people took a once-weekly Wegovy injection alongside structured lifestyle support, including guidance on food choices and physical activity. This was against a group who were making the same lifestyle changes but taking a placebo. 

Average weight loss in the trial 

By week 68, people taking Wegovy 2.4 mg achieved: 

• 14.9% average weight loss with Wegovy 
• 2.4% average weight loss with placebo 

This means Wegovy supported an additional 12.4% weight loss compared with lifestyle changes alone. 

How many people saw meaningful results? 

The study also looked at how many people reached commonly used weight-loss milestones: 

• Losing at least 5% of body weight: 86.4% on Wegovy vs 31.5% on placebo 
• Losing at least 10%: 69.1% vs 12.0% 
• Losing at least 15%: 50.5% vs 4.9% 
• Losing at least 20%: 32.0% vs 1.7% 

These results show that many people achieved clinically meaningful weight loss, with around one in three losing 20% or more of their starting weight. 

Side effects 

The most commonly reported side effects were gastrointestinal, including nausea, diarrhoea, vomiting, and constipation. These were typically mild to moderate, happened most often during dose increases, and improved over time. 

A small number of participants stopped treatment due to side effects, most commonly related to digestion. There were no long-term side effects reported. 

Wegovy and heart health 

Weight loss is often linked to improved heart health, but newer research suggests Wegovy may support cardiovascular health beyond weight loss alone. 

A large study known as the SELECT trial looked at people living with overweight or obesity who already had cardiovascular disease, but did not have diabetes. The results showed that weekly semaglutide injections were associated with a 20% reduction in the risk of major cardiovascular events, including heart attack, stroke, and cardiovascular-related death. 

This indicates that Wegovy may help support heart health not only through weight reduction, but also through wider effects on inflammation, blood pressure, and metabolic health. 

What this means in practice 

For some people, Wegovy may offer benefits that go beyond the number on the scales. While weight loss remains an important goal, the evidence suggests cardiovascular protection may still be present even when weight loss is more modest. 

As always, Wegovy is prescribed following a clinical assessment to make sure it is appropriate, safe, and suited to your individual health needs. 

Managing side effects 

In clinical trials and in real life, both Wegovy and Mounjaro can cause side effects for some people. The most common are digestive, such as nausea, diarrhoea, constipation, bloating, and reflux, and they often happen during dose increases while your body adjusts. 

The important thing to know is that side effects do not affect everyone. Many people only experience mild symptoms, or none at all. But understanding what to look out for can help you feel more prepared and know when to reach out. 

Throughout your journey, we monitor how you’re getting on and support you with side effect management. Where clinically appropriate, we can also provide treatments and advice to help ease symptoms at no extra cost, so you can stay comfortable and keep moving forward. 

Every journey is different 

It’s helpful to look at clinical trial results because they show what these medications can do in controlled studies. But they are not a guarantee of the same outcome for everyone. 

Trial results are averages across large groups. Some people lose more weight than the average, and some lose less. Your results can be influenced by things like your starting weight, your dose, how your body responds to treatment, and the lifestyle changes you build alongside it. 

How we’re here to help you 

Our clinicians are here to make sure treatment is safe, suitable, and properly supported. They review your consultation, guide you through dose changes, and help you manage side effects if they show up. 

Because medication does not work alone. Lasting weight loss comes from the habits you build over time, including how you eat, how you move, and how you support your health day to day. We’ll help you put those foundations in place, so you can get the most from treatment and keep your progress going long term. 

Links to clinical trials 

• Tirzepatide Once Weekly for the Treatment of Obesity | New England Journal of Medicine 
• Once-Weekly Semaglutide in Adults with Overweight or Obesity | New England Journal of Medicine 
• Weight loss drugs: Semaglutide boosts heart health regardless of how much weight patients lose, study suggests | The BMJ