Mounjaro or Wegovy? Understanding the evidence behind modern weight loss medication

At Edinburgh GP, we recognise that obesity is a complex, chronic medical condition – not simply a question of willpower. As a GP with a particular interest in obesity management and lifestyle medicine, I’m passionate about helping patients achieve sustainable weight loss using evidence-based treatments alongside realistic lifestyle change.

Mounjaro® (tirzepatide) and Wegovy® (semaglutide) have transformed obesity treatment over the past few years. But how effective are they, and what should you expect?

How do they work?
Both medications are given as a once-weekly injection and mimic naturally occurring gut hormones that regulate appetite and fullness. By reducing hunger, slowing stomach emptying and improving blood sugar regulation, they help people naturally eat less without the constant struggle against cravings.

Mounjaro activates both the GLP-1 and GIP receptors, while Wegovy acts on the GLP-1 receptor alone.

What does the research show?
The results from modern clinical trials have been remarkable.

In the landmark STEP-1 trial, people taking Wegovy alongside lifestyle support lost an average of around 15% of their starting body weight over 68 weeks.

Even greater weight loss was seen in the SURMOUNT-1 trial of Mounjaro. Participants lost an average of:
• 15% on the 5 mg dose
• 19.5% on the 10 mg dose
• 20.9% on the 15 mg dose

Perhaps most impressively, almost one in three people taking the highest dose lost at least 25% of their starting body weight.

These are average results, and individual responses vary. Around 90% of people lose at least 5% of their starting weight, although the greatest success is seen when medication is combined with dietary, behavioural and physical activity changes.

Benefits beyond weight loss
The benefits extend well beyond the number on the scales.

Weight loss with GLP-1 medications is associated with improvements in blood pressure, cholesterol and blood sugar control, while reducing harmful abdominal fat. In the landmark SELECT trial, Wegovy reduced the risk of heart attack, stroke or cardiovascular death by 20% in people living with obesity and established cardiovascular disease. Emerging research also suggests benefits for conditions including obstructive sleep apnoea, metabolic dysfunction-associated steatotic liver disease (MASLD) and chronic kidney disease.

What about side effects?
Most side effects occur while the dose is gradually increased and are mild and temporary. Nausea, constipation and diarrhoea are the most common, and simple measures such as eating smaller meals, drinking plenty of fluids and eating slowly usually help.

Serious side effects such as pancreatitis or gallbladder disease are uncommon, and we’ll discuss these carefully before treatment begins so you know what symptoms to watch for.

Our approach at Edinburgh GP
Medication is only one part of successful weight management. At Edinburgh GP, I work with patients to understand the underlying causes of weight gain, optimise nutrition, physical activity and sleep, and provide ongoing medical support as treatment progresses.

The aim isn’t simply to prescribe Mounjaro or Wegovy – it’s to help you achieve meaningful, sustainable improvements in your health for years to come.