Comparing Weight Loss Injections: Wegovy, Saxenda, Mounjaro, and the Future of Retatrutide

With obesity and weight-related health conditions on the rise, medical solutions are becoming an increasingly important part of the wider weight management conversation. Among these, weight loss injections have gained considerable attention due to their effectiveness in supporting individuals who struggle to lose weight through diet and exercise alone.

Currently, three main injectable medications are widely discussed in the UK: Wegovy, Saxenda, and Mounjaro. Each works in a slightly different way and offers varying degrees of weight loss efficacy. This article provides a detailed, side-by-side comparison to help individuals understand the differences and decide—together with a healthcare professional—what might be the most suitable option.

1. Wegovy (Semaglutide)

Mechanism of Action:

Wegovy contains semaglutide, a medication that mimics the hormone GLP-1 (glucagon-like peptide-1). This hormone is naturally produced in the gut and plays a key role in regulating appetite and food intake. By activating GLP-1 receptors in the brain, semaglutide increases feelings of satiety and reduces hunger, which can lead to reduced calorie intake over time.

Dosage and Administration:

Wegovy is administered via a once-weekly subcutaneous injection, making it relatively low-maintenance compared to daily alternatives. The dosage is gradually increased over the first 16–20 weeks to minimize gastrointestinal side effects.

Efficacy:

Clinical trials, such as the STEP 1 study, have shown that Wegovy can lead to an average weight loss of around 15% of total body weight over 68 weeks when used alongside lifestyle interventions. Some individuals may experience even greater results.

Availability in the UK:

As of 2025, Wegovy is approved by the MHRA and available via the NHS for patients meeting specific criteria, including those with a BMI ≥35, or BMI ≥30 with comorbidities like type 2 diabetes or hypertension. It is also available privately through weight loss clinics and pharmacies.

Common Side Effects:

- Nausea

- Diarrhoea or constipation

- Headaches

- Fatigue

- Mild injection site reactions

2. Saxenda (Liraglutide)

Mechanism of Action:

Saxenda contains liraglutide, which, like semaglutide, is a GLP-1 receptor agonist. It functions similarly by helping regulate appetite and reduce food intake, but it has a shorter half-life, which means it must be taken more frequently.

Dosage and Administration:

Saxenda requires a daily injection. The dose is titrated up over several weeks to reduce the likelihood of side effects. This daily commitment can be a drawback for some users but may suit those who prefer more consistent, day-to-day management.

Efficacy:

In clinical trials, Saxenda has been shown to produce average weight loss of 5% to 10% of body weight over a one-year period, when combined with a reduced-calorie diet and increased physical activity.

Availability in the UK:

Saxenda has been available in the UK for several years and is approved for use in adults with a BMI ≥30, or BMI ≥27 with a weight-related condition. It is available on the NHS in limited circumstances and is also widely available through private healthcare providers.

Common Side Effects:

- Nausea

- Vomiting

- Bloating

- Diarrhoea

- Dizziness

3. Mounjaro (Tirzepatide)

Mechanism of Action:

Mounjaro represents a new class of weight loss medication. It contains tirzepatide, a dual-action compound that targets both GLP-1 and GIP (glucose-dependent insulinotropic polypeptide) receptors. These hormones work together to improve insulin sensitivity, reduce appetite, and enhance energy expenditure.

Dosage and Administration:

Like Wegovy, Mounjaro is administered as a once-weekly injection, with a gradual titration to minimize side effects. It has been praised for its strong impact on both weight loss and glycaemic control.

Efficacy:

Clinical studies, such as the SURMOUNT trials, indicate that Mounjaro can help patients lose up to 20% or more of their body weight, making it potentially more effective than either Wegovy or Saxenda. This makes it an especially promising option for individuals with obesity and type 2 diabetes.

Availability in the UK:

As of early 2025, Mounjaro is approved in the UK for managing type 2 diabetes, but it is not yet formally licensed for weight loss alone. However, off-label use is becoming increasingly common in private clinics, and full approval for obesity treatment is expected in the near future.

Common Side Effects:

- Nausea and vomiting

- Constipation or diarrhoea

- Reduced appetite

- Low energy levels (initially)

- Headaches

4. Retatrutide: The Emerging Contender

Mechanism of Action:

Retatrutide is a novel investigational drug currently in development for obesity treatment. It is a triple hormone receptor agonist, targeting **GLP-1, GIP, and glucagon receptors**. This triple-action approach is designed to deliver enhanced appetite suppression, improved glucose control, and increased energy expenditure — a combination that may significantly outperform current single or dual-agonist therapies.

- GLP-1 and GIP: Reduce appetite, enhance satiety, and support insulin sensitivity.

- Glucagon receptor activation: Promotes energy use by increasing resting metabolic rate and fat oxidation.

Dosage and Administration:

Retatrutide is expected to be administered as a once-weekly subcutaneous injection, following a titration schedule similar to existing GLP-1-based therapies. Full dosing protocols are still being refined in clinical trials.

Efficacy:

Early-phase clinical trials have produced remarkable results, with patients experiencing weight loss of up to 24% of body weight over 48–72 weeks. This makes it potentially the most effective weight loss injection to date, pending further studies and long-term safety evaluation.

Availability in the UK:

As of 2025, Retatrutide is not yet available in the UK. It is currently in Phase 3 clinical trials, and regulatory approval is expected to be sought once those trials conclude—possibly by late 2025 or early 2026. If approved, it could become available in the UK by 2027, according to early forecasts.

Common Side Effects (Preliminary Data):

- Gastrointestinal discomfort (nausea, diarrhoea, constipation)

- Fatigue

- Mild headache

- Potential metabolic side effects still under investigation

Because retatrutide is still under investigation, all efficacy and safety data should be interpreted with caution until peer-reviewed results and regulatory reviews are complete.

Summary Comparison Table

Feature	Wegovy	Saxenda	Mounjaro	Retatrutide
Active Ingredient	Semaglutide	Liraglutide	Tirzepatide	Retatrutide
Hormone Targets	GLP-1	GLP-1	GLP-1 & GIP	GLP-1, GIP & Glucagon
Injection Frequency	Weekly	Daily	Weekly	Weekly
Average Weight Loss	~15% (68 weeks)	~5-10% (1 year)	Up to 20%+	Up to 24% (trials ongoing)
NHS Availability	Yes (BMI Criteria)	Yes (Limited)	Not yet (weight loss)	Not available
Private Access	Widely available	Widely available	Limited but growing	Not available
Common Side Effects	GI upset, fatigue	GI upset, dizziness	GI upset, low energy	GI upset, fatigue, unknown

Final Considerations

The rise of injectable medications for weight management is changing how obesity is treated—not just in the UK, but globally. While Wegovy, Saxenda, and Mounjaro are currently accessible (to varying degrees), the development of Retatrutide signals the next generation of treatment—one that could offer even greater results through multi-hormone action.

However, as with all medications, these treatments are not substitutes for lifestyle changes, and their long-term success often depends on integrating them with a structured plan involving diet, movement, and behavioural support.

As newer treatments move closer to approval, it's essential to stay informed and work closely with qualified healthcare professionals to explore the safest and most effective options.