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What are the most effective weight loss drugs available in the UK?

Simon Edward • 13 January 2025

Cut through the noise and hype with our guide to the most effective weight loss drugs available in the UK.

If you're interested in losing weight, you've probably heard of the new generation of weight loss medications that are available in the UK.

The latest weight loss drugs are clinically proven to be highly effective when combined with diet and exercise. However, there's lots of misinformation out there that can make it hard to know what you're getting into and which medication might work for you.

Some social media influencers make lofty claims about the medications that aren't rooted in scientific fact. Others give incorrect advice that, at its worst, is unsafe. And news outlets sometimes prioritise scare stories and attention-grabbing headlines over balanced and unbiased reporting.

It can all be a bit overwhelming. But we're here to help you make sense of it.

In this article, we look at the most effective weight loss medications available in the UK and share the key facts you need to know. That includes information on their effectiveness, their potential side effects and where to get them.

What is the best medication for weight loss?

That depends on how you define 'best'.

Remember that everyone reacts to drugs differently. You might find that you get better results with a specific medication or experience fewer side effects. In some cases, a person can't safely take a drug if they have certain allergies or a history of certain health conditions.

What is the best weight loss medication? By one measure, the answer is 'Whichever medication works best for you'.

That's why we're not looking at the 'best' medications – we're ranking the most effective weight loss drugs based on results from clinical trials.

We can't say for sure which medication will suit you best. However, we can look at clinical data and determine which drugs, on average, work best for most people.

So, that's what we'll do – starting with the most effective weight loss drug currently available in the UK.

1. Mounjaro (tirzepatide)

What is it?

Mounjaro is an injectable weight loss medication with the active ingredient tirzepatide. It comes in a pre-filled cartridge called a 'KwikPen', which makes it easy for users to inject themselves.

Each pen contains four doses. Users inject themselves once per week for four weeks. They then switch to another pen with a higher dose of the medication.

After 16 weeks, users switch to the highest (15mg) dose. They continue taking this dose until the treatment ends.

How effective is it?

In clinical trials, participants lost an average of 21% of their starting body weight when taking Mounjaro with a balanced diet and increased exercise. ¹ ² This makes it the best prescription weight loss drug currently available in terms of total average weight loss.

How does it work?

Mounjaro is part of a class of drugs known as 'incretin mimetics'. These drugs work by mimicking natural hormones called 'incretins'. These hormones are released after we eat and are responsible for helping us feel full and controlling blood sugar levels, among other things.

When you take Mounjaro, your body acts like it's released more incretins. This has several benefits:

It makes you feel fuller for longer, so you're less likely to eat between meals.
It controls blood sugar, so you're less likely to crave high-energy snacks.
It slows down the digestion process.

Incretin mimetics are also known as 'GLP-1 agonists' after the GLP-1 hormone they mimic. However, Mounjaro also mimics another hormone called 'GIP'. This is one reason why it's the most effective drug of its class.

What are the potential side effects?

Side effects are fairly common with Mounjaro. The most widely reported side effects are:

Nausea
Diarrhoea
Constipation
Vomiting
Headaches or dizziness
Indigestion and heartburn

Many users who report side effects find they improve as they continue treatment. However, the intensity of the side effects often temporarily increases as they move to a higher dose.

Other side effects include making the contraceptive pill work less effectively and – in some patients – hair loss or hair thinning. Serious side effects are rare. To learn more, read our guide to the side effects of weight loss drugs.

How can I access it?

At the time of writing (January 2025), Mounjaro is not yet available on the NHS. However, plans are in place to roll out the drug on a restricted basis starting in March 2025.

The rollout will only make the drug available to patients with a BMI of 35 or greater and one weight-related health condition. However, you can also buy Mounjaro online in the UK via private healthcare providers like SemaPen. Our Mounjaro programme is available to patients with a BMI of 30 or greater, regardless of whether they have a weight-related health condition.

2. Wegovy (semaglutide)

What is it?

Like Mounjaro, Wegovy is an injectable weight loss drug that comes in a pen-shaped cartridge for easy injection.

Wegovy has a similar dosing schedule to Mounjaro – patients take one dose a week for four weeks before switching to a higher dose. However, the individual doses are different. Wegovy users also reach the maximum dose more quickly than Mounjaro users (12 weeks versus 16 weeks).

Wegovy uses the active ingredient semaglutide. This is the same active ingredient as another drug called 'Ozempic'. However, Ozempic is only licensed in the UK for the treatment of type 2 diabetes, not obesity.

How effective is it?

Wegovy is the second most effective weight loss medication in terms of total average weight loss. In the main Wegovy trial, users lost 15% of their starting weight, on average.³ As with Mounjaro, participants took the drug alongside lifestyle changes relating to diet and exercise.

How does it work?

As an incretin mimetic, Wegovy works in a similar way to Mounjaro. However, Wegovy only mimics one incretin hormone (GLP-1), while Mounjaro mimics two (GLP-1 and GIP).

What are the potential side effects?

Wegovy is similar to Mounjaro in this regard, too. The most common Wegovy side effects are nausea, diarrhoea, constipation, vomiting, headaches or dizziness, and indigestion and heartburn.

Side effects are more common with Wegovy than with Mounjaro – and Wegovy users tend to report stronger side effects. However, certain side effects seen with Mounjaro are rare or absent with Wegovy. This includes hair thinning and hair loss, as well as interaction with the contraceptive pill.

This is not an exhaustive list. Read our guide to the side effects of Wegovy and Mounjaro to learn more.

How can I access it?

Wegovy is available on the NHS in the UK – but only through specialist weight management clinics. The NHS offers Wegovy to those with a weight-related health condition and a BMI of 35 or greater. In exceptional circumstances, it may be offered to those with a BMI of 30 or greater.

You can also purchase Wegovy on private prescription through a healthcare provider like SemaPen. Our Wegovy programme is available to people with a BMI of 30 or greater, regardless of whether they have a weight-related health condition.

3. Saxenda (liraglutide)

What is it?

Saxenda, like Wegovy and Mounjaro, is an injectable weight loss medication that comes in a pre-filled pen. It uses the active ingredient liraglutide.

A big difference with Saxenda is the dosing schedule. Rather than taking one dose per week, Saxenda users inject themselves once per day. This means Saxenda users reach the highest dose more quickly than Wegovy or Mounjaro users – as quickly as four weeks.

How effective is it?

In a clinical trial, liraglutide users lost an average of 8% of their starting body weight after 56 weeks of treatment.⁴ As with the Wegovy and Mounjaro trials, participants took the medication along with positive lifestyle changes.

How does it work?

Saxenda, like Wegovy, is a GLP-1 agonist – it mimics the GLP-1 hormone that's partly responsible for making us feel full and regulating blood sugar levels.

What are the potential side effects?

The most common Saxenda side effects are similar to those experienced with other GLP-1 agonists. The main Saxenda website lists the following (this is not an exhaustive list):

Nausea
Diarrhoea
Constipation
Vomiting
Injection side reactions
Low blood sugar
Headache
Upset stomach
Tiredness
Dizziness
Stomach pain

For the complete list and more information, visit the Saxenda website.

How can I access it?

Like Wegovy, Saxenda is available on the NHS – but only through specialist weight management clinics, not GPs. To be eligible for NHS treatment, you must have a BMI of 35 or greater. You must also have high blood sugar and a high risk of heart problems.

You can also purchase Saxenda from private clinics. The criteria for accessing the drug may be less restrictive when it's purchased privately.

As part of Phoenix Health, SemaPen has been helping people with obesity lose weight for more than 20 years. If you'd like to learn more about our Wegovy and Mounjaro weight loss programmes, start with our programme overview page or read our expert guides.

Sources

1. Jastreboff, A.M. et al. (2022) "Tirzepatide Once Weekly for the Treatment of Obesity" New England Journal of Medicine, 387(3) https://doi.org/10.1056/NEJMoa2206038

2. Aronne, L.J. et al. (2023) "Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity" JAMA, 331(1) https://doi.org/10.1001/jama.2023.24945

3. Wilding, J.P.H. et al. (2021) "Once-weekly semaglutide in adults with overweight or obesity" New England Journal of Medicine, 384(11) https://doi.org/10.1056/NEJMoa2032183

4. Pi-Sunyer, X. et al (2015) "A Randomized, Controlled Trial of 3.0 mg of Liraglutide in Weight Management" New England Journal of Medicine, 373(1) https://doi.org/10.1056/NEJMoa1411892

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