Weight Loss Injections: Are They Safe, and Do They Actually Work?

By Irfan Mahmud, Pharmacist & Independent Prescriber

Weight loss injections such as Mounjaro and Wegovy are transforming the way we treat obesity — and many people are seeing incredible results from them.

But with so many people using such powerful medication, it’s worth exploring how safe they are and whether they work as well as people say.

Let’s see what the science says about the safety and efficacy of weight loss injections like Mounjaro and Wegovy.

How Does Wegovy Work?

Weight loss medication has worked in a few different ways over the years, but the most common was probably the mechanism found in the likes of Alli and Orlistat. They work by preventing your body from absorbing fat. Instead, the fat passes through your body with your stools (faeces).

Wegovy works a little differently. It mimics the GLP-1 receptor in your brain, which is responsible for letting your body know when it’s full. So, when you inject Wegovy, your brain sends more signals to your body to let it know when it’s full. That mechanism helps you feel fuller for longer, which reduces hunger cravings and regulates your appetite.

Because Wegovy was originally designed as a treatment for type 2 diabetes, it helps to regulate your blood sugar levels and increase insulin sensitivity — two processes which also help you manage your weight. But it is the appetite-controlling part of Wegovy that is the most important and effective part of the medication.

How Does Mounjaro Work?

Mounjaro works in a very similar way to Wegovy, regulating your appetite by mimicking certain receptors in your brain. But whilst Wegovy just mimics the GLP-1 receptor, Mounjaro also mimics the GIP receptor.

Whilst it’s not entirely clear how this extra mechanism may lead to even more weight loss than Wegovy, it’s thought that this double-pronged approach also slows the emptying of your stomach. This mechanism helps you feel fuller for even longer.

Essentially, both Wegovy and Mounjaro help you to reduce your food intake by controlling your hunger cravings.

Are Weight Loss Injections Safe?

The short answer is yes: Mounjaro and Wegovy are considered to be safe long-term treatments for obesity. But as with any medication, they are not entirely without risk.

In this context, ‘safe’ means that the potential benefits of the medication outweigh the potential risks. In the UK, it is the responsibility of the Medicines and Healthcare products Regulatory Agency (MHRA) to decide whether or not a medication is safe enough to be sold here. To receive approval from the MHRA, a medication must first go through comprehensive clinical trials.

Both Wegovy and Mounjaro have been approved for use in the UK by the MHRA. This means that they’re considered effective treatments for obesity and are also considered safe when used appropriately.

Side Effects

As with any medication, weight loss injections have some potential side effects to be aware of. The most common, which affect more than 1 in 10 people, include:

• Nausea
• Diarrhoea
• Vomiting
• Stomach pain
• Indigestion

These side effects tend to subside as your body gets used to the medication, but it’s still important to be aware of them. Wegovy and Mounjaro are powerful medications that are used to treat long-term obesity. With this in mind, side effects like these are possible and should not be taken lightly.

There are other, rarer side effects to be aware of too. Up to 1 in 100 people may experience an inflamed pancreas, kidney stones, or bladder stones. If you start experiencing any of these symptoms, you should inform a medical professional immediately.

Frequently Asked Questions

How can I be sure my medication is what it’s described as?

If approved for treatment, you’ll receive either Wegovy or Mounjaro, both of which have been approved for use in the UK by the MHRA. You can search for their registration on the MHRA website.

We are licensed to prescribe Wegovy and Mounjaro in the UK and regulated by the General Pharmaceutical Council (GPhC). You can verify our registration here.

Additionally, each piece of medication we prescribe comes with a Patient Information Leaflet (PIL), which is required by UK law. It details the ingredients in the medication, possible side effects, and recommended dosage.

Is there anything I can do to support the effects of weight loss medication?

Yes. Although Wegovy and Mounjaro can lead to significant weight loss, you will only achieve long-term results by making long-term changes to your lifestyle. This could include incorporating regular exercise into your routine and eating a balanced diet.

What will happen if I stop taking weight loss medication altogether?

If you stop taking weight loss medication, there’s a good chance you’ll regain weight if you haven’t made positive changes to your lifestyle.

Weight loss medication like Wegovy and Mounjaro can help you reset your relationship with food. Your time taking the medicine should be a chance to implement positive lifestyle changes, such as regular exercise and a balanced diet. Without these changes, there’s a chance you could regain weight after stopping your medication.

Wegovy Weight Loss Results: Does It Work?

All of the clinical evidence so far suggests that Wegovy is an effective treatment for obesity.

• In the largest clinical trial carried out so far (involving 1,961 participants), once-weekly semaglutide (Wegovy) users lost an average of 14.9% of their starting body weight over 68 weeks.
• Another trial involving 304 participants found that Wegovy patients lost an average of 15.2% of their starting body weight over 104 weeks.
• A separate trial, limited to participants over 45 years old, showed an average weight loss of 9.4% over 104 weeks.
• Finally, a shorter 12-week trial found an average weight loss of 5 kg among those taking Wegovy every week.

The differences in these results can be attributed to the dosage strength, individual body responses, and whether participants made long-term lifestyle changes such as improved diet and increased activity.

Mounjaro Weight Loss Results: Does It Work?

Like Wegovy, all available evidence suggests that Mounjaro is also an effective weight loss medication. In fact, clinical trials have found Mounjaro to be even more effective than Wegovy.

• One study found that participants receiving tirzepatide (Mounjaro) experienced mean weight reductions of -12.3% (5mg dose), -18.7% (10mg dose), and -19.7% (15mg dose).
• Another study found that over 72 weeks, tirzepatide 5mg users lost 15.0%, 10mg users lost 19.5%, and 15mg users lost 20.9% of their body weight.
• A comparison trial between semaglutide (Wegovy) and tirzepatide (Mounjaro) found that, on the strongest dose of each, the average weight loss was -20.2% with tirzepatide and -13.7% with semaglutide.

Again, differences in results can be explained by dose strength, individual responses, and lifestyle changes.

Summary

Weight loss injections such as Wegovy and Mounjaro are safe and effective treatments for obesity. They have passed multiple clinical trials and have been approved for use in the UK by the MHRA. However, like all medications, they are not entirely without risk.

Potential side effects include nausea, diarrhoea, vomiting, stomach pain, and indigestion. Rarer side effects include inflamed pancreas, kidney stones, or bladder stones.

All available evidence suggests that both medications are effective: trials show that patients can lose up to 20.9% of their body weight with the strongest dose of Mounjaro and up to 15.2% with the strongest dose of Wegovy.

Reviewed by:
Irfan Mahmud, Pharmacist & Independent Prescriber (GPhC Reg. No: 2080386)

Irfan is a UK-registered pharmacist and independent prescriber with expertise in weight management, primary care, and lifestyle medicine. As the founder of Cuva Health, he is dedicated to providing safe, clinically approved treatments and clear, trustworthy health guidance.