How to safely use weight loss medications

Are Wegovy and Mounjaro safe?

The short answer

If you're considering weight loss treatment with Wegovy or Mounjaro, you probably have questions about their safety.


We understand. That's why we've created this guide. We want you to feel confident in your decision before you start treatment.


So, are weight loss injections safe?


The short answer is 'yes'. Weight loss injections like Wegovy and Mounjaro are considered safe when prescribed by a doctor and taken according to their instructions.


However, no medication is 100% safe. All drugs can cause side effects and may become unsafe if used incorrectly.


With that in mind, let's look at the straight facts so you can judge for yourself.

What does 'safe' mean in medicine?

People often use the words 'safe' and 'unsafe' as if they're complete opposites. Things are either safe or unsafe – just like things are either black or white or on or off, right?


The truth is that safety is a spectrum. In our daily lives, we consider things to be safe if they meet a certain threshold for safety and unsafe if they're unacceptably risky.


But this comes with a problem. We often use our gut instincts to determine whether something is safe – and our gut instincts aren't always right.



Taking the bus, for instance, is usually considered very safe. On the other hand, many people are scared of flying and consider it unsafe. However, you're actually safer on a plane than you would be on a bus.


In the world of medicine, safety is also a spectrum. But the difference is that doctors never rely on gut instinct. Before a drug can be licensed for use in the general population, it must go through years of testing and clinical trials.


By the time those tests and trials are finished, doctors have lots of data to tell them exactly how safe the drug is.


In medicine, a drug is considered safe if the benefits have been shown – time and time again, through extensive clinical trials – to outweigh the potential risks.

Wegovy: is it safe?

Wegovy is a fairly new weight loss drug. It was approved in the UK as an obesity treatment in September 2023.


Despite its relative newness, Wegovy has been proven to be safe and effective for most patients in clinical trials and case studies.


One of the biggest Wegovy trials is known as 'STEP 1'. The results of this trial were published in The New England Journal of Medicine under the title, 'Once-Weekly Semaglutide in Adults with Overweight or Obesity'.


The trial involved 1,961 adults with obesity. The participants were randomly selected to begin treatment with a 2.4mg dose of Wegovy or a placebo (a harmless treatment that has no medical effects). They took the drug or the placebo for 68 weeks.


Here's what the trial found:


  • Wegovy users lost 14.9% of their body weight on average.
  • Over 86% of Wegovy users lost 5% or more body weight.
  • The most common side effects were nausea and diarrhoea. These effects were typically 'mild-to-moderate in severity and subsided with time'.
  • About 7% of Wegovy users stopped treatment because of side effects.

Mounjaro: is it safe?

Mounjaro is slightly newer than Wegovy. The drug was approved in the UK as a weight loss treatment in November 2023.


However, like Wegovy, Mounjaro has been through extensive tests and trials to prove that it's safe for most patients.


The SURMOUNT-1 Mounjaro trial took place between 2019 and 2022. It involved 2,539 participants with obesity, who were randomly assigned one of the following:


  • A 5mg dose of Mounjaro
  • A 10mg dose of Mounjaro
  • A 15mg dose of Mounjaro
  • A placebo


The results were published in The New England Journal of Medicine in June 2022 as 'Tirzepatide Once Weekly for the Treatment of Obesity'.



Here's what the trial found:


  • Participants on the highest Mounjaro dose lost 20.9% of their body weight on average.
  • Within the same group, 91% lost at least 5% of their body weight.
  • The side effects were similar to those seen with Wegovy. They were typically mild to moderate and usually happened when users moved to a higher dose.
  • Around 6% of 15mg Mounjaro users stopped treatment because of side effects.

Are there any serious or long-term side effects?

Serious side effects of Wegovy and Mounjaro are rare. However, they do happen from time to time. Here are some of the more severe side effects you should be aware of:


  • Pancreatitis (1 in 500 risk)
  • Gallbladder-related disorders (about a 1 in 500 risk)
  • Injection site reactions (1 in 100 risk)
  • Gastroparesis (rare)
  • Mental health disturbance (rare)


For more information, see our guide to Wegovy and Mounjaro side effects  or visit the manufacturers' websites:


Are weight loss injections safe for people with diabetes?

It depends.


Wegovy and Mounjaro both began as diabetes medications. To take Wegovy as an example, we know its active ingredient (semaglutide) is safe for people with diabetes because Ozempic uses the same active ingredient. Ozempic has been used to treat diabetes in the United States since 2017.


However, there are a few special precautions for people who take certain diabetes drugs. This includes:


  • Insulin
  • Sulphonylureas, such as glipizide and gliclazide
  • DPP4 inhibitors (these drugs usually have names that end in 'gliptin')
  • GLP-1 agonists (drugs that work in a similar way to Wegovy and Mounjaro)


These drugs can interact with Wegovy and Mounjaro in dangerous ways. If you're currently taking one of these drugs, make sure you let us know when you have your online consultation.

Tips for taking Mounjaro or Wegovy safely

  • Get a prescription

    It's crucial that you have a consultation with a medical professional to make sure it's safe and appropriate for treatment to begin. Getting hold of weight loss medications without a prescription is illegal and potentially very risky.

  • Do your research

    Your doctor will determine whether it's safe for you to take Wegovy or Mounjaro. But are you sure you know exactly what's involved? It's a good idea to do your own research into the medications and their side effects so you can be sure they're right for you.

  • Stick to your dose

    Remember that losing weight with Mounjaro or Wegovy is a marathon, not a sprint. Don't be tempted to increase your dose too early or take the medication more regularly than you should.

  • Keep to a schedule

    You can take Wegovy or Mounjaro at any time of day, with or without meals. However, it's important to stick to the same day each week. If you do miss a day, you can find out what to do in our guide to the best time to take Wegovy or Mounjaro.

  • Listen to feedback

    When you join the SemaPen programme, you can expect real-time, tailored feedback from our clinical team. It's their job to keep you safe and give you the best chance of success, so make sure you take their advice on board.

  • Report anything unusual

    If you have side effects that are too strong to cope with, make sure you report them to your SemaPen support team. The same goes for any unusual side effects. We may recommend moving to a lower dose or sticking to the same dose for a little longer. 

Where next?

Our programmes

Discover SemaPen's expert-led Wegovy and Mounjaro treatment programmes.

Explore programmes

What to expect

Learn how much weight you can expect to lose after one week, one month and beyond.

Read guide

Side effects

Get clued up on the potential side effects of Wegovy and Mounjaro.

Read guide

Diet and exercise tips

Start here if you want to learn about the Wegovy and Mounjaro weight loss diets.

Read guide
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