How Do Weight Loss Injections Work?

Weight loss injections have become a popular way to lose more weight than through dieting and exercise alone. However, it’s important to do your research before starting any treatment so you know what to expect. In this guide, we explore how weight loss injections work, how long they take to work, and how you can get the best results from your chosen weight loss treatment.

Brenda Ikeji

Medically reviewed by

Dr Brenda Ikeji

Last reviewed: 03 Mar 2025

What's on This Page?

What are weight loss injections?

Weight loss injections, like Wegovy and Mounjaro, are medications designed to help you lose weight. Originally developed to help manage type 2 diabetes, the active ingredients in these treatments were also found to be effective in promoting weight loss and are now approved for use in the UK for weight loss specifically.

When used for weight loss, these injections are usually taken once a week (or once a day for liraglutide) at home via an injectable pen that comes pre-filled with your required dose.

There are currently 3 approved weight loss injections available in the UK. These are:

How do weight loss injections work?

Weight loss injections work by mimicking a natural hormone in your body called GLP-1 (glucagon-like peptide-1). Your body releases this hormone after you eat, and by doing so, it changes how full you feel and your response to food, helping you lose weight.

Slows down digestion

Normally, when you eat food, your stomach breaks it down and sends it into your intestines quite quickly. But when you take a weight loss injection, it slows down this process. This means food stays in your stomach for longer, so you feel fuller for longer and aren’t as likely to get hungry again soon. This stops you from eating more, helping you lose more weight.

Reduces hunger and cravings

GLP-1 sends signals to your brain that tell it you're full. Think of it as an "off switch" for hunger. When your brain gets this message, it reduces your appetite, meaning you eat less without feeling hungry or deprived. This means that weight loss injections can make you feel more satisfied with smaller portions, helping you cut back on snacking and overeating. This helps you better control your caloric intake to lose more weight.

Regulates blood sugar levels

Another benefit of weight loss injections is that they help control blood sugar levels. When you eat, your body normally releases insulin to manage blood sugar. GLP-1 injections help keep your blood sugar levels stable and prevent energy crashes that can lead to cravings. Less cravings means less eating and more weight loss. It also makes it easier to stick to healthier foods as you do not need the same energy boost from sugars.

In order to be effective, weight loss injections should be used alongside a healthy diet and lifestyle changes. You’ll need to commit to a long-term diet and exercise plan to get the most out of them.

How long do weight loss injections take to work?

Weight loss injections start working in your body soon after you begin treatment. Many people notice a decrease in appetite within the first week. However, significant weight loss usually takes more time. How quickly injections work can vary from person to person, and how quickly you lose weight will also depend on your diet, exercise regime, and the treatment you’re taking.

Here's a simple breakdown of what to expect while taking Wegovy:

First month: You might feel less hungry and start to see minor weight changes.

By 3 months (12 weeks): It's common to lose about 5% of your starting weight.

By 6 months (24 weeks): Many people reach a 10% weight loss.

By 12 months (52 weeks): Average weight loss of around 15%.

Wegovy weight loss graph showing an average loss of 15.6% by 68 weeks

Here’s a breakdown of what to expect while taking Mounjaro:

First month: You should feel less hungry and start seeing small changes in your weight. In clinical studies, patients lost just under 1% of their starting weight per week, totalling approximately 4% by the end of the first month.

By 3 months (12 weeks): It’s common to see a weight loss of about 8% from your starting weight.

By 6 months (24 weeks): Patients lose approximately 12 to 15% of their initial body weight by this time.

By 12 months (52 weeks): Patients achieved an average weight loss of around 15.3% of their starting weight.

By 18 months (72 weeks): Studies have shown that individuals can lose up to 22.5% of their initial body weight when taking a 15 mg weekly dose.

mounjaro weight loss graph showing weight loss for the 5mg, 10mg, and 15mg maintenance doses

Remember: These are only guidelines and averages of what people have achieved in studies. Your weight loss results might differ depending on your starting weight, metabolism, diet, lifestyle, exercise regime, and age.

How much weight can I lose on weight loss injections?

Weight loss injections have been clinically proven to help you lose a significant amount of weight when combined with diet and lifestyle changes. How much weight you can lose will depend on the treatment you’re on, how you respond to it, and how dedicated you are to making healthier diet changes and exercising.

Wegovy (semaglutide):

Clinical studies have shown that over 68 weeks (a little over a year), patients on Wegovy tend to lose an average of 15% of their starting weight.

Approximately one-third of the participants achieved a weight loss of 20% or more.

Mounjaro (tirzepatide):

Clinical studies have shown that in a 72-week study (around a year and a half), patients taking Mounjaro experienced significant weight loss. Those on a 10 mg dose lost about 13.4% of their body weight, while those on a 15 mg dose lost approximately 15.7%. In another study, patients achieved an average weight loss of nearly 26% after 84 weeks.

How to get the best results from weight loss injections

Weight loss injections work best when combined with healthy habits. Here’s how to get the most out of your treatment:

1. Eat a balanced diet

What you eat makes a big difference. Try to:

  • choose healthy foods – lean proteins (like chicken, fish, and tofu), whole grains, fruits, and vegetables will keep you feeling full and give your body the nutrients it needs
  • avoid processed and high-fat foods – fast food, fried food, and sugary snacks can slow down your progress

2. Stay active

Exercise helps your body burn extra calories and supports your metabolism. Try to:

  • do physical activities you enjoy – walking, swimming, cycling, or even dancing can all help
  • mix it up – a combination of cardio (like running or brisk walking) and strength training (like lifting weights or bodyweight exercises) gives the best results
  • start small and build up – even doing 10 to 15 minutes a day can make a difference

3. Take your medication as prescribed

You need to take your weight loss injections exactly as prescribed for the best results. Try to:

  • stick to your schedule – make sure you take it at the same time each week to make it easier to remember
  • follow your dosage instructions – your doctor will start you on a lower dose and slowly increase it over time as you get used to your medication
  • avoid skipping doses – while missing one dose isn’t going to cause you many problems, missing multiple doses can lead to slower progress or even needing to restart your treatment altogether

4. Set realistic goals

Losing weight takes time, so be patient with yourself. Try to:

  • aim for steady progress – even small losses (like 1 to 2 pounds per week) add up over time
  • track your progress – keep a record of your progress as this can help you see how well you’re doing and support your doctor in monitoring your progress
  • celebrate every win (no matter how small) – fitting into an old pair of jeans or even sticking to your diet in a challenging moment are wins that deserve to be celebrated

5. Manage side effects

Mild side effects like nausea or stomach issues are common with weight loss injections, especially when first starting out. Here’s how to make things easier:

  • eat small, balanced meals – large meals can make nausea worse so try eating smaller portions throughout the day
  • stay hydrated – drink water regularly to prevent headaches and dehydration
  • avoid greasy or sugary foods – these can upset your stomach and make side effects worse
  • take it easy on alcohol (or avoid it altogether) – it can increase nausea and affect blood sugar levels, plus most alcoholic drinks are packed with calories, slowing weight loss
  • give your body time – most side effects improve as your body gets used to the medication, but if symptoms don’t go away or get worse, check in with your doctor

What to eat when taking weight loss injections

What you eat while using weight loss injections can make a big difference in how you feel and how well your treatment works. Eating the right foods can help boost weight loss, reduce side effects, and keep you feeling good throughout your journey.

Focus on healthy, nutritious foods

Aim to eat nutritious, balanced meals that keep you full and energised. In particular, focus on the following healthy food groups:

  • lean proteins
  • whole grains
  • fruits and vegetables
  • healthy fats

Lean proteins

These help build muscle and keep you feeling full for longer. Some great examples include:

  • chicken, turkey, fish, tofu, and eggs
  • low-fat Greek yoghurt, and cottage cheese
  • beans, lentils, and chickpeas

Whole grains

These provide long-lasting energy and help with digestion. Some great examples include:

  • brown rice, quinoa, whole wheat bread, and whole grain pasta
  • oats, whole wheat crackers, and barley

Fruits and vegetables

These are packed with vitamins, fibre, and antioxidants. Some great examples include:

  • leafy greens (like spinach, kale, and lettuce)
  • cruciferous veggies (such as broccoli, cauliflower, and Brussels sprouts)
  • berries, apples, pears, and oranges

Healthy fats

These (in moderation – a tablespoon with some of your meals) help support heart health and brain function. Some great examples include:

  • avocados, olive oil, nuts, and seeds
  • fatty fish like salmon and mackerel

➤ For diet tips, ideas on healthy swaps, and recipes you can use while taking weight loss injections, see our Mounjaro diet plan guide.

Don’t forget to drink plenty of water. Staying hydrated is essential to prevent dehydration and help manage side effects like nausea.

What to avoid

Some foods can slow down weight loss or make you feel worse while taking injections. Try to limit or avoid the following food groups:

High-fat foods

These can be hard to digest and may cause nausea or stomach discomfort. They include:

  • fried foods (like chips, fried chicken, and doughnuts)
  • fast food, and processed meats like bacon and sausages
  • heavy cream, butter, and full-fat cheese

Sugary foods and drinks

These can cause blood sugar spikes and lead to cravings. They’re also often very high in calories and include:

  • cakes, cookies, sweets, and pastries
  • sugary drinks (like soda, fruit juice, and energy drinks)

Processed foods

These are often packed with unhealthy fats, sugars, and additives. They include things like:

  • white bread, sugary cereals, and instant noodles
  • packaged snacks like crisps and biscuits

Alcohol

This can affect how your body processes the medication and may increase side effects like nausea or dizziness. Alcoholic drinks also contain a lot of calories and can lead to binge eating, so it’s best to cut down or avoid them altogether.

➤ For more information on foods to limit or avoid while taking weight loss medications, see our in-depth guide on foods to avoid while taking Wegovy.

Eat at a calorie deficit

To lose weight, it’s important to burn more calories than you consume. This is called a calorie deficit. Weight loss injections make it easier to stick to a calorie deficit as you’ll be less hungry from eating less food.

If your body burns 2,000 calories per day, eating 1,500 calories per day puts you in a 500-calorie deficit. A 500-calorie daily deficit can lead to a weight loss of about 1 pound per week.

It’s not just about eating fewer calories but choosing nutrient-dense foods that support your health. Protein helps you preserve muscle while still losing weight. Aim for around 20 to 30g of protein per meal.

Finally, extreme calorie cutting isn’t necessary. Too few calories can leave you feeling tired, weak, and unmotivated.

Are weight loss injections safe?

Yes, weight loss injections have been thoroughly tested to ensure they are safe and effective for helping people lose weight. They have passed all necessary clinical trials and have been approved by health authorities for weight management long term. So, as long as a doctor has approved treatment for you, weight loss injections should be safe.

However, like all medications, they can cause side effects. Common ones include nausea, diarrhoea, constipation, and headaches. These usually improve as your body gets used to the medication. Your doctor will ensure weight loss treatments are safe and suitable for your body and health before prescribing. This is why getting weight loss injections from a trusted provider is crucial.

At ZAVA, we have a rigorous screening process and a team of expert doctors who review every request we receive personally.

Medically reviewed by

brenda-ikeji.png Dr Brenda Ikeji

Brenda studied medicine at St Georges University of London and has most recently worked in John Radcliffe Oxford University Hospitals, covering acute and general medicine.

Last reviewed: 03 Mar 2025

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