Prescription weight loss medications come in two different forms - injections and tablets - and the right treatment for you will depend on your personal preferences and medical history. Whether you choose weight loss injections or tablets, they are most effective when taken alongside lifestyle changes such as eating healthily and exercising.
Who is eligible for weight loss medication?
Weight loss medications can only be prescribed at ZAVA if you have a body mass index (BMI) over 30, or if you have a BMI of 27 to 30, with one or more weight-related health conditions. Some examples of weight-related medical conditions include prediabetes, type 2 diabetes, high blood pressure, and cardiovascular disease.
Weight loss injections like Mounjaro and Wegovy help you lose weight by reducing your appetite and helping you feel fuller after a meal. This means you’ll be more satisfied after a meal with fewer calories. Weight loss injections are the most effective weight loss treatment for most people, and can help you lose on average between 8% and 22.5% of your starting body weight depending on which treatment you are on.
Weight loss injections come as pens that you inject yourself with at home. You usually inject yourself in the stomach, thigh, or upper arm (with help) – wherever is most comfortable for you.
For patients who do not want to inject themselves or can't take weight loss injections for medical reasons, weight loss tablets might be a suitable alternative.
Weight loss tablets
Weight loss tablets help you lose weight in one of two ways: by reducing how much fat your body absorbs from food (orlistat and Xenical) or suppressing your appetite (Mysimba). Weight loss tablets are less effective than weight loss injections according to studies, but may be a good option if you are uncomfortable with injections.
Which weight loss treatment is right for me?
When choosing which weight loss treatment is right for you, it's important to consider:
whether you'd prefer taking tablets or using an injectable pen
whether you will remember to take a daily injection like liraglutide, or if you'd find it easier taking a once-weekly injection like Wegovy or Mounjaro
your current BMI, as this will determine which medications you are suitable for
how you want your weight loss medication to work: reducing appetite (Mounjaro, Wegovy, liraglutide, and Mysimba) vs reducing how much fat you absorb (orlistat products)
your budget
how much weight you want to lose, as some medications have been proven to show a larger percentage of weight loss than others
"It's important to read about each weight loss medication and how they work to see which you’d prefer. Your doctor will also check your medical history and make sure your chosen treatment is suitable, or offer an alternative if possible. Weight loss injections often cause more weight loss in clinical trials, however, all weight loss medications can be effective when taken alongside a reduced-calorie, balanced diet, and enough exercise.” – Dr Brenda Ikeji
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How do weight loss treatments work?
Each weight loss medication works in a slightly different way, as they contain different active ingredients. Wegovy, Mounjaro, Saxenda, generic liraglutide and Mysimba all work on the brain and other organs to reduce appetite levels and cravings, making it easier to eat less and lose weight.
Orlistat, alli, and Xenical work by changing how your body digests fat. alli is actually an over-the-counter weight loss pill, but a doctor or pharmacist still needs to check your BMI and medical history to make sure it’s suitable.
How do weight loss injections work?
Weight loss injections work similarly because they all belong to a class of medications called glucagon-like peptide-1 (GLP-1) agonists. This means they mimic GLP-1, a naturally occurring hormone that:
reduces appetite, helping you to eat less
delays the emptying of your stomach, helping you to stay fuller between meals
regulates your blood sugar levels which can help manage diabetes (if you have it) and reduces food cravings
Mounjaro also mimics a second hormone called glucose-dependent insulinotropic peptide (GIP). This is thought to enhance the effects of GLP-1 while improving how your body breaks down sugar and fat.
How do weight loss tablets work?
Orlistat (Xenical, alli) is a type of medication called a lipase inhibitor. This means when you eat a meal containing fat it blocks the absorption of about a third of it, and instead, this is excreted from your body in your stools. It’s recommended to take orlistat 3 times a day and consume about 30% of your daily calories in the form of healthy fats, split evenly across each of these 3 meals.
Mysimba contains 2 active ingredients called naltrexone and bupropion. These ingredients work on the parts of your brain that control appetite, reducing hunger and the desire to snack.
Side effects of weight loss treatment
Weight loss treatment can have side effects, just like any other medication. Not everyone will experience them, and if they do appear, they usually only last for a few weeks. Side effects can also come back after increasing your dose. If you get side effects whilst taking weight loss treatment and they do not go away or start affecting your daily life during treatment, you can speak to our doctors.
Some treatments are increased gradually over time to reduce the chances of side effects. You can find the full list of known side effects in the patient information leaflet that comes with your treatment pack.
Weight loss injection side effects
Here you can find out more about the common side effects of weight loss injections. If you want to learn more about specific side effects for a particular medication, you can read more in the patient information leaflet included with your treatment.
Wegovy
The most common side effects of Wegovy are:
headaches
feeling sick
being sick
diarrhoea
constipation
stomach pain
feeling weak or tired
The side effects usually go away over time.
Mounjaro
Very common side effects of Mounjaro are:
diarrhoea
constipation
feeling sick (nausea)
being sick (vomiting)
when used for type 2 diabetes treatment in combination with certain diabetes medications: low blood sugar
stomach pain
Side effects that affect the digestive system are usually not severe. They mostly show up after starting the treatment or after increasing the dosage. Once your body gets used to the medication, these side effects usually disappear.
Liraglutide
Very common side effects of liraglutide include:
nausea
vomiting
diarrhoea
constipation
headaches
These side effects usually go away after a few days or weeks.
Weight loss tablets side effects
Read more on the most common side effects of weight loss tablets. If you would like to discover more about rare or uncommon side effects of any of these treatments, information is available in the patient information leaflet included with your treatment.
Mysimba
Common Mysimba side effects include:
hot flushes
anxiety
trouble sleeping
headaches
dizziness
restlessness
heart palpitations
These usually go away once your body gets used to the medication. You cannot take Mysimba if you have certain medical conditions, but our doctors will check your suitability before prescribing.
Orlistat, alli and Xenical
The most common side effects of orlistat, alli, and Xenical are:
needing to poo urgently and more frequently
oily or fatty poo
oily discharge from your rectum (anus)
flatulence (farting)
Sticking to a low-fat diet will decrease the risk of these side effects.
Who shouldn't use weight loss medication?
People can’t take GLP-1 agonists found in weight loss injections like Wegovy and Mounjaro if they have existing health conditions that make them unsafe.
Weight loss injections may not be safe for:
people taking certain medications, such as diabetes medicines (including insulin) and immunosuppressants
pregnant or breastfeeding women
those with pancreatic disorders, such as pancreatitis
those with a history of thyroid cancer
those with a history of gallbladder disease
How else can I lose weight?
Alongside medication, you can support your weight loss journey with healthy eating and regular exercise. Surgery is sometimes considered when other weight loss methods haven’t worked and should generally be a last resort.
Diet
To lose weight at a safe pace of 0.5 to 1 kg a week, you should reduce your calorie intake by 600 kcal a day. Your doctor will help you with a diet plan if you need it. This is not about following a ‘fad’ diet, but about making healthier long lasting choices that will improve your eating habits for the rest of your life.
For women, weight loss can be achieved by consuming no more than 1,400 calories a day. Men should be eating no more than 1,900 calories daily. To lose weight, you should swap high-energy and unhealthy foods for healthier choices. This means lowering your intake of processed food, fast food, and sugary drinks like fizzy drinks like sodas, and alcohol.
Exercise
Although a healthy and balanced diet can help you to lose weight, you also need to burn energy through regular exercise. The UK government recommends that adults do at least 150 minutes of moderate intensity exercise weekly. This can be split into 30 minute sessions 5 times a week. Some exercise is better than none, so if you struggle to exercise, you can increase this slowly, starting with 10 minutes a day.
Exercise has many health benefits and can reduce the risk of other illnesses that can affect your weight. According to the NHS, regular exercise can reduce the risk of getting type 2 diabetes by up to 30%.
Surgery
Weight loss surgery can sometimes be used as a treatment for obesity. It is only available through the NHS if you are severely obese and:
are fit enough to have surgery
have a BMI over 40, or between 35 and 40 and certain health conditions that weight loss could improve, such as type 2 diabetes
are committed to the long term follow up of your surgery
you have tried all other weight loss strategies but they have not achieved or maintained weight loss
are receiving intensive management of your weight, usually through your GP
In rare cases, surgery may be recommended straight away if you have a BMI over 50.
FAQs
Are weight loss injections safe?
Weight loss injections are safe when prescribed by a doctor and used correctly. A doctor needs to review your medical history and make sure weight loss injections are safe for you. They’ll also discuss any side effects, risks, and benefits of using weight loss injections. This can help you make an informed decision about using treatment.
Weight loss injections, like all prescription medications, have the chance to cause some serious side effects. Most people will not experience these but they can happen. You should read the patient information leaflet that comes with your medication and let your doctor know if you’re getting any side effects from your treatment. Although it’s common to get mild side effects, these should go away as you continue treatment.
➤ For more information on weight loss injection side effects, who can safely be prescribed with weight loss medication, and what to do if you get side effects, we recommend reading our doctor-reviewed weight loss injection safety guide.
What's the best weight loss pill?
The best weight loss pill for you depends on many factors, including your preferences, suitability, and BMI. The most popular weight loss pills in the UK are orlistat (also called Xenical and alli) and Mysimba.
Weight loss pills containing orlistat can be a great choice if you’re happy to follow a reduced-fat diet and want a pill that can help you lose weight by reducing how much fat you absorb from your meals. Mysimba is another great choice if you’d prefer a weight loss pill that works on your appetite to help you reduce how many calories you consume.
Which weight loss injection is the best?
Mounjaro has shown the highest rate of weight loss in clinical studies at 22.5%. However, results vary from person to person and other factors like side effects may also affect which injection works best for you.
All weight loss injections work in a very similar way, so which one is best for you usually comes down to preference and suitability. Liraglutide, Wegovy, and Mounjaro can all reduce your appetite and leave you feeling fuller for longer. Mounjaro is the only weight loss injection that mimics 2 hormones which is thought to enhance appetite suppression and digestion further.
Wegovy has an average weight loss of 21%, while liraglutide has an average weight loss of about 8%.
Who is eligible for weight loss medication?
At ZAVA, you may be eligible for weight loss medication if you have a BMI over 30 and have no condition or current medications that interact with your chosen treatment. You may also be eligible if you have a BMI between 27 and 30 and also have a weight-related health condition.
Eligibility for weight loss medication is different on the NHS and usually, you need at least a BMI over 30.
What should I eat on weight loss medicaiton?
Regardless of the weight loss medication you are taking, you should be eating a healthy, balanced diet and reducing your daily calories for safe and sustainable weight loss.
This usually involves eating 600 calories less than your recommended daily intake and should include:
plenty of fruit and veg
some complex carbohydrates
some healthy, lean sources of protein from meat, fish, or plant-based alternatives
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: 04 Aug 2025
Real results from ZAVA weight loss patients
Fiona, Newcastle
“It feels good to look in the mirror...”
“... and I feel much happier than before. Everything – so life in general – feels better.”
Neil, H., Scotland
“I use the phrase genuinely life changing...”
“... I feel normal again and not embarrassed about my size.”
A. P., London
“At my heaviest, I simply didn't recognise myself...”
“... Today, I feel like my old (youthful) self.”
Anna, Scotland
“I have regained so much more stamina and energy...”
“... since I started taking the medication - the entire experience is definitely worth it!”
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