For many people in the UK, finding a weight loss treatment that genuinely works can feel overwhelming. 

With three clinically proven medications widely available – Wegovy, Mounjaro and Orlistat – it pays to understand each treatment.

This guide is designed to help you compare the three most commonly prescribed weight loss medications in the UK, so you can have a more informed conversation with a prescriber. 

Every treatment must be tailored to your individual health profile, so the information below should sit alongside, not replace, advice from a qualified clinician.

Wegovy (semaglutide)

Wegovy is a once-weekly injection that mimics a natural gut hormone called GLP-1. It works by reducing appetite, slowing the rate at which your stomach empties and helping you feel fuller for longer.

Expected results. In clinical trials, adults taking Wegovy alongside diet and lifestyle changes lost an average of around 15% of their body weight over 12 months. 

In January 2026 a higher 7.2mg dose was made available in the UK, with trial data showing average losses of up to 21% over 72 weeks for patients who can tolerate the increased strength.

Common side effects. Nausea, vomiting, diarrhoea, constipation, headache, tiredness and tenderness at the injection site are the most frequent. Most settle within a few weeks as your body adjusts. Rarer but more serious risks include pancreatitis, gallstones and severe allergic reactions, and you should seek urgent advice if you develop persistent abdominal pain.

Drug interactions. Wegovy should not be combined with other GLP-1 medicines such as Mounjaro, or with Orlistat. Because it slows digestion, it can affect the absorption of some oral medicines, including certain antibiotics and the oral contraceptive pill. If you take insulin or a sulfonylurea for type 2 diabetes, your risk of low blood sugar is higher and your dosing may need to be reviewed.

Mounjaro (tirzepatide)

Mounjaro is also a once-weekly injection, but it acts on two gut hormone pathways at once; GLP-1 and GIP. This makes it the most potent of the three treatments currently licensed for weight management in the UK.

Expected results. In the SURMOUNT trials, adults without diabetes taking the highest dose alongside lifestyle changes lost an average of around 20% of their starting body weight after 72 weeks. From April 2026 Mounjaro became available on the NHS through GP practices under new Quality and Outcomes Framework indicators, although access varies considerably by area and is not guaranteed even where eligibility criteria are met.

Common side effects. The side effect profile is broadly similar to Wegovy – nausea, vomiting, diarrhoea, constipation, indigestion and tiredness  and is most noticeable when the dose is increased. Less common effects include gallstones and injection-site reactions. As with all GLP-1 medications, persistent severe abdominal pain should be assessed urgently.

Drug interactions. Mounjaro can reduce the effectiveness of oral contraceptives, particularly during the first four weeks of treatment and after any dose increase, so an additional barrier method is recommended during these periods. 

It should not be combined with Wegovy or Orlistat. Anyone using insulin or a sulfonylurea will need closer blood glucose monitoring to reduce the risk of hypoglycaemia.

Orlistat (Xenical and Alli)

Orlistat is a capsule taken three times a day with meals. Unlike Wegovy and Mounjaro, it does not act on appetite. Instead, it blocks roughly a third of the fat in your meals from being absorbed, so that fat is passed out in your stool.

Expected results. The clinical evidence suggests Orlistat helps people lose around 5–10% of their starting body weight over 6–12 months when it is combined with a calorie-controlled, low-fat diet. Results are typically more modest than those seen with GLP-1 injections, but Orlistat suits people who prefer an oral treatment, or for whom injections are not appropriate.

Common side effects. Most side effects are gastrointestinal and directly linked to how the medicine works. They include oily or loose stools, urgent bowel movements, faecal incontinence, flatulence with discharge and abdominal discomfort. 

These tend to worsen if your meal is too high in fat and ease once your diet is adjusted. Orlistat can also reduce absorption of fat-soluble vitamins (A, D, E and K), so a daily multivitamin taken at bedtime is usually recommended.

Drug interactions. Orlistat can reduce the absorption of several important medicines, including levothyroxine, ciclosporin, certain anti-epileptics, warfarin, the oral contraceptive pill and some HIV antiretroviral treatments. 

It should be separated from these medicines by several hours where possible, and your prescriber should review your full medication list before you start.

How to choose the right option for you

The right choice depends on your starting weight, your medical history, your personal preferences and the level of weight loss you are realistically aiming for. As a broad guide:

  • Mounjaro typically delivers the greatest weight loss and may suit people with a higher BMI, or those who have not achieved their goals with other treatments.
  • Wegovy is also highly effective and is the most established GLP-1 option in the UK, with a substantial real-world evidence base.
  • Orlistat offers a non-injectable alternative with more modest results, often suited to people who prefer a tablet or who cannot use a GLP-1 medication.

None of these medications work in isolation. 

Sustained results depend on combining treatment with a calorie-controlled diet, regular physical activity and ongoing clinical support. 

Certain health conditions, including a personal or family history of medullary thyroid cancer, pancreatitis, severe gastrointestinal disease or pregnancy, may rule particular options out. 

This is why a proper clinical assessment is essential before starting any treatment.

Speak to a UK clinician

At onlinemeds we specialise exclusively in safe, effective weight loss medication. Our UK-registered pharmacist prescribers will review your medical history, talk you through the most suitable options and provide ongoing support throughout your treatment.

Check your eligibility today and take the first step towards a treatment plan that is right for you.