Types and Classes of Prescription Weight Loss Medicines in the UK

Weight loss medicines are usually discussed one brand at a time, which makes the field look like a list of competing products. It is more useful to look at it as three classes that work in completely different ways.

Once you know which class a medicine belongs to, most of its characteristics follow: how it is taken, what the side effects tend to be, who it suits and who it does not.

This page sets out the classes licensed in the UK, how each one works, what is not available here, and what a prescriber weighs when choosing between them.

Medicines are referred to by their active substance throughout rather than by brand name, since the active substance is what determines the class.

Key things to know

• Licensed UK weight management medicines can be grouped into three broad classes: gut hormone medicines, fat absorption inhibitors, and centrally acting combination medicines.
• Gut hormone medicines reduce appetite. They include semaglutide, liraglutide, tirzepatide and orforglipron.
• Within that class, tirzepatide acts on two receptors rather than one, and orforglipron is a non-peptide molecule that can be taken as an ordinary tablet.
• Orlistat works in the gut on dietary fat rather than on appetite.
• Naltrexone with bupropion acts centrally on appetite and reward pathways.
• Several medicines used elsewhere are not licensed in the UK.
• All are prescription medicines except one lower-strength orlistat product, and all are licensed for use alongside diet and activity changes.

Why the class matters more than the brand

Two medicines in the same class behave similarly in the ways that affect you. Two medicines in different classes have almost nothing in common beyond the outcome they aim at.

Knowing the class tells you where the side effects will show up, whether the medicine acts on hunger or on digestion, whether it is taken daily or weekly, and which existing health conditions are relevant.

It also explains why a prescriber might choose one over another for reasons that have nothing to do with which produces the largest average result.

Class one: gut hormone medicines

This is the class that has changed the field, and it contains most of the current options.

These medicines imitate hormones your gut releases after eating. Those hormones reduce appetite, slow how quickly the stomach empties and prompt insulin release when blood glucose is raised. Medicines in this class produce a longer-lasting version of that signal.

The practical effect is reduced appetite and earlier fullness.The practical effect is reduced appetite and earlier fullness, which can make it easier to reduce food intake.

Within the class there are three distinct groups.

GLP-1 receptor agonists

These act on one receptor, for the hormone glucagon-like peptide-1.

Semaglutide is licensed for weight management as a weekly injection and, since June 2026, as a daily tablet. The tablet form has specific administration requirements because a peptide has to survive the stomach to be absorbed.

Liraglutide is a daily injection and was among the earlier medicines in this class licensed for weight management.

A point of frequent confusion: semaglutide is also sold under a different product licensed only for type 2 diabetes. That is a separate medicine with a separate licence, and using it for weight management is off-label. The Department of Health and Social Care has strongly discouraged the off-label use of medicines licensed for other indications to manage obesity when an appropriately licensed option is available.

Dual GIP and GLP-1 receptor agonists

Tirzepatide acts on two receptors rather than one, adding GIP, another gut hormone released after eating. It is given as a weekly injection and is licensed for weight management and, separately, for type 2 diabetes.

The reasoning behind dual action is that engaging two arms of the same system may produce a larger effect than engaging one. The effects people notice are the same in kind.

Non-peptide GLP-1 medicines

Orforglipron is the newest addition, authorised by the MHRA in August 2026. It acts on the GLP-1 receptor but is a small-molecule, non-peptide medicine rather than a peptide.

That chemistry matters practically. It can be taken as an ordinary daily tablet at any time of day, with or without food, without the fasting window that peptide-based oral treatment requires.

Injection or tablet

Within this class you now have both. The mechanism is the same; what differs is delivery, frequency and the routine each asks of you.

Weekly injections require one action a week and some storage planning. Daily tablets require a daily habit, and in the case of oral semaglutide a fasted window that anchors your morning.

Neither is inherently better. Which suits you depends on your routine, your other medicines and your preference, alongside the clinical factors.

What this class has in common

• Side effects are mostly digestive: nausea, vomiting, diarrhoea and constipation, worst when starting and after each dose increase
• The dose is built up in steps to reduce those effects
• Weight loss can include reductions in both fat mass and lean mass, so maintaining adequate nutrition and physical activity is important during treatment.
• Appetite and weight can increase again after treatment stops, and weight regain is common without continued support and lifestyle measures.
• These medicines have important safety considerations, including gastrointestinal side effects and specific advice relating to pregnancy. Individual medicines also have their own warnings and precautions, including those concerning pancreatitis and interactions with other medicines.

Class two: fat absorption inhibitors

Orlistat is the only medicine in this class and it works in an entirely different way.

It acts in the intestine, reducing the amount of fat from your food that is absorbed. The unabsorbed fat passes through the digestive system. It does not affect appetite, it is not absorbed to act elsewhere in the body, and it has no effect on how hungry you feel.

Because it acts on dietary fat, what you eat directly determines both how well it works and what side effects you get. A high-fat meal produces more side effects, which are predominantly bowel-related: oily stools, urgency and changes in bowel habit.

It is taken with meals containing fat, usually during or shortly after each main meal.

Orlistat is available at two strengths. The higher strength is prescription only. A lower strength is available from pharmacies under a pharmacist's supervision, which makes it the only weight loss medicine in the UK obtainable without a prescription.

There is a defined review point: treatment at the prescription strength should be stopped after twelve weeks if at least 5% of starting body weight has not been lost.

Class three: centrally acting combination medicines

Naltrexone with bupropion is a combination tablet that acts on the brain rather than the gut.

Naltrexone is used in other contexts for dependence, and bupropion is used in other contexts for mood and smoking cessation. Together they act on appetite regulation and reward pathways, with the aim of reducing both hunger and the drive to eat for reasons other than hunger.

It holds a UK marketing authorisation for weight management. Because both components act centrally, it has a different safety profile from the other classes, with considerations around blood pressure, seizure risk, mood and interactions with several other medicines. It is not suitable for everyone and the assessment is correspondingly careful.

Commonly reported side effects include nausea, headache, constipation and dry mouth.

The classes at a glance

What is not licensed in the UK

People frequently ask about medicines they have read about elsewhere, usually American content.

Phentermine, and combinations containing it, are not licensed for weight management in the UK. It appears constantly in US material and is unavailable here.

Several older medicines have been withdrawn, including sibutramine, rimonabant and lorcaserin, after safety concerns emerged following their introduction. That history is worth knowing, because it is the reason current medicines carry ongoing safety monitoring rather than being considered settled once licensed.

Anything sold without a prescription that claims to work like these medicines is not a licensed alternative. The MHRA has also warned about illegally supplied weight-loss products and pens that may contain the wrong or undeclared active ingredients, creating serious health risks

How a prescriber chooses between classes

This is rarely a question of which class is strongest. The factors that actually decide it:

1. Your medical history, including conditions that rule out a particular class
2. Other medicines you take, which matters most with the centrally acting combination
3. Whether you have type 2 diabetes, which changes what is appropriate
4. How you have responded to previous treatment, and why anything stopped
5. Tolerability, since a medicine you cannot stay on produces no result
6. Route and frequency, and whether you can realistically follow what a given option requires
7. Pregnancy plans, which affect timing and choice
8. Your eligibility against the licensed criteria for each medicine

A medicine with a larger average effect that you cannot tolerate, cannot take alongside your other medicines, or cannot stay on consistently is not the better option for you. Treatment suitability depends on an individual clinical assessment, and your prescriber will determine the appropriate treatment.If you are considering weight-loss treatment, you can explore the weight-loss treatments available through EverydayMeds and check whether you may be eligible for a clinical consultation.

What they all have in common

• All are licensed for use alongside a reduced-calorie diet and increased physical activity. That is part of the licensed indication, not a recommendation attached to it
• All require appropriate eligibility and safety checks before use. Pharmacy-strength orlistat is supplied under the supervision of a pharmacist.
• Each medicine has defined eligibility criteria, including BMI thresholds and, for some products, specified weight-related health conditions.
• All require review rather than indefinite unsupervised use
• Results vary considerably between individuals in every class

What none of them does

None burns fat. They reduce intake, in different ways.

None targets a specific area of the body. Where fat is lost is determined by your body, not by the medicine.

These medicines do not prevent all loss of lean mass during weight loss. Adequate nutrition and resistance exercise can help support muscle mass during treatment.

None permanently changes how your body regulates weight. Weight commonly returns after stopping unless the wider changes have been established.

These medicines are licensed to be used alongside dietary and physical-activity measures rather than as a replacement for them.

When to seek advice

Speak to a pharmacist or prescriber if:

• You are considering treatment and want to know which class might suit you
• You take regular medicines, particularly if considering the centrally acting combination
• You have tried one class without success, since another may work differently for you
• You have a condition affecting the digestive system, the liver, the kidneys, or your mental health
• You are planning a pregnancy

Seek urgent medical attention for severe and persistent stomach pain that may spread through to your back, swelling of the face, lips, tongue or throat, difficulty breathing, or signs of significant dehydration.

Working out where you fit

If you are trying to make sense of the options, the class is the right level to start at. Do you want something that reduces hunger, something that acts on what you eat, or something that acts on cravings as well as appetite? That question narrows the field faster than comparing brands does.

The rest is clinical: what your history allows, what your other medicines permit, and what you can realistically stay on. A pharmacist or prescriber can work through that with you in a single conversation, which is more useful than any amount of comparing products online.

Medical Disclaimer

This article is for general information only. It is not medical advice, it is not a complete list of side effects, and it does not replace the patient information leaflet supplied with your medicine or an individual assessment by an appropriately qualified healthcare professional.

With the exception of the lower-strength orlistat product, which is supplied under a pharmacist's supervision, these are prescription-only medicines. They should only be used under the supervision of an appropriately qualified prescriber, following a clinical assessment that considers your medical history, current medicines and other conditions. Treatment suitability depends on an individual clinical assessment, and results vary between individuals.

Licensing and availability change. Check current product information for any medicine you are considering, and do not obtain prescription medicines outside the regulated system.

Seek urgent medical attention for severe and persistent stomach pain that may spread to your back, swelling of the face, lips, tongue or throat, difficulty breathing, or signs of significant dehydration. Suspected side effects can be reported through the Yellow Card scheme, which the MHRA uses to monitor the safety of medicines.