The most popular weight loss injections available in the UK right now

Step 1, Understanding what these medicines actually do

Both tirzepatide and semaglutide mimic hormones your gut releases after eating. They slow the rate at which your stomach empties, reduce appetite signals in the brain, and help regulate blood sugar. The result, for most people, is eating less without a constant battle against hunger.

There is a misconception worth clearing up gently: these injections are not fat-dissolving or metabolism-boosting in the way some social-media posts imply. They work primarily through appetite and satiety signals. The weight loss follows from eating less, consistently, over many months alongside a reduced-calorie diet and increased activity. The medicine supports that process; it does not replace it.

Tirzepatide goes a step further by also activating GIP receptors, which is why it is described as a dual-agonist. In the SURMOUNT-5 trial, published in the New England Journal of Medicine in 2025, tirzepatide produced greater average weight loss than semaglutide 2.4mg over 72 weeks in a direct head-to-head study. That does not make it the automatic right choice for everyone, which medicine suits a particular person is a clinical question. You can read more about how weight loss injections compare across several practical dimensions.

Step 2, What the licensed treatment schedule looks like

Neither of these injections starts at full strength. Treatment begins at a low starter dose to give the body time to adjust, particularly to the gastrointestinal effects that are most common in the early weeks. For tirzepatide, that is 2.5mg; for semaglutide (Wegovy), 0.25mg. Dose increases happen roughly every four weeks, guided by the prescriber, until the maintenance dose is reached or the highest tolerated dose is found.

The injections are self-administered once a week, usually into the abdomen, thigh, or upper arm, rotating sites each time. The pens are pre-filled and designed to be used at home. The Mounjaro injection guide covers the practical technique in detail if you want the specifics of how tirzepatide is administered.

Storage matters: both medicines require refrigeration between 2°C and 8°C. The exact window for use at room temperature varies by product and is stated in each medicine's Patient Information Leaflet. The NHS tirzepatide information page is a reliable first reference for storage and administration details for Mounjaro.

Step 3, Who these injections are licensed for

The licence for both medicines covers adults aged 18 and over with a BMI of 30 or above, or a BMI of 27 to 29.9 alongside at least one weight-related condition such as high blood pressure, high cholesterol, obstructive sleep apnoea, or type 2 diabetes. BMI thresholds are reduced by 2.5 kg/m² for certain ethnic backgrounds under UK clinical guidance.

Having a BMI that meets the threshold is a starting point, not a guarantee. A prescriber reviews the whole picture: current medicines, medical history, kidney and liver function markers, and whether there are any contraindications. Certain conditions, including a personal or family history of medullary thyroid carcinoma, are listed as contraindications in the prescribing information. The consultation exists precisely to catch those things. If you are unsure whether your BMI puts you in range, the NHS BMI calculator is a straightforward tool to check before you enquire.

For a fuller picture of eligibility criteria (including the NHS rollout phases and what the private route requires) see what weight you need to be for weight loss injections.

Step 4, Side effects to know about before starting

The most common side effects of both popular weight loss injections are gastrointestinal: nausea, loose stools, constipation, indigestion, and burping. These tend to be most noticeable after the first injection or after a dose increase, and for many people they settle within a week or two as the body adjusts. Fatigue and headaches are also reported, particularly early on.

A smaller number of people experience more significant reactions. The MHRA issued a Drug Safety Update in January 2026 specifically flagging acute pancreatitis as an infrequent but serious risk with GLP-1 medicines: severe abdominal pain that spreads to the back, with or without vomiting, warrants urgent medical attention and treatment should be paused until a doctor has assessed the situation.

Anyone on these injections can report a suspected side effect directly to the MHRA via the Yellow Card scheme, a step that actively contributes to ongoing medicine safety monitoring in the UK.

Both injections are not recommended during pregnancy, while breastfeeding, or for anyone actively trying to conceive. They are not licensed for under-18s. For a fuller breakdown of what to watch for and when to seek help, the side effects and safety guide for weight loss injections covers the topic in more depth. And if cost is a factor in your thinking, the weight loss injection cost page sets out what private treatment typically involves.

If you would like a prescriber to review your situation, start your free consultation with nume and a GPhC-registered prescriber will review your answers the same day. You can also find out more about how the MJ injection is used for weight loss if tirzepatide is the option you are considering.