What is a weight-loss pen?

A "weight-loss pen" is a pre-filled injector pen that delivers a small dose of medicine into the fat just under the skin — a subcutaneous injection into the abdomen, thigh or upper arm.[1] The needle is short and hidden, and the pens are designed for people to inject themselves at home after being shown how. What is inside the pen is the important part: a GLP-1 receptor agonist, or in one case a medicine that acts on two gut-hormone receptors at once. These medicines reduce appetite and slow the rate at which the stomach empties, so you feel full for longer and tend to eat less.[1]

The terms people search for — "weight loss injections", "skinny jab pen", "mounjaro pen", "wegovy pen" — all point at the same category of medicine: serious treatments for overweight and obesity, used alongside changes to diet and activity, not cosmetic quick fixes.

Key takeaway

A weight-loss pen injects a GLP-1 (or dual GIP/GLP-1) medicine under the skin. The two most discussed in the UK — Mounjaro and Wegovy — are injected once a week and are prescription-only. The medicine, not the pen, does the work.

The pens on the UK market

Two once-weekly pens dominate the conversation. Mounjaro is the brand name for tirzepatide, made by Eli Lilly, and is supplied both as a multi-dose KwikPen and as single-dose pre-filled pens.[2] Wegovy is the brand name for semaglutide, made by Novo Nordisk, and comes as a FlexTouch pre-filled pen.[1] The MHRA authorised Mounjaro for weight management on 8 November 2023.[4]

The two headline UK weight-loss pens, from the electronic Medicines Compendium Summaries of Product Characteristics.
PenMedicineTypeMakerHow oftenDevice
MounjaroTirzepatideDual GIP & GLP-1 receptor agonistEli LillyOnce weeklyKwikPen (multi-dose) & single-dose pens
WegovySemaglutideGLP-1 receptor agonistNovo NordiskOnce weeklyFlexTouch pre-filled pen

Two other pens are worth naming plainly. Saxenda (liraglutide) is an older, licensed weight-management pen, but it is injected once a day rather than weekly.[3] Ozempic contains the same medicine as Wegovy (semaglutide) but at lower doses, and is licensed only for type 2 diabetes — not for weight loss. Using Ozempic to lose weight is off-label, and the MHRA advises that the licensed weight-loss route for semaglutide is Wegovy.[6] Our Mounjaro pen and Wegovy pen pages go through each device in detail.

How the medicines work

GLP-1 is a hormone your gut releases in response to food. It helps control blood sugar and appetite, but the natural hormone lasts only minutes in the bloodstream before the body breaks it down.[11] The medicines inside these pens are engineered, long-acting versions designed to keep working across a whole week, which is what makes a once-weekly injection possible.[1] That half-life engineering — extending a molecule that would normally survive only minutes into one dosed weekly — is the basis of the whole modern pen category.[11]

Semaglutide (Wegovy) is a single-receptor GLP-1 agonist. Tirzepatide (Mounjaro) is different: it is a dual agonist that mimics two gut hormones at once — GIP as well as GLP-1 — which together reduce appetite and slow gastric emptying.[2] That is why it is inaccurate to call Mounjaro simply "a GLP-1"; it acts on both receptors.

In practice, both medicines are started at a low dose and stepped up slowly. Wegovy follows a 16-week escalation — 0.25 mg for four weeks, then 0.5 mg, 1 mg, 1.7 mg and finally a 2.4 mg once-weekly maintenance dose.[1] Mounjaro starts at 2.5 mg once weekly for four weeks, moves to 5 mg, and can be increased in 2.5 mg steps no sooner than every four weeks, up to a maximum of 15 mg.[2] The gradual climb is deliberate: it reduces gastrointestinal side effects. A higher 7.2 mg once-weekly maximum dose of semaglutide was authorised in the UK in January 2026, with a single-dose 7.2 mg pen approved by the MHRA on 14 April 2026.[5]

What the research actually shows

The pens reached the market on the back of large, randomised trials, and it helps to quote them honestly rather than reaching for the biggest number.

  • Semaglutide (Wegovy) — STEP 1. A 68-week trial in 1,961 adults without diabetes found a mean weight change of −14.9% with semaglutide 2.4 mg versus −2.4% on placebo, and 86% of participants lost at least 5% of their body weight.[9]
  • Tirzepatide (Mounjaro) — SURMOUNT-1. A 72-week trial in 2,539 adults without diabetes produced mean weight reductions the MHRA cites as −16.0% (5 mg), −21.4% (10 mg) and −22.5% (15 mg) versus −2.4% on placebo, with at least 5% loss in 89–96% of participants.[4] The trial's intention-to-treat analysis reports somewhat lower figures — around −15.0%, −19.5% and −20.9% — because it answers a slightly different question; both are legitimate ways to read the same study.[10]

Keep those numbers honest: they are trial averages achieved alongside diet and activity support, and individual results vary widely. Trial results in people with type 2 diabetes have generally shown smaller average weight loss than in people without diabetes — a pattern seen across this class of medicine.

A prescription-only reality

Under their UK licences, Wegovy is for adults with a body-mass index (BMI) of 30 or more, or 27 to under 30 with at least one weight-related health problem;[1] Mounjaro is licensed on a similar basis, for a BMI of 30 or more, or 27–30 with a weight-related condition such as raised blood pressure or prediabetes.[4] A licence, though, only means the medicine may be prescribed — it does not automatically mean the NHS funds it.

For NHS use in England and Wales, NICE sets narrower rules. Wegovy is recommended only within a specialist weight-management service and for a maximum of two years, with treatment stopped if a person has not lost at least 5% of their weight after six months on the maintenance dose.[7] Mounjaro is recommended for adults with a BMI of 35 or more plus at least one weight-related condition, through a phased three-year NHS rollout; NICE also lowers the BMI thresholds (usually by 2.5) for people from several ethnic backgrounds in whom risk occurs at a lower BMI.[8] Many people access the pens privately instead, but even then a prescriber still has to decide the medicine is appropriate.

Sourcing a pen safely: illegal sellers and #FakeMeds

Because demand is high, weight-loss pens have become a magnet for unregulated online sellers, social media adverts and "skinny jab" offers. This is the part of the subject where getting it wrong can genuinely harm you, so it is worth being blunt.

Selling these pens without a prescription is illegal

Mounjaro and Wegovy are prescription-only medicines. Selling one to the public without a prescription is against the law in the UK, and medicines bought this way may be falsified — the wrong drug, the wrong dose, contaminated, or nothing active at all. Legitimate supply only ever comes from a GPhC-registered pharmacy dispensing against a prescription written after a consultation. The MHRA's #FakeMeds campaign explains how to spot a fake and check a seller. This site never sells, supplies, prescribes or links to any provider of these medicines.

A legitimate route has a recognisable shape: a proper consultation, a named prescriber who decides whether the medicine is appropriate, and a registered pharmacy that dispenses it in sealed manufacturer packaging with a patient information leaflet. The offers regulators warn against tend to share a few tell-tale signs:

  • No prescription needed. If a seller will supply a pen without any prescriber assessment, they are breaking the law — and you cannot know what is in the box.
  • Sold through social media, messaging apps or a marketplace listing rather than a registered pharmacy.
  • Claims that seem too good to be true, "beauty" or "skinny jab" branding, or pressure to buy quickly.
  • Vials, unbranded pens or loose needles instead of a sealed, brand-name manufacturer pen.

Side effects, in brief

The most common side effects are gastrointestinal — nausea, diarrhoea, vomiting and constipation — usually mild to moderate, most likely during dose increases, and they tend to settle over weeks.[1] In Wegovy's pooled trial data, nausea was reported by 43.9% of users and diarrhoea by 29.7%.[1] Rarer risks flagged in the UK product information include pancreatitis and gallbladder problems, and the medicines should not be used in pregnancy.[2] Our side effects page has the full picture. If you experience a side effect from any medicine, report it through the MHRA Yellow Card scheme and speak to your GP or pharmacist.

Frequently asked questions

What are weight loss pens?

A weight-loss pen is a pre-filled device that injects a small dose of medicine just under the skin of the abdomen, thigh or upper arm.[1] The medicine is a GLP-1 receptor agonist — or, in Mounjaro's case, a dual GIP and GLP-1 agonist — which reduces appetite and slows how quickly the stomach empties, so you feel full for longer and tend to eat less. They are prescription-only medicines in the UK.

Which weight loss pens are available in the UK?

The two most talked-about are Mounjaro (tirzepatide), supplied as a KwikPen and single-dose pens,[2] and Wegovy (semaglutide), a FlexTouch pre-filled pen[1] — both injected once a week. An older once-daily pen, Saxenda (liraglutide), is also licensed for weight management.[3] Ozempic contains the same medicine as Wegovy but is licensed only for type 2 diabetes, so weight-loss use is off-label.[6]

How much weight can you lose with a weight loss pen?

In STEP 1, adults on semaglutide 2.4 mg lost on average about 15% of body weight (−14.9%) over 68 weeks versus −2.4% on placebo, and 86% lost at least 5%.[9] In SURMOUNT-1, tirzepatide produced mean reductions the MHRA cites as −16.0% to −22.5% depending on dose,[4] with the intention-to-treat figure at the top dose around −20.9%.[10] These are trial averages; individual results vary.

Can you buy weight loss pens without a prescription?

No. Mounjaro and Wegovy are prescription-only medicines. A qualified prescriber must decide they are suitable after a consultation, and lawful supply only comes from a GPhC-registered pharmacy dispensing against that prescription. Any website or social media seller offering these pens without a prescription is acting unlawfully, and what they supply may be falsified — the MHRA's #FakeMeds campaign explains how to check.

Is a "skinny jab" the same as a weight loss pen?

"Skinny jab" and "skinny jab pen" are informal nicknames for GLP-1 weight-loss injections such as Mounjaro and Wegovy. They are not a separate product and not a beauty treatment. The nickname turns up a lot in adverts from unregulated sellers, so treat "skinny jab" offers with particular caution and only ever use a medicine prescribed for you and dispensed by a registered pharmacy. Our FAQ page answers more questions like this.

Next: the Mounjaro pen, the Wegovy pen, using your pen, or the full side effects.

References

  1. Wegovy 2.4 mg FlexTouch pre-filled pen — Summary of Product Characteristics. Electronic Medicines Compendium (emc), product 13803. medicines.org.uk
  2. Mounjaro KwikPen / pre-filled pen (tirzepatide) — Summary of Product Characteristics. Electronic Medicines Compendium (emc), product 15481. medicines.org.uk
  3. Saxenda 6 mg/mL pre-filled pen (liraglutide) — Summary of Product Characteristics. Electronic Medicines Compendium (emc), product 2313. medicines.org.uk
  4. MHRA. "MHRA authorises diabetes drug Mounjaro (tirzepatide) for weight management and weight loss" (8 November 2023). GOV.UK. gov.uk
  5. MHRA. "Single-dose 7.2 mg semaglutide (Wegovy) pen approved to treat adult patients with obesity" (14 April 2026). GOV.UK. gov.uk
  6. MHRA. "MHRA updates guidance for semaglutide prescribers and patients." GOV.UK. gov.uk
  7. NICE. Technology appraisal TA875: Semaglutide for managing overweight and obesity — Recommendations. nice.org.uk
  8. NICE. Technology appraisal TA1026: Tirzepatide for managing overweight and obesity — Recommendations (published 23 December 2024). nice.org.uk
  9. Wilding JPH, et al. "Once-Weekly Semaglutide in Adults with Overweight or Obesity" (STEP 1). New England Journal of Medicine, 2021. nejm.org
  10. Jastreboff AM, et al. "Tirzepatide Once Weekly for the Treatment of Obesity" (SURMOUNT-1). New England Journal of Medicine, 2022. nejm.org
  11. GLP-1 receptor agonists — drug discovery and development review (native GLP-1 half-life of minutes; fatty-acid acylation and DPP-4-resistant half-life extension to weekly dosing; class lineage). PMC, National Library of Medicine, article PMC11441540. ncbi.nlm.nih.gov