Weight loss pens: frequently asked questions

In their clinical trials, yes — used alongside diet and activity, not instead of them. In the 68-week STEP 1 trial of semaglutide (Wegovy), 1,961 adults lost a mean of about 15% of their body weight (14.9%) against 2.4% on placebo, and 86% lost at least 5%.^[4] In the 72-week SURMOUNT-1 trial of tirzepatide (Mounjaro) in 2,539 adults, the MHRA-cited mean reductions were 16.0%, 21.4% and 22.5% at 5mg, 10mg and 15mg, against 2.4% on placebo.^[5] And in the 56-week SCALE trial of liraglutide (Saxenda) in 3,731 adults, the mean loss was 8.4kg against 2.8kg on placebo.^[6]

Two honest caveats: these are averages, so individual results vary; and weight loss is generally smaller in people who also have type 2 diabetes — in the companion SURMOUNT-2 trial the mean reductions were 13.4% at 10mg and 15.7% at 15mg.^[5]

Each pen delivers a medicine that mimics gut hormones the body naturally releases after eating. Semaglutide (Wegovy) and liraglutide (Saxenda) are GLP-1 receptor agonists; tirzepatide (Mounjaro) is a dual GIP and GLP-1 receptor agonist that acts on two receptors at once.^[1][2] In each case the medicine reduces appetite and slows the rate at which the stomach empties, so people tend to feel full sooner and for longer.^[1] The medicine is injected under the skin — into the abdomen, thigh or upper arm — not into a vein or muscle.^[1] Our using your pen guide walks through the routine in more detail.

They are prescription-only precisely because they carry real risks that need a prescriber's judgement. The most common side effects are gastrointestinal — nausea, diarrhoea, vomiting and constipation — usually mild to moderate and most common while the dose is being increased.^[3] With Wegovy, the product information reports nausea in 43.9% of users, diarrhoea in 29.7%, vomiting in 24.5% and constipation in 24.2%.^[1] With Saxenda, nausea is reported in around 40% of users, especially during dose escalation.^[3]

The labels also carry specific warnings: stop and seek advice if acute pancreatitis is suspected; be alert to gallbladder problems and to dehydration from vomiting or diarrhoea; and note the risk of low blood sugar when combined with insulin or a sulfonylurea. These medicines should not be used in pregnancy.^[1][2] If you experience any side effect from any medicine, report it through the MHRA Yellow Card scheme at yellowcard.mhra.gov.uk and speak to your GP or pharmacist.^[14] Our side effects page covers this in full.

The only contraindication listed in the UK labels is a known allergy to the medicine or the pen's other ingredients, and none of these pens should be used in pregnancy.^[1][2] Beyond that, suitability depends on your full medical history — which is exactly why a prescriber has to assess you rather than a website. One point worth flagging: the US prescribing information carries a boxed thyroid C-cell tumour warning and lists a personal or family history of medullary thyroid carcinoma or MEN 2 as contraindications,^[15] but the UK and EU labels do not — they handle thyroid risk as a precaution instead.^[2] So US warnings you may read online are not the UK position.

No. Weight loss injection pens are prescription-only medicines in the UK, so there is no lawful way to buy one without a prescription. Legitimate supply only ever comes through a GPhC-registered pharmacy dispensing against a prescription after a consultation with a qualified prescriber, who decides whether the medicine is appropriate for you.

Three injectable pens are licensed for weight management in UK adults. Wegovy (semaglutide) is licensed for a body mass index of 30 or above, or 27 to under 30 with a weight-related health problem, and for some adolescents aged 12 and over.^[1] Mounjaro (tirzepatide) was authorised by the MHRA on 8 November 2023 for a BMI of 30 or above, or 27 to 30 with a weight-related condition.^[5] Saxenda (liraglutide) is licensed for a BMI of 30 or above, or 27 or above with a weight-related condition.^[3] You can read more on our Mounjaro pen and Wegovy pen pages.

Sometimes, but NHS access is narrower than a licence alone suggests, because a pen must first be licensed by the MHRA and then recommended by NICE for NHS use.^[12] NICE recommends Wegovy within a specialist weight-management service for a maximum of two years (TA875).^[9] It recommends tirzepatide (Mounjaro) for adults with a BMI of 35 or above plus a weight-related condition, through a phased three-year rollout (TA1026), with BMI thresholds lowered — usually by 2.5 — for people from several ethnic backgrounds.^[10] Saxenda is recommended within a specialist (tier 3) service for defined patients (TA664).^[11] Many people who use these pens do so privately, where a prescriber still has to agree they are suitable.^[10]

Ozempic contains semaglutide — the same drug as the Wegovy pen — but it is licensed in the UK only for type 2 diabetes, at lower doses of up to 1mg or 2mg, and not for weight loss.^[8] Using it for weight loss is off-label, and the MHRA advises that the licensed weight-loss route for semaglutide is Wegovy. The MHRA also notes it has not assessed the safety or effectiveness of these medicines outside their licensed use.^[8]

It depends on the medicine. Wegovy and Mounjaro are injected once a week, on the same day each week; Saxenda is injected once a day.^[1][2][3] The weekly-versus-daily distinction is the most practical difference between them. All three start at a low dose that a prescriber increases in stages — for example, Mounjaro starts at 2.5mg once weekly for four weeks, then 5mg — so the routine settles over the first weeks or months.^[2]

These are subcutaneous self-injection pens designed with the needle kept out of sight, using a fine, short needle that goes just under the skin of the abdomen, thigh or upper arm — not into a vein or muscle.^[1][2] Many people who feel anxious about needles find the pen easier than they expected, but needle fear is a genuine, common reason to speak to your prescriber or pharmacist — a healthcare professional shows you the technique before you start. Our using your pen page explains what to expect step by step.

Both are once-weekly injection pens, but the medicine inside differs. The Wegovy pen contains semaglutide, a GLP-1 receptor agonist made by Novo Nordisk.^[1] The Mounjaro pen contains tirzepatide, a dual GIP and GLP-1 receptor agonist made by Eli Lilly that acts on two receptors rather than one — so it is not accurate to call Mounjaro simply "a GLP-1" without the dual qualifier.^[2] That dual action is positioned as producing greater weight loss than single-receptor GLP-1 medicines,^[7] but which medicine is appropriate for a given person is a decision for a prescriber, not something that can be judged from the drug class alone.

Storage details come from each medicine's product information. Mounjaro is kept refrigerated at 2 to 8 °C and, once in use, may be kept below 30 °C for up to 30 days before being discarded; it must not be frozen.^[2] Saxenda is also kept refrigerated and, once in use, can be kept for one month below 30 °C or in the fridge, with the cap on to protect it from light.^[3] The patient information leaflet in your pack is always the definitive guide.

These medicines work by reducing appetite while they are being taken, so their effect on appetite is not permanent.^[1] Stopping is a clinical decision, not something to do abruptly on your own — speak to your prescriber first. On the NHS, treatment is also reviewed against results: NICE recommends stopping Wegovy if less than 5% of body weight is lost after six months on the maintenance dose, and its NHS use is capped at a maximum of two years.^[9] Whether or not a medicine is being used, day-to-day weight management still rests on diet and physical activity.

A tablet route now exists alongside the injection pens: the UK's first GLP-1 weight-loss tablet — an oral form of semaglutide (Wegovy) — was approved by the MHRA on 11 June 2026.^[12] Tablets are a different category from the pens this site covers — they are swallowed, not injected — and an approval alone does not put a medicine on the NHS: an oral product faces the same two gates as any pen, an MHRA licence and then a NICE recommendation for NHS use.^[12] Whatever the format, the same buying-safety rule applies — obtain any of these medicines only through a legitimate pharmacy dispensing against a prescription, and use the MHRA's FakeMeds campaign to check any online seller before handing over money.^[13] For readers following the tablet story more broadly — including other oral products such as Foundayo (orforglipron), which are tablets and therefore outside this site's pen-focused scope — the sibling site foundayotablet.co.uk tracks how oral GLP-1 medicines are developing in the UK. For now, the licensed injectable pens remain the established route, and which option suits a person is a decision for a prescriber.