Weight loss drugs: what the evidence says about Mounjaro

What the SURMOUNT-1 trial actually found

The headline figures for tirzepatide come from SURMOUNT-1, a 72-week randomised controlled trial published in the New England Journal of Medicine. The trial enrolled 2,539 adults with obesity but without type 2 diabetes. At the 15 mg maintenance dose, participants lost an average of roughly 20–21% of their starting body weight. To put that in everyday terms: someone starting at 100 kg would typically have lost around 20 kg by the end of the study period.

Those numbers were enough to prompt NICE to issue a positive recommendation (technology appraisal TA1026) for adults with a BMI of 35 or more and at least one weight-related comorbidity. Lower BMI thresholds apply for people from South Asian, Chinese, Middle Eastern, Black African or African-Caribbean backgrounds. Importantly, NICE's criteria govern NHS access; the licensed indications for private prescribing are broader, starting at a BMI of 30.

In 2025, a further head-to-head trial called SURMOUNT-5 compared tirzepatide directly against semaglutide 2.4 mg. Tirzepatide produced greater average weight reduction over 72 weeks. That comparison matters when considering the landscape of available weight-loss treatments, though which medicine suits a particular person is always a clinical judgement.

The biology behind weight loss drugs like tirzepatide

Mounjaro's mechanism is worth understanding, because it explains both its effectiveness and its side-effect profile. Tirzepatide activates two receptors: GIP (glucose-dependent insulinotropic polypeptide) and GLP-1 (glucagon-like peptide-1). Both are gut hormones released after eating. Activating them together slows how quickly the stomach empties, signals satiety to the brain more powerfully, and reduces appetite.

Most other GLP-1 drugs in this class (including semaglutide, the active ingredient in Wegovy) work on the GLP-1 pathway alone. Tirzepatide's dual action is what separates it from earlier weight-loss medicines and is widely cited as the reason for its stronger average results. You can read more about the underlying biology on the tirzepatide overview page.

Gastric slowing also explains the most common side effects: nausea, diarrhoea, constipation, indigestion and burping. These tend to be most noticeable after starting treatment or after a dose increase, and they usually settle within a week or two. Some people also notice heart rate changes, and if that happens to you, our page on Mounjaro palpitations explains what is known and when to seek advice. The slow titration schedule (stepping up no faster than every four weeks, beginning at the 2.5 mg starter dose) exists partly to reduce that early discomfort. Details of the full dosing schedule are in the Mounjaro guide.

Storage, routine and what to expect day to day

Each Mounjaro pen contains four weekly doses. The pen lives in the fridge (2 to 8°C) until you open it. For the exact window during which a pen can be kept at room temperature, the Patient Information Leaflet is the definitive source; storage rules differ slightly by strength and should not be guessed at.

The once-weekly injection is given under the skin of the abdomen, thigh or upper arm, rotating sites each time. Most people settle into a routine quickly. The NHS medicines page for tirzepatide covers practical administration guidance in plain English and is worth bookmarking alongside the Patient Information Leaflet.

A few safety points are worth knowing before starting. People using oral contraceptive pills should use an additional barrier method for the first four weeks of treatment and for four weeks after each dose increase, because the pill's absorption may be reduced. Anyone due to have surgery should tell their anaesthetist they are taking a GLP-1 medicine. Tirzepatide is not recommended during pregnancy, while breastfeeding, or for anyone trying to conceive. Prescribers weigh all of this during the assessment. You can read more about the access process and what the consultation involves on the how to get Mounjaro page.

Cost context and the prescription requirement

Mounjaro is a prescription-only medicine. In practice, that means no reputable supplier (pharmacy or clinic) can send it without a valid prescription issued after clinical assessment. Eli Lilly revised its UK list prices in September 2025; private market prices have typically ranged from around £149 to £375 per month depending on dose and provider since then, figures widely reported at the time of the change.

For a medicine at this price point, it is worth being clear about what any quoted figure includes. Some providers charge separately for the consultation, the prescription, delivery, and ongoing clinical review. At nume, one transparent price covers all of that. We are not the cheapest option, that is a deliberate choice. A lower number that excludes clinical review, aftercare, or a legitimate supply chain is not actually a saving. A fuller breakdown of what to look for when comparing private providers is on our Mounjaro private prescription page. If you have questions before starting, the FAQs are a useful first stop.