How Effective Is GLP-1? What the UK Evidence Shows (October 2026)

01 · The core answer

Which GLP-1 options are actually effective?

Only licensed, prescription-only medicines have trial evidence behind them. Everything marketed as a natural or over-the-counter GLP-1 product sits in a different category.

For a closer look at the unregulated products, see our guides on whether GLP-1 supplements work and how GLP-1 drops compare.

02 · Your decision

What should you do next?

If you meet the licensed criteria, the realistic choice is between regulated treatments, not supplements. Our weight loss clinic explains the options, and every order starts with a free consultation reviewed by a GPhC-registered prescriber.

On cost: your first 2.5mg Mounjaro pen costs £129.99 with code FIRST40 (standard price £169.99). Prices checked 10 October 2026.

Related guides: Cost effective glp 1 and Are oral glp 1 effective and Cheapest most effective glp 1.

03 · Why they work

Why are GLP-1 medicines so effective?

They act on gut-hormone receptors that regulate appetite. Tirzepatide is the first treatment of its kind to activate two natural gut-hormone receptors at once: GIP and GLP-1. Together these help regulate appetite, slow how quickly the stomach empties, and improve the way the body responds to insulin.¹

In practice, most people feel fuller sooner and less hungry between meals, which supports a reduced-calorie diet. This dual action is what distinguishes Mounjaro from single-action GLP-1 medicines such as semaglutide.¹

04 · Injections versus tablets

Injections versus GLP-1 tablets

Tablets are effective, but their evidence comes from different trials. In the OASIS 4 trial, adults taking semaglutide 25mg tablets for 64 weeks lost on average 13.6% of their body weight, compared with 2.2% on placebo.²²

In the ATTAIN-1 trial of 3,127 adults without type 2 diabetes, people taking the highest dose of orforglipron for 72 weeks lost on average 12.4% of their body weight, compared with 0.9% on placebo.²³

No trial has compared Mounjaro directly with either tablet, so these results should not be set against injection results. SURMOUNT-5 is the trial that compared tirzepatide directly with injected semaglutide.¹⁵

05 · Boosters and natural options

Are GLP-1 boosters, capsules and "natural" options effective?

No. "Natural GLP-1" supplements such as berberine are not medicines, are not regulated for this use and have no comparable trial evidence.¹ That covers boosters, capsules, vitamins and drops sold with dramatic weight-loss claims.

These products are cheaply available without assessment, and dosing is not standardised. Orlistat, a tablet that blocks some dietary fat absorption, is a licensed option but produces smaller average losses.¹

06 · Realistic expectations

What results should you expect?

Weight loss on Mounjaro is gradual by design. The first weeks on 2.5mg are an adjustment phase and losses are usually modest; most measurable progress builds from 5mg onward, month by month.¹

Trial averages do not predict your own result. Treatment response is reviewed by your prescriber: if you have lost less than 5% of your starting weight after 6 months on the highest dose you can tolerate, your prescriber will discuss whether continuing or stopping is right for you.¹

07 · Most effective medicine

Is there head-to-head evidence between GLP-1 medicines?

The first large head-to-head evidence is SURMOUNT-5, which compared tirzepatide with injected semaglutide.¹⁵ Beyond that, comparing separate trials is unreliable because the people, durations and doses differ. The Wegovy pen is the semaglutide injection option.