The King Kong of Weight Loss Drugs: What It Is (October 2026)

01 · The short answer

What is the “King Kong” weight loss drug?

It is Mounjaro, the brand name for tirzepatide, a once-weekly injectable prescription medicine made by Eli Lilly.¹ In the UK it is licensed for weight management in adults and for type 2 diabetes. It comes as a pre-filled multi-dose pen (KwikPen) in six strengths: 2.5mg, 5mg, 7.5mg, 10mg, 12.5mg and 15mg.¹

The nickname reflects how much attention it has had in the press compared with earlier injections.

02 · Your decision

Is it relevant to you?

It may be if you are an adult living with overweight or obesity and want a licensed, clinician-supervised treatment. Mounjaro is used alongside a reduced-calorie diet and increased physical activity, and it is supplied only after a consultation reviewed by a GPhC-registered prescriber. Eligibility is a clinical judgement, so a nickname or a headline should never be the reason to start.

If you want to see whether you are suitable, start with the free consultation on the Mounjaro pen page. At the starting 2.5mg dose the standard price is £169.99 per month. Prefer to look around first? Our weight loss clinic explains how the service works.

Related guide: King kong of weight loss jabs.

03 · How it works

How the “King Kong” jab works

Tirzepatide is the first treatment of its kind to activate two natural gut-hormone receptors at once: GIP and GLP-1.¹ 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.¹ This dual action is what distinguishes Mounjaro from single-action GLP-1 medicines such as semaglutide (Wegovy and Ozempic).¹

Tirzepatide has a half-life of about five days. After a final dose it takes roughly four to five weeks to clear the system. Nothing needs doing while it clears, and there is no way to speed it up.¹

04 · Does the hype hold up?

Does the hype hold up?

Partly. Headlines often quote trial averages as if they were promises. They are not: trial averages do not predict your own result, and individual results vary.

What is fair to say is that Mounjaro works differently from semaglutide, and that it is a licensed, well-studied treatment when used as directed with diet and activity changes.¹ Our page of reviews of weight loss drugs compares the main options in plain terms, and three weight loss drugs sets out how the main options differ.

05 · Illegal jabs

Warnings about illegal “King Kong” jabs

Several news stories have warned about unlicensed weight-loss injections sold online under tempting nicknames. The MHRA publishes counterfeit medicine alerts and runs the Yellow Card scheme for reporting problems.⁷

Treat any seller offering to supply without a consultation or prescription as a red flag. Check that a pharmacy is on the GPhC register; ours is GPhC 9012878. We supply genuine Mounjaro KwikPens through official LillyDirect supply from Eli Lilly's UK distribution. You can report any suspected side effect through the Yellow Card scheme.¹⁹

06 · NHS access

Can you get it on the NHS?

Mounjaro is available on the NHS, but access is being phased in over several years.⁴ NICE recommends it for adults with a BMI of at least 35 plus at least one weight-related condition, and the NHS primary-care rollout began in June 2025 with the highest-need groups first (initially BMI 40+ with four or more qualifying conditions).⁶

Full rollout is expected to take years, which is why many people who meet the licensed criteria choose regulated private treatment rather than waiting. Your GP can tell you where you sit in the local rollout.

07 · Other contenders

Other drugs in the headlines

Retatrutide is a medicine in development and is not licensed in the UK, so we do not supply it. Anyone offering it for sale is operating outside the licensed system. Liraglutide is a daily injection supplied at 6mg/ml, unlike once-weekly Mounjaro and Wegovy.

Low-cost supplements sold as “nature's Ozempic” are not equivalent to any of these. They are available cheaply without assessment, with no clinical oversight, and dramatic weight-loss claims made for them should be treated with caution. For older stimulant-style options, see our page on amphetamine drugs for weight loss.

08 · Serious side effects

Serious side effects: when to get help straight away

Serious side effects are uncommon but need urgent attention. Seek immediate medical help for: severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis); pain in the upper right abdomen, fever or yellowing of skin/eyes (possible gallbladder problems); or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing. If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion) — your prescriber may need to adjust those medicines.¹