Retatrutide vs Mounjaro (UK): the simple, evidence‑based guide

If you're on Mounjaro (tirzepatide) and you've started hearing people talk about retatrutide, you're not alone. Retatrutide has had some seriously attention‑grabbing trial results — but it's also not approved yet, which changes what's possible (and what's safe).

This article is for normal humans, not scientists. I'll explain what retatrutide is, how it compares to Mounjaro, what the side effects look like, why the "grey market" is such a big deal, and how to track your progress properly inside Healthcount.

Quick facts (30 seconds)

What retatrutide is

Retatrutide is an investigational (not‑approved‑yet) medicine being tested for obesity and related conditions.

In a published Phase 2 trial (48 weeks), adults with obesity who took retatrutide lost, on average, more weight at higher doses, with the 12 mg group showing −24.2% average weight change at 48 weeks (compared with −2.1% in the placebo group).

It's usually described as a triple agonist because it activates three receptors:

• GLP‑1 receptor
• GIP receptor
• Glucagon receptor

What Mounjaro is

Mounjaro is the brand name for tirzepatide. In the UK, it has an official SmPC (Summary of Product Characteristics) that explains how it should be prescribed and used.

Mounjaro is a dual agonist, meaning it activates two receptors:

• GLP‑1 receptor
• GIP receptor

The UK SmPC also lays out how dosing usually works (start low and increase slowly): starting dose 2.5 mg once weekly, then move to 5 mg, then step up in 2.5 mg increments if needed (with at least 4 weeks at each step).

Tiny biology lesson: receptors and hormones

You don't need a biology degree for this — you just need the right mental picture.

What's a receptor?

A receptor is like a doorbell on a cell. When the right "finger" (a hormone or a medicine) presses it, the cell gets a message and reacts.

What's a hormone?

A hormone is a message chemical your body makes. Many hormones travel in your bloodstream (so yes, they're "in your blood"), but they aren't a separate liquid — they're molecules carried by the blood.

Why does this matter for weight and blood sugar?

Because drugs like Mounjaro and retatrutide are basically messages you inject, designed to press certain "doorbells" that affect:

• hunger and fullness
• how your body handles sugar after meals
• sometimes energy use

If you've ever wondered why these meds can change appetite so strongly… that's why.

Retatrutide vs Mounjaro: the big difference

Here's the headline:

• Mounjaro = dual agonist (2 targets)
• Retatrutide = triple agonist (3 targets)

So retatrutide has an "extra lever" (the glucagon receptor) on top of the GLP‑1 and GIP effects.

Important: you can't compare them by "milligrams" alone

This is the part that saves you from a lot of confusion:

• 1 mg of Drug A is not automatically "stronger" or "weaker" than 1 mg of Drug B
• They're different molecules, hitting different receptors, with different potencies and different dose–response curves
• So there is no official Mounjaro‑to‑retatrutide conversion chart
• What you can do is compare trial averages at specific timepoints — with a big "rough comparison" label

"What dose of retatrutide equals my Mounjaro dose?"

Let's answer this the way your readers are actually asking it.

The honest answer (the one you can trust)

There is no evidence‑based, safe, "same impact" conversion from Mounjaro mg to retatrutide mg — especially because retatrutide isn't approved yet.

What we can do is compare published trial results at different doses and see what looks similar on average (not person‑to‑person).

A clear, rough comparison table (trial averages, not a dosing guide)

Not a conversion chart. This is only to help you understand why "mg" doesn't translate cleanly. Different studies, different timeframes, different people.

Let's explain:

• In trials, retatrutide 4 mg at 48 weeks landed around the same average weight‑loss range as Mounjaro 5 mg at 72 weeks — but that does not make them equivalent doses.
• And for higher doses: retatrutide 8–12 mg shows higher average loss at 48 weeks, while Mounjaro 10–15 mg shows strong results at 72 weeks.

Will you have to "dose ladder" again if you switch from Mounjaro to Retatrutide?

Very likely, yes.

Why? Because the slow step‑up is how these medicines reduce side effects, especially nausea and vomiting.

• Mounjaro's UK SmPC describes step‑ups in 2.5 mg increments after at least 4 weeks at a dose.
• In the Phase 2 retatrutide trial, some groups used a lower starting dose (2 mg vs 4 mg) and the study notes side effects were "partially mitigated" with a lower starting dose.

To be on the safe side: Even if you've used Mounjaro before, a new medicine will usually mean a fresh titration schedule (if and when it's approved), because tolerability matters.

What retatrutide is being studied for

Retatrutide isn't just being studied for "weight loss". The Phase 3 TRIUMPH programme includes multiple conditions linked to obesity (and the complications that come with it).

Here's the clean list that's been publicly described:

Current status in the UK and what might happen next

Is retatrutide available in the UK right now?

Not as a normal prescription medicine.

As of December 2025, retatrutide is still an investigational medicine in Phase 3 trials.

What's new recently?

Lilly's first Phase 3 readout (TRIUMPH‑4) reported strong results in people with obesity/overweight and knee osteoarthritis, including:

• ~28.7% average weight loss at 68 weeks at the 12 mg dose (topline reporting), plus improvements in pain and function measures

The important phrase there is "topline". It's encouraging, but the full peer‑reviewed dataset is what eventually sets the standard.

What might happen next year?

More Phase 3 readouts are expected in 2026 (again, that's based on public reporting around the trial programme).

Key Takeaway: Retatrutide is moving quickly through trials, but approval timing is still uncertain until regulators review the full evidence.

Side effects: what's common (and what's worth watching)

Most of the side effects people talk about for these meds are… honestly… stomach‑related. That's not because the drug is "bad", it's because these hormone pathways affect appetite and digestion.

Common side effects seen in retatrutide reporting

In the Phase 2 trial, the most common adverse events were gastrointestinal and dose‑related, mostly mild to moderate.

In TRIUMPH‑4 reporting, GI effects like nausea, diarrhoea, constipation, vomiting, and decreased appetite were also common.

One side effect that gets people asking questions: dysesthesia

TRIUMPH‑4 reporting mentions dysesthesia (odd, uncomfortable sensations like tingling/burning) and says it was generally mild and infrequently led to discontinuation.

The grey market: what it is and why it matters here

What does "grey market" mean?

In this context, it usually means products sold outside regulated pharmacies — often labelled "for research only".

Why it's especially relevant for retatrutide

Because retatrutide isn't approved yet, any "retatrutide for sale" to the general public is not coming through normal, regulated medicine supply chains.

And in the UK, this risk is not theoretical:

The MHRA announced it dismantled an illicit facility and seized more than 2,000 unlicensed retatrutide and tirzepatide pens, plus raw ingredients and equipment, and warned people not to buy weight‑loss meds from unregulated sources.

The simple truth

If it isn't coming from a legitimate clinic/pharmacy route, you cannot reliably know:

• what's in it,
• how strong it is,
• whether it was stored correctly,
• whether it's sterile.

That's a big deal for anything you inject.

What to track in Healthcount (so this doesn't become guesswork)

If you only do one thing after reading this, do this: track the basics consistently.

Not because you need to "obsess". Because your memory lies to you. (Mine does too.)

Here's a simple routine that fits real life.

The Healthcount tracking plan (simple and sustainable)

FAQs

Can I switch to retatrutide right now in the UK?

Not through normal prescribing routes. Retatrutide is still investigational, so the clean path is clinical trials (or waiting for approval).

Is retatrutide "better" than Mounjaro?

It may end up being more effective for some people, but we're still waiting for more Phase 3 results and real‑world safety data. The early numbers are impressive, but "better" depends on tolerability, safety, and what you personally need.

If I'm doing well on Mounjaro, should I chase what's next?

Not automatically. If you're losing weight, feeling okay, and your health markers are improving, that's already a win. Newer isn't always better for you — sometimes it's just newer.

Do I have to titrate again if I switch medicines?

Very likely, yes. Both Mounjaro's prescribing info and retatrutide trial design reflect the idea of starting lower and stepping up to reduce side effects. For accurate dosing, check our conversion guide.

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