Retatrutide vs. Tirzepatide: What the Trial Data Actually Shows

Retatrutide and tirzepatide are two of the most-discussed compounds in metabolic research right now, both developed by Eli Lilly, and both frequently pitted against each other in comparisons that quote a single number from each side's trial data. That framing skips the most important caveat: the two have never actually been tested against each other in the same trial.

The mechanism difference

Tirzepatide is a dual agonist, activating GLP-1 and GIP receptors. Retatrutide goes a step further as a triple agonist, adding glucagon receptor activation on top of GLP-1 and GIP. That third pathway is the entire premise behind retatrutide's development — glucagon receptor activity is associated with increased energy expenditure, which is a mechanism the dual-agonist approach doesn't directly engage.

What each trial actually found

Tirzepatide's pivotal weight-loss trials (the SURMOUNT program) reported average weight loss up to approximately 20.9% over 72 weeks.¹ Retatrutide's Phase 2 trial, published in the New England Journal of Medicine in 2023, reported a mean weight reduction of 24.2% at 48 weeks with the 12mg dose — and notably, the weight-loss curve had not yet plateaued when the 48-week treatment period ended, meaning the trial ended before participants stopped losing weight.² Participants with a BMI of 35 or higher saw even larger reductions, around 26.4–26.5%.

Regulatory status

Tirzepatide is an approved medicine in several markets — as Mounjaro (type 2 diabetes, since 2022) and Zepbound (weight loss since 2023, obstructive sleep apnea since 2024) in the US, for example. Retatrutide remains investigational everywhere; it's still in Phase 3 trials and isn't approved as a therapeutic drug in any market yet.

In South Africa specifically, both compounds sit in the same practical bucket for most people: neither is a SAHPRA-registered finished medicine, and both are accessed through the research-peptide or GP-led compounding pathway rather than a standard pharmacy prescription. Tirzepatide's overseas approval status doesn't change how it's currently accessed locally.

Side effects

Both compounds share a broadly similar side-effect profile, predominantly gastrointestinal — nausea, vomiting, diarrhea, and constipation, typically dose-dependent and most pronounced during dose escalation. Neither has shown a materially different safety signal from the other in their respective trials so far.

So which one should you research?

This isn't a question with a single right answer, and it isn't one we're going to answer for you in the abstract. Tirzepatide has a substantially larger body of trial data and real-world use behind it. Retatrutide's early numbers are genuinely higher, but they come from a smaller, shorter, Phase 2 dataset — Phase 3 data and, eventually, the direct head-to-head trial will matter more than the numbers currently circulating.

If you're weighing the two, the conversation is better had with a GP who can review your specific health history against both compounds' current evidence — not settled by whichever percentage looks bigger in a comparison article, including this one.