What it is
Retatrutide is an investigational peptide that activates three receptors at once: GIP, GLP-1 and glucagon. That third target, glucagon, is the key difference from tirzepatide, and researchers believe it adds an energy-expenditure effect on top of appetite suppression. It is sometimes nicknamed a "triple-G" or triagonist.
How it works
It combines the appetite and glucose effects of GLP-1 and GIP with glucagon receptor activity. Glucagon can increase energy expenditure and influence liver fat metabolism, which is the rationale for why a triple agonist might outperform a dual one.
What the evidence shows
The phase 2 obesity trial, published in the New England Journal of Medicine in 2023, produced the largest weight reductions yet recorded in a peptide trial at this stage.
A striking detail from the phase 2 data: participants' weight had not plateaued when the 48-week trial ended, suggesting the full effect may be larger over longer periods. Early phase 3 data has been reported at around 28.7% average weight loss at 68 weeks on the 12mg dose, though phase 3 results are still being completed and peer-reviewed.
A separate phase 2a sub-study in people with fatty liver disease found liver fat reductions of up to 82-84% at the higher doses, with most participants reaching normal liver-fat levels.
Reported side effects
The side-effect profile in trials was similar to GLP-1 and GIP/GLP-1 drugs: mostly gastrointestinal, dose-related, and more frequent at higher doses and faster escalation. A dose-dependent rise in heart rate was observed, peaking around 24 weeks then declining. Some participants reported mild, transient changes in skin sensation.
Where it stands
The numbers are genuinely impressive, but the important caveat is timing. Phase 3 trials (the TRIUMPH programme) are still running, with potential FDA review and approval not expected before late 2026 or 2027. Until then, anything sold as retatrutide is a research chemical of unknown quality, outside any safety oversight.