Retatrutide vs tirzepatide — one is approved, one is investigational, and that gap is the headline
This comparison gets made constantly and the single most important fact usually gets buried.
Tirzepatide is approved for specific indications and has completed phase 3 programmes.
Retatrutide is investigational. It has not completed phase 3. It is not approved anywhere, for anything.
Mechanistically retatrutide adds glucagon receptor agonism to the GLP-1/GIP combination, making it a triple agonist. The glucagon receptor addition is counterintuitive — glucagon raises blood glucose — but the proposed rationale involves energy expenditure rather than glycaemic control alone.
The phase 2 results attracted a lot of attention and the reported weight reduction was large. Two caveats that matter enormously:
1. Phase 2 results routinely attenuate in phase 3. Larger, longer, more heterogeneous populations. This happens repeatedly and is the norm, not the exception. 2. Safety signals emerge with scale. A phase 2 population is not large enough to characterise uncommon events.
Evidence tier: tirzepatide — approved, phase 3 complete. Retatrutide — phase 2 published, phase 3 ongoing, no approval.
Comparing them as though they were peers is the mistake I see most often.