Retatrutide: what is still unknown

Retatrutide has been studied in thousands of people for up to two years. That is a lot, and it is also not very much. Lilly does not plan to ask the US regulator for approval until early 2027, and several of the questions below will not be answered until years after that. This page lists the gaps, calmly, so that "the trials say it is safe" can be read for what it does and does not cover.

Long-term outcomes for the heart

The trials so far show that retatrutide improves blood pressure, cholesterol and inflammation markers.^[3] They do not show that it prevents heart attacks or strokes, because none of them was designed or large enough to. TRIUMPH-3 included people with established cardiovascular disease, but Lilly said plainly that it was not powered to measure major cardiovascular events.^[4]

That job belongs to TRIUMPH-Outcomes, which is following about 10,000 people with cardiovascular or kidney disease and is expected to finish around February 2029.^[1] Until then, the dose-dependent rise in resting heart rate covered on the heart rate and rhythm page is a question mark, not an answer. It may be harmless. Nobody has shown that yet.

Pregnancy and breastfeeding

Every retatrutide trial has excluded people who are pregnant, breastfeeding or planning a pregnancy.^[5] There is no human safety data. The licensed relatives give the class position: the Wegovy SmPC says semaglutide must not be used in pregnancy and should be stopped at least two months before a planned pregnancy because it stays in the body so long; the Mounjaro SmPC says the same for tirzepatide, with at least one month, and that animal studies showed harm to the developing baby.^[6][7] Both say the drug should not be used while breastfeeding. Retatrutide is, if anything, less known.

Older people

The trials recruited adults, and Lilly's topline announcements have not said how many participants were over 75; trials of this kind typically include few. The Mounjaro SmPC notes only very limited data in the oldest age groups and that older people are more susceptible to dehydration from gut side effects.^[7] Older adults also lose muscle more readily during rapid weight loss, which the preserving muscle page covers. For retatrutide, the honest position is that the effect in people over 75 is unstudied.

Severe kidney or liver disease

The Phase 2 trial excluded people with an eGFR below 45 and anyone with liver disease other than fatty liver.^[5] One trial, TRANSCEND-T2D-3, is now testing retatrutide in people with type 2 diabetes and moderate or severe kidney impairment; it is due to complete in late 2026.^[8] Nothing equivalent exists for serious liver disease. Details on the liver and kidney page.

Thyroid C-cell tumours

Every GLP-1-class medicine carries a warning because these drugs caused thyroid C-cell tumours in rats and mice. The SmPCs describe this as a class effect, note that rodents are particularly sensitive, and say the relevance to humans is thought to be low but cannot be excluded.^[6][7] Because of it, retatrutide's trials excluded anyone with a personal or family history of medullary thyroid cancer or the genetic syndrome MEN2.^[9] Whether retatrutide behaves like its relatives here is assumed, not shown, and human data on any of these drugs will take decades.

Regain after stopping

No retatrutide data yet. The class evidence is consistent:

• Semaglutide (STEP 1 extension): a year after stopping, participants had regained about two-thirds of the weight they lost.^[2]
• Tirzepatide (SURMOUNT-4): most people who stopped regained at least a quarter of their lost weight within a year, and the improvements in blood pressure and blood sugar reversed as the weight came back.^[10]

TRIUMPH-6, a dedicated weight-maintenance trial of retatrutide, is running and is expected to finish around April 2028.^[11] Until then, assume retatrutide is like its relatives: the weight loss lasts while the drug does. What that means for someone with a finite supply of an unlicensed product is covered in plateaus and maintenance.

Interactions with other medicines

Lilly has published no interaction data for retatrutide. The class example is worth knowing: because tirzepatide slows the stomach, the Mounjaro SmPC reports that it reduced the absorption of a combined oral contraceptive pill, and advises people on the pill to switch to a non-oral method or add a barrier method for four weeks after starting and after each dose increase.^[7] Whether retatrutide does the same is untested but plausible, given that it slows the stomach at least as much. Anyone relying on the pill and using any product of this kind has an unaddressed risk, on top of the pregnancy warning above.

Mental health

In 2023 the European Medicines Agency reviewed reports of suicidal thoughts in people taking GLP-1 medicines. Its safety committee concluded in April 2024 that the available evidence did not support a causal link between the GLP-1 medicines it examined (which included semaglutide and liraglutide) and suicidal or self-harming thoughts or actions.^[12] That review did not cover retatrutide, which had not been in large trials then. The Phase 2 trial excluded anyone with a major problem with depression or other mental illness in the previous two years,^[5] so the trials cannot tell you much about people with existing mental-health conditions. If your mood changes on any product, tell a GP; when to seek help lists crisis contacts.

The dysesthesia mechanism

Up to one in five people on the top dose of TRIUMPH-4 developed tingling, burning or oversensitive skin.^[13] It was mostly mild and rarely a reason to stop,^[14] and nobody has explained it. The dysesthesia page has the details. Until the full trial papers are published, "not yet explained" is the accurate statement.

People the trials left out

Trial results describe the people who were in the trials. Retatrutide's excluded, among others: people with diabetes (in the obesity trials), a recent heart attack, stroke or heart-failure admission, uncontrolled blood pressure, a history of pancreatitis or symptomatic gallbladder disease, significant kidney disease, liver disease other than fatty liver, active cancer, recent major mental illness, heavy alcohol use, recent cannabis use, and pregnancy.^[5] If you are in one of those groups, nothing on this site's side-effect pages was measured in people like you.