The most commonly reported retatrutide side effects in clinical trials have been gastrointestinal, particularly nausea, diarrhoea, constipation and vomiting.

In the peer-reviewed Phase 2 obesity trial, these adverse events were generally mild to moderate, occurred mainly while doses were being increased and were more frequent at higher doses. Researchers also observed increased heart rate and altered skin sensations.

More recent Phase 3 research has considerably expanded the safety evidence. In the TRIUMPH-1 obesity trial, more than 2,300 participants were randomised to retatrutide or placebo, giving researchers a much larger dataset for examining how often different adverse events occur.

The overall picture is becoming clearer: gastrointestinal side effects remain the main tolerability issue, some adverse events become more common at higher doses, and other signals such as dysesthesia, an abnormal or altered sensation, are now being characterised more closely.

But retatrutide remains investigational. It is not an approved therapeutic good in Australia, and its complete long-term and rare-adverse-event profile has not yet been established.

If you are looking for the broader picture of what retatrutide does, including its GIP, GLP-1 and glucagon receptor mechanism and the latest efficacy results, we cover that separately in our guide to the current research.

What Are the Most Common Retatrutide Side Effects?

Across the clinical evidence available so far, the clearest side-effect pattern is gastrointestinal.

The peer-reviewed Phase 2 obesity study identified nausea, diarrhoea, vomiting and constipation as the most frequently reported adverse events. These occurred more often with retatrutide than placebo and were predominantly mild to moderate.  

The larger TRIUMPH-1 Phase 3 trial provides a more detailed picture:

Source: Eli Lilly’s May 2026 TRIUMPH-1 Phase 3 topline results. The complete TRIUMPH-1 results were subsequently published in the New England Journal of Medicine on 29 September 2026.

The dose pattern is particularly clear for some side effects.

Nausea increased from 28.6% at 4 mg to 42.4% at 12 mg, while vomiting increased from 10.6% to 25.3%. Dysesthesia rose from 5.1% at 4 mg to 12.5% at 12 mg.

Not every adverse event increased perfectly with every dose. Diarrhoea, for example, was slightly more common in the 9 mg group than the 12 mg group.

So the evidence supports saying that dose influences retatrutide tolerability, rather than suggesting every individual side effect rises in a straight line with dose.

What Did the Phase 2 Trial Find About Retatrutide Side Effects?

The original obesity study remains one of the most useful sources for understanding retatrutide side effects because its full results were peer reviewed and published in the New England Journal of Medicine.

The randomised, double-blind, placebo-controlled trial involved 338 adults with obesity, or overweight plus at least one weight-related condition, and followed participants for 48 weeks.

At least one adverse event was reported in 70% of placebo participants and 73% to 94% of participants across the different retatrutide groups.

That does not mean 73% to 94% experienced a serious reaction caused by retatrutide.

An adverse event is a medical event recorded during a clinical trial after treatment begins. It does not automatically establish that the treatment caused the event. Researchers therefore also look at the type of event, its severity, timing, dose pattern and how frequently it occurred with placebo.

For retatrutide, gastrointestinal events showed a clear pattern. They:

• occurred mainly during dose escalation;
• were predominantly mild to moderate;
• were more frequent at higher doses; and
• were the most common adverse events leading participants to stop treatment. 

The study also found that gastrointestinal adverse events were partially reduced when some participants began at 2 mg rather than 4 mg.

That is a clinical-trial finding, not dosing advice. Retatrutide remains investigational and does not have a TGA-approved dosing schedule in Australia.

Can Retatrutide Increase Heart Rate?

The Phase 2 obesity trial found dose-dependent increases in heart rate among participants receiving retatrutide.

Heart rate increased through approximately week 24 and then declined.

This is worth distinguishing from an isolated adverse event because it was a measurable physiological effect seen across treatment groups.

It does not, however, tell us by itself whether the increase has long-term cardiovascular consequences.

The currently available public TRIUMPH-1 results do not provide enough detailed heart-rate data to make a stronger Phase 3 conclusion. A dedicated Phase 3 outcomes trial, TRIUMPH-Outcomes, is separately examining cardiovascular and kidney outcomes in people with obesity, but no results have yet been posted.

So the evidence currently supports a fairly narrow conclusion: retatrutide increased heart rate in the Phase 2 obesity trial, while the longer-term clinical significance remains under study.

Can Retatrutide Cause Skin Sensitivity or Dysesthesia?

Altered skin sensation is one of the more distinctive findings to emerge from the retatrutide trials.

In Phase 2, cutaneous hyperesthesia and skin-sensitivity adverse events occurred in 7% of participants receiving retatrutide compared with 1% receiving placebo. The study reported that none was severe or serious and none resulted in treatment discontinuation.  

The signal became clearer in TRIUMPH-1.

Lilly reported dysesthesia in:

• 5.1% of participants receiving 4 mg;
• 12.3% receiving 9 mg;
• 12.5% receiving 12 mg; and
• 0.9% receiving placebo.  

Dysesthesia is a broad medical term for an abnormal or altered sensation.

Lilly reported that these events were generally mild to moderate, that most resolved during treatment and that most affected participants continued receiving retatrutide.  

What makes this finding particularly interesting is the progression of the evidence: a skin-sensation signal appeared in Phase 2 and remained visible in a much larger Phase 3 population.

What Does Phase 3 Add to What We Knew About Retatrutide Side Effects?

The biggest advantage of Phase 3 is scale.

TRIUMPH-1 is a randomised, double-blind, placebo-controlled Phase 3 study in adults with obesity or overweight without type 2 diabetes. ClinicalTrials.gov records 2,335 actual enrolments, while Lilly reports that 2,339 participants were randomised in the master trial.  

Rather than overturning the Phase 2 safety picture, the newer evidence mostly strengthens and refines it.

There is also an important evidence distinction.

The Phase 2 obesity study has been fully peer reviewed. Detailed TRIUMPH-1 safety figures were initially reported by Lilly, with the complete trial results subsequently published in the New England Journal of Medicine on 29 September 2026.

That does not make the Phase 3 evidence unimportant. It means we should describe it accurately according to its current publication status rather than treating every source as equivalent.

Do Other Phase 3 Trials Show the Same Side-Effect Pattern?

Yes.

The TRANSCEND-T2D-1 trial gives us an important second Phase 3 dataset because it studied a different population: adults with type 2 diabetes whose blood glucose was inadequately controlled with diet and exercise.

The randomised, double-blind, placebo-controlled trial involved 537 participants and was published in The Lancet in June 2026.  

The most frequent adverse events with retatrutide were again gastrointestinal and generally mild to moderate.

The study also reported that these gastrointestinal events subsided over time. Treatment was discontinued because of adverse events in approximately 2% to 5% of retatrutide participants, compared with none receiving placebo. No severe hypoglycaemia was reported.  

This matters because the gastrointestinal side-effect pattern is no longer confined to one obesity trial or to company-reported Phase 3 results. It has now appeared in a peer-reviewed Phase 3 study in a separate population.

The subsequent TRANSCEND-T2D-2 trial also provides comparative safety data, examining retatrutide alongside semaglutide (Ozempic). Our retatrutide vs Ozempic analysis examines the reported differences in gastrointestinal side effects and treatment discontinuation rates.

Do Retatrutide Side Effects Cause People to Stop Treatment?

For some trial participants, yes.

In the Phase 2 obesity study, adverse events resulted in treatment discontinuation in 6% to 16% of participants receiving retatrutide, compared with none receiving placebo. Gastrointestinal adverse events were the most common reason.  

In TRIUMPH-1, Lilly reported discontinuation because of adverse events in:

• 4.1% with 4 mg;
• 6.9% with 9 mg;
• 11.3% with 12 mg; and
• 4.9% with placebo.  

Lilly’s subsequent scientific presentation reported that gastrointestinal events were the most common reason for stopping treatment, accounting for discontinuation in 2.2% to 4.6% of retatrutide participants compared with 1.2% receiving placebo.  

This provides useful context when trial reports describe most gastrointestinal side effects as mild to moderate.

Mostly mild to moderate does not mean inconsequential for every participant. For some people in the trials, adverse events were significant enough to stop treatment.

Were Serious Adverse Events Reported?

Yes, serious adverse events have occurred during retatrutide trials.

But serious adverse event does not automatically mean serious side effect caused by retatrutide.

The Phase 2 obesity trial reported 15 serious adverse events in 13 participants. Serious adverse events occurred at the same overall frequency, 4%, in the combined retatrutide and placebo groups.  

One participant receiving retatrutide died from drowning. The investigator assessed the death as unrelated to retatrutide.

TRIUMPH-1 adds a larger dataset. Lilly’s 2026 scientific presentation reported serious adverse events in 7.7% to 10.5% of participants across the retatrutide groups, compared with 5.5% receiving placebo.  

Those figures show what happened during treatment. They do not establish that retatrutide caused every serious event recorded.

A full peer-reviewed TRIUMPH-1 publication will be particularly useful for examining the individual serious events and their assessed relationship to treatment in greater detail.

Was Pancreatitis Reported With Retatrutide?

One serious adverse event of acute pancreatitis was reported in the Phase 2 obesity trial.

Researchers also observed increases in the pancreatic enzymes amylase and lipase, which were asymptomatic apart from that single pancreatitis event.  

That event deserves to be reported, but one case cannot establish the size of any causal pancreatitis risk.

This is an important distinction when interpreting side-effect information online. The evidence supports saying that acute pancreatitis occurred in one Phase 2 participant. It does not support reducing that finding to the much broader statement that “retatrutide causes pancreatitis”.

What About Urinary Tract Infections?

Urinary tract infections, or UTIs, appeared as another numerical imbalance in TRIUMPH-1.

Lilly’s May 2026 results reported UTIs in:

• 7.5% with 4 mg;
• 8.8% with 9 mg;
• 8.4% with 12 mg; and
• 5.3% with placebo.  

Lilly reported that these events were generally mild to moderate, that most resolved during treatment and that most affected participants continued receiving retatrutide.  

The appropriate interpretation is that UTIs occurred somewhat more often in the retatrutide groups than placebo.

The currently reported data do not establish why that difference occurred or prove a specific causal mechanism.

When Do Retatrutide Side Effects Tend to Occur?

For gastrointestinal adverse events, the strongest evidence points to dose escalation as an important period.

In the Phase 2 obesity study, gastrointestinal adverse events occurred primarily while doses were being increased.  

TRIUMPH-1 also used stepwise escalation. Participants assigned to retatrutide began at 2 mg and increased every four weeks until reaching their assigned 4 mg, 9 mg or 12 mg target dose.  

The peer-reviewed TRANSCEND-T2D-1 Phase 3 study further reported that its most frequent gastrointestinal adverse events subsided over time.  

Taken together, the evidence suggests that gastrointestinal tolerability changes over the course of treatment and is particularly relevant during dose escalation.

Again, these are observations from controlled clinical trials, not instructions for self-administering an investigational drug.

What Don’t We Know Yet About Retatrutide Side Effects?

The safety evidence is much stronger in 2026 than it was when the first Phase 2 results appeared, but important questions remain.

Long-term effects

The Phase 2 obesity trial followed participants for 48 weeks. TRIUMPH-1 extended the controlled evidence considerably further, including an 80-week main study and a prespecified extension for some participants.  

That is useful long-term trial exposure, but it is not the same as years of widespread post-approval use.

Rare adverse effects

Even studies involving thousands of participants can struggle to identify very rare events.

An adverse effect that occurs only once in many thousands of people may not appear clearly until a much larger population has been exposed.

That means not observed is not the same as proven not to occur.

The significance of emerging signals

Dysesthesia is now visible across Phase 2 and Phase 3 evidence. Increased heart rate was identified in Phase 2. Serious adverse events have also occurred during the Phase 3 programme.

Researchers still need to determine what some of these findings mean over longer periods and across larger populations.

Not all evidence has reached the same publication stage

This is especially important with retatrutide because the evidence is developing quickly.

The original obesity Phase 2 study, TRANSCEND-T2D-1 and TRIUMPH-1 have undergone peer review. TRIUMPH-1 was published in the New England Journal of Medicine on 29 September 2026.

A careful reading of the evidence therefore requires more than saying “clinical studies show”. It matters which study, which population and what type of evidence produced the finding.

Is Retatrutide Approved in Australia?

No.

As of August 2026, retatrutide remains investigational and is not an approved therapeutic product on the Australian Register of Therapeutic Goods. Our analysis of whether retatrutide is available in Australia explains the current TGA status and Australian availability in more detail.

The Therapeutic Goods Administration specifically identifies retatrutide among examples of unapproved peptide products and states that products not included in the ARTG have not been evaluated by the regulator for safety, quality or effectiveness.  

This distinction matters when discussing side effects of retatrutide.

The evidence covered in this article comes from controlled clinical trials using defined investigational products, study protocols and participant monitoring.

It cannot automatically be applied to an unapproved product purchased online that claims to contain retatrutide.

The TGA has warned that imported unapproved peptide products may be poorly labelled and may lack reliable information about their identity, concentration or contents.  

In June 2026, the TGA and Australia’s Chief Medical Officer also reported serious adverse effects associated with the broader use of unapproved peptide products, including liver damage, severe allergic reactions and inflammatory complications requiring medical attention. Retatrutide was listed among examples of unapproved peptides, but the statement did not attribute all of those reported effects specifically to retatrutide.  

For our broader Australian research overview and current regulatory context, see Retatrutide Australia.

What Does the Evidence Currently Tell Us About Retatrutide Side Effects?

The retatrutide side-effect picture has become considerably clearer as the clinical programme has moved from Phase 2 into Phase 3.

Gastrointestinal adverse events are the most consistent finding. Nausea, diarrhoea, constipation and vomiting have repeatedly appeared across obesity and type 2 diabetes trials.

Dose matters for tolerability. Several adverse events become more common at higher doses, although the relationship is not perfectly linear for every individual effect.

Altered skin sensation has become a reproducible signal. Skin-sensitivity events appeared in Phase 2, while dysesthesia was reported considerably more often with retatrutide than placebo in TRIUMPH-1.

Some participants stop treatment because of side effects. In TRIUMPH-1, Lilly reported adverse-event discontinuation rates ranging from 4.1% at 4 mg to 11.3% at 12 mg.

Some questions remain unresolved. The long-term significance of the heart-rate increase seen in Phase 2, uncommon adverse effects and the complete long-term safety profile all require further evidence.

So the evidence does not support reducing retatrutide to either “safe” or “unsafe”. We examine the broader question of whether retatrutide is safe separately, bringing together the Phase 2 and Phase 3 evidence, serious adverse events and the remaining uncertainties.

A more accurate conclusion as of August 2026 is:

Retatrutide’s most established side effects are gastrointestinal, tolerability is influenced by dose and dose escalation, additional signals including dysesthesia and increased heart rate have emerged, and its complete long-term and rare-adverse-event profile is still being established.

Retatrutide Side Effects: Frequently Asked Questions

What are the most common side effects of retatrutide?

The most consistently reported retatrutide side effects are nausea, diarrhoea, constipation and vomiting. Gastrointestinal adverse events were the most common events in the Phase 2 obesity trial and remained prominent in the larger TRIUMPH-1 Phase 3 study.  

Does retatrutide cause nausea?

Nausea is one of the most frequently reported adverse events. In TRIUMPH-1, Lilly reported nausea in 28.6% of participants receiving 4 mg, 38.4% receiving 9 mg and 42.4% receiving 12 mg, compared with 14.8% receiving placebo.  

Can retatrutide cause vomiting or diarrhoea?

Both have been repeatedly reported. In TRIUMPH-1, vomiting occurred in 10.6% to 25.3% of participants across the retatrutide groups versus 4.8% with placebo. Diarrhoea occurred in 25.2% to 34.1% versus 13.5% with placebo.  

Are retatrutide side effects worse at higher doses?

Several important adverse events become more common at higher doses, although the relationship is not identical for every side effect. Phase 2 found gastrointestinal events were more frequent at higher doses, while TRIUMPH-1 showed particularly clear dose differences for nausea, vomiting and dysesthesia.  

Can retatrutide increase heart rate?

The Phase 2 obesity trial found a dose-dependent increase in heart rate that increased through approximately week 24 and then declined. The long-term clinical significance of that finding has not yet been established.  

What is dysesthesia with retatrutide?

Dysesthesia means an abnormal or altered sensation. In TRIUMPH-1, Lilly reported dysesthesia in 5.1%, 12.3% and 12.5% of participants receiving 4 mg, 9 mg and 12 mg respectively, compared with 0.9% receiving placebo.  

Has pancreatitis occurred in a retatrutide trial?

Yes. One serious adverse event of acute pancreatitis was reported in the Phase 2 obesity trial. A single event is not enough by itself to establish how large any causal pancreatitis risk may be.  

Do we know the long-term side effects of retatrutide?

Not completely. Retatrutide now has substantially more Phase 3 safety data, but it remains investigational and does not yet have the long-term post-market safety history available for medicines that have been widely used for years.

Research Sources

Jastreboff AM, et al.  ⁠Triple-Hormone-Receptor Agonist Retatrutide for Obesity: A Phase 2 Trial. New England Journal of Medicine, 2023.

Bajaj HS, et al.  ⁠Efficacy and Safety of Retatrutide in People With Type 2 Diabetes and Inadequate Glycaemic Control With Diet and Exercise TRANSCEND-T2D-1. The Lancet, 2026.

ClinicalTrials.gov.  ⁠TRIUMPH-1, NCT05929066.

Eli Lilly and Company.  ⁠TRIUMPH-1 Phase 3 Topline Results, May 2026.

Eli Lilly and Company.  ⁠TRIUMPH-1 Phase 3 Scientific Presentation, ADA 2026.

ClinicalTrials.gov.  ⁠TRIUMPH-Outcomes, NCT06383390.

Therapeutic Goods Administration.  ⁠Understanding Your Responsibilities When Importing, Compounding and Supplying Unapproved Peptide Products, April 2026.

Therapeutic Goods Administration and Australian Chief Medical Officer.  ⁠Concerns Regarding the Public Health Risks Associated With Unapproved Peptide Products, June 2026.