Retatrutide: Clinical Evidence, Safety and Side Effects

Retatrutide, also known as LY3437943, is an investigational drug developed by Eli Lilly and Company. It belongs to a new group of medications called “triple-agonist” metabolic drugs because it activates three hormone receptors that help control appetite and metabolism.

These hormones help regulate hunger, blood sugar and energy use. By activating all three together, the drug may reduce appetite, help burn more calories and improve blood sugar control.

Researchers believe this triple-hormone approach may explain why retatrutide appears to cause more weight loss than older drugs that target only one or two of these receptors.

Like other incretin-based drugs, retatrutide is given as a weekly injection during clinical trials.

Retatrutide is part of a growing group of weight-loss drugs called GLP-1-based medications. This group also includes approved drugs such as Semaglutide and Tirzepatide.

Clinical trials have shown promising results so far, but research is still ongoing. There is limited information on the drug’s long-term safety.

Phase 1 and Early Trials

The first human studies began in late 2019. Early trials suggested retatrutide might help reduce appetite and improve markers of metabolic health.

A Phase 1 study involving dozens of participants tested weekly and saw decreases in mean body weight at all dose levels over 0.1 mg.

These early studies also suggested the drug was generally safe when used with medical supervision.

Phase 2 Obesity Trial

One of the most widely cited studies on retatrutide was a randomized Phase 2 clinical trial published in the New England Journal of Medicine in 2023.

Researchers studied 338 adults with obesity or overweight who did not have diabetes. Participants were randomly assigned to receive weekly injections of retatrutide at doses of 1 mg, 4 mg, 8 mg or 12 mg, or a placebo for 48 weeks.

The trial found significant weight loss across all treatment groups, with the highest dose producing the largest effect.

These results represented one of the largest average weight reductions reported for an antiobesity medication in a randomized clinical trial.

Researchers also observed improvements in several metabolic health markers, including blood pressure, cholesterol levels and insulin sensitivity.

Studies in Type 2 Diabetes

Retatrutide has also been studied in patients with Type 2 diabetes.

In a Phase 2 clinical trial published in The Lancet, the drug significantly improved blood-sugar control and reduced body weight in people with Type 2 diabetes compared with a placebo.

These findings suggest retatrutide may have potential as a treatment for both diabetes and obesity, though larger trials are needed to confirm these effects.

Phase 3 Clinical Trials

Retatrutide is currently being evaluated in a series of Phase 3 clinical trials known as the TRIUMPH program.

These trials are designed to evaluate the drug’s safety and effectiveness in larger populations of people with obesity and other metabolic conditions.

Preliminary results from a late-stage trial involving adults with obesity and knee osteoarthritis showed average weight loss of about 28.7% after 68 weeks, suggesting the drug may be even more effective than earlier treatments.

Additional Phase 3 trial results are expected through 2026.

In clinical trials, the most commonly reported side effects were gastrointestinal symptoms, which are similar to those seen with other GLP-1–based medications.

Researchers reported that these side effects were usually mild to moderate and most commonly occurred during the dose-escalation phase of treatment.

Some participants discontinued treatment due to adverse effects, and approximately 12–18% of trial participants dropped out, often due to side effects.

Because the drug is still being studied, researchers are continuing to monitor potential long-term risks such as cardiovascular effects and pancreatic complications.

Mental Health Effects and Mood Changes

In addition to physical side effects, researchers are studying whether drugs in the GLP-1 class may affect mood, depression or other psychiatric symptoms.

GLP-1 receptor agonists — including semaglutide, tirzepatide and investigational drugs like retatrutide — interact with brain pathways involved in appetite, reward and emotional regulation. GLP-1 receptors are present in brain regions associated with motivation and mood, including the ventral tegmental area and the nucleus accumbens. Because of this, scientists are investigating whether these medications could influence mental health in some patients.

Potential Mood Benefits

Some early research suggests GLP-1–based medications may improve mood for certain patients. Weight loss, improved blood-sugar control and better metabolic health can lead to increased energy and improved self-esteem, which may help reduce depressive symptoms.

A systematic review and meta-analysis of clinical trials reported that patients treated with GLP-1 receptor agonists experienced greater improvements in depression scores compared with control groups.

Researchers believe this could be related to both improved physical health and potential neurochemical effects in the brain.

Reports of Depression and Suicidal Thoughts

At the same time, some studies and case reports have raised concerns about possible psychiatric side effects.

For example, a published case report described two patients who developed significant depressive symptoms after starting semaglutide, including one patient with no prior history of depression. In both cases, symptoms improved after the medication was discontinued.

Because of reports like these, regulators have investigated whether GLP-1 drugs may be linked to depression or suicidal thoughts. In 2023, both U.S. and European regulators reviewed safety reports involving patients using GLP-1 medications.

However, the U.S. Food and Drug Administration has stated that its preliminary review of available data did not find evidence of a causal link between GLP-1 drugs and suicidal thoughts or actions, though researchers say rare risks cannot be completely ruled out.

What This Means for Retatrutide

Retatrutide has not yet been approved and limited data exist on its psychiatric effects. However, because it activates the GLP-1 pathway — along with GIP and glucagon receptors — researchers believe similar brain-related effects could occur.

More research is needed to determine whether these drugs directly affect mood or whether changes are related to factors such as rapid weight loss, hormonal changes or other metabolic effects.

Other Possible Serious or Concerning Side Effects

Because the drug affects several hormone pathways involved in metabolism, researchers say long-term safety data will be essential before approval.

Retatrutide is often compared with popular weight-loss medications such as Wegovy, Ozempic, Mounjaro and Zepbound.

While these medications are commonly grouped together, they work differently and have shown varying weight loss results in clinical trials.

Key Differences in How the Drugs Work

Most currently approved weight-loss drugs belong to the GLP-1 receptor agonist class. These medications mimic the hormone GLP-1, which helps regulate appetite and blood sugar.

Researchers believe stimulating multiple hormone pathways may produce stronger effects on appetite, metabolism and energy expenditure.

Weight-Loss Results in Clinical Trials

Clinical trials suggest retatrutide may lead to greater average weight loss than earlier medications, though direct head-to-head studies are limited.

In a Phase 2 clinical trial involving adults with obesity, participants receiving the highest dose of retatrutide lost an average of about 24.2% of their body weight after 48 weeks.

Later studies have reported weight loss approaching 28% in some participants, which is comparable to results sometimes seen with bariatric surgery.

Researchers caution that these results come from controlled clinical trials and may not translate directly to real-world use.

Side Effects Compared With Other GLP-1 Drugs

The side effects reported in retatrutide trials are similar to those seen with other GLP-1-based drugs, but some studies suggest they may occur more frequently at higher doses.

Recent trial results indicate that retatrutide may cause higher rates of gastrointestinal side effects compared with some existing obesity medications.

Researchers are continuing to study whether these side effects can be managed through dosing strategies.

Another key difference is regulatory approval.

Currently:

• Wegovy, Ozempic, Mounjaro and Zepbound are FDA-approved medications.
• Retatrutide is still an experimental drug in clinical trials.

The medication is currently being studied in the TRIUMPH Phase 3 clinical trial program, which is typically the final stage of testing before companies apply for FDA approval.

Analysts estimate that, if trials are successful, retatrutide could be approved by 2027, though it’s not guaranteed.

Even though retatrutide is still being studied, it is already being sold online through peptide websites and other unregulated sellers.

Medical experts warn that these products can be dangerous for several reasons:

Retatrutide is part of a new generation of metabolic drugs created to treat obesity and diabetes by targeting several hormonal pathways at the same time.

If ongoing trials confirm the early results, researchers think the drug could become one of the most powerful weight-loss medications ever developed. However, there are still questions about:

• Long-term safety
• Tolerability and side effects
• Cardiovascular outcomes
• Cost and accessibility

Until clinical trials are finished and regulators review the evidence, retatrutide will remain an experimental therapy.