Retatrutide heart rate and arrhythmia: what the trials have measured and what they have not

By Retatrutide Risk editorial team. Published September 21, 2026. Sources at the end of the page.

Of all the retatrutide side effects, the one with the least public attention and the most long-term uncertainty is the effect on heart rate. Gastrointestinal side effects are unpleasant and visible; a resting heart rate a few beats higher is neither, but it is the kind of change whose consequences, if any, only appear over years and across thousands of people. This page sets out exactly what the trials have reported, how it compares with the approved drugs of the class, why the glucagon receptor makes retatrutide a special case, and which trial will answer the open question.

What the phase 2 obesity trial reported

The published abstract of the 48-week phase 2 obesity trial ends its results section with one sentence on the cardiovascular system: dose-dependent increases in heart rate peaked at 24 weeks and declined thereafter.

Three things can be read from that sentence and nothing more should be.

• The effect is dose-dependent: higher doses produced larger increases.
• The effect peaked at 24 weeks. That is the trial's primary time point and, in the weight-loss curves, the point of most rapid change.
• The effect declined after the peak, over the second half of the trial, while participants remained on treatment. Weight loss continued during this period (the 12 mg arm went from -17.5% at 24 weeks to -24.2% at 48 weeks), so the decline in heart rate was not a consequence of stopping the drug.

The abstract does not give the magnitude of the increase per arm, and this site does not quote figures it has not verified against the published source. The full paper and its supplementary appendix contain the values; the citation is at the end of this page.

What the other trials reported

The absence of a heart rate statement in a topline announcement is not evidence of absence. Announcements list the most frequent adverse events and the discontinuation rates. Laboratory and vital-sign changes appear in the full publication.

How this compares with the approved drugs of the class

An increase in resting heart rate is a known effect of GLP-1 receptor agonists and of tirzepatide. The FDA prescribing information for tirzepatide (Zepbound), in its adverse reactions section, states that in a pool of its two obesity trials treatment resulted in a mean increase in heart rate of 1 to 3 beats per minute compared with no increase in placebo-treated patients.

Retatrutide differs from tirzepatide in one respect that is relevant here: it also activates the glucagon receptor. Glucagon has direct effects on the heart, including increases in heart rate and contractility, which is why glucagon is used as an emergency treatment in some cases of beta-blocker overdose. Whether glucagon receptor agonism at retatrutide's doses adds to the class effect on heart rate, and whether the "dose-dependent" description in the phase 2 abstract reflects a larger effect than tirzepatide's 1 to 3 beats per minute, cannot be determined from the abstracts. The published comparison, once the phase 3 papers are out, is the one to read.

Arrhythmia: what is and is not known

No published retatrutide abstract and no phase 3 announcement reports the rate of cardiac arrhythmia adverse events. The word does not appear in any of the sources cited on this page. That means:

• It is not possible to state, from public sources, how many trial participants developed atrial fibrillation, supraventricular tachycardia or any other rhythm disturbance, or whether the rate differed from placebo.
• It is not possible to say the rate was zero.
• Reports of palpitations on forums cannot be checked against any published trial figure.

Trials of this size record electrocardiograms at scheduled visits and code any rhythm abnormality as an adverse event. Those data exist and will appear in the full phase 3 publications and, if the drug is approved, in the FDA's review documents. Until then, "retatrutide causes arrhythmia" and "retatrutide does not cause arrhythmia" are both unsupported statements.

The trial that will answer the question

Heart rate is a surrogate. The question that matters is whether people on retatrutide have more, fewer or the same number of heart attacks, strokes, cardiovascular deaths and kidney events as people on placebo. That question is being tested directly.

"Event-driven" means the trial ends when a predetermined number of events has occurred, not on a calendar date; the February 2029 estimate depends on the event rate. Its result will arrive years after any first approval, which Lilly plans to seek in the first quarter of 2027.

A second source of cardiac data is TRIUMPH-3 (NCT05882045), which enrolled 1,946 participants with severe obesity and established cardiovascular disease and reported weight loss of 21.6% (9 mg) and 22.6% (12 mg) at 80 weeks, with discontinuation because of adverse events of 9.8% and 13.5% versus 4.8% on placebo. That is a population in which any adverse cardiac effect would be expected to show up, and its full safety tables are the most awaited item in the retatrutide record.

Reading the risk proportionately

• A dose-dependent rise in heart rate that peaks and then declines is consistent with the class effect seen with GLP-1 receptor agonists and tirzepatide, and by itself did not prevent those drugs from being approved.
• Retatrutide's glucagon component is a real reason for closer scrutiny, not a reason to assume harm.
• The people most exposed to any cardiac risk are those using grey-market product without electrocardiograms, without exclusion criteria and at doses of their own choosing. The trials excluded or monitored such risks; the Reddit article and the where-to-buy article explain why forum experience cannot substitute for trial data.
• Anyone with a history of arrhythmia, structural heart disease or heart failure has no trial data to rely on; the retatrutide trials have not reported on these subgroups.

For the full list of adverse events see the side effects article. For interactions and contraindications see alcohol, pregnancy and interactions.