The short answer: retatrutide belongs to the GLP-1 family, and that family changes how the stomach and small intestine move. At the top dose in Lilly's trials, diarrhea showed up in 24% to 34% of people and constipation in 13% to 26%, usually during dose increases.
Why one person gets one and someone else the other hasn't been worked out. Nothing I could read says how long an episode lasts on reta. On semaglutide, diarrhea tends to pass in days, while constipation drags on for weeks.
Status as of November 6, 2026: Retatrutide is not approved by the FDA or any regulator. Lilly plans to file for U.S. approval in Q1 2027. It has no label, so the warning signs below come from the labels of approved drugs in the same family. Nothing here is medical advice, a dosing guide or a treatment plan.
Key takeaways
At 12 mg, diarrhea ran 24.4% to 33.6% and constipation 13.1% to 26.1% across five retatrutide trials.
Lilly said phase 2 gut side effects "usually occurred during the dose escalation period."
No reta episode length is published in anything I could read. On semaglutide 2.4 mg, the median was 3 days for diarrhea and 47 days for constipation.
Approved labels say to contact a provider for severe or persistent gut symptoms. Dehydration from them has been tied to kidney injury.
Only magnesium oxide has randomized trials for chronic constipation. For citrate and glycinate, a major U.S. guideline says the evidence is "unknown."
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Why does reta cause diarrhea?
The class effect is known. The exact reason for diarrhea isn't. The closest approved labels say "Tirzepatide delays gastric emptying" (Zepbound label, Aug 2026) and "Semaglutide delays gastric emptying" (Wegovy label, June 2026). A 2024 review adds that GLP-1 drugs "slow gastric emptying and motility of the small intestine" (Jalleh et al., JCEM, 2024).
For diarrhea specifically, the best lead is a pattern. An expert consensus noted long-acting drugs have "less nausea and vomiting but with more diarrhoea," which "might be explained by a more sustained effect" on GLP-1 receptors in the gut (Gorgojo-Martínez et al., J Clin Med, 2023). Reta is a once-weekly, long-acting drug. Note the verb: "might."
Why can reta cause constipation too?
Slower movement is the likely reason. The main semaglutide analysis says constipation lasted longer and "may be because of the effects of GLP-1RAs on GI motility" (Wharton et al., Diabetes Obes Metab, 2022). The consensus adds a behavior piece: feeling full, people "tend to reduce water intake."
Retatrutide's third target, glucagon, is the open question. One FDA-approved glucagon product is labeled "a gastrointestinal motility inhibitor" for radiology, and the label says glucagon relaxes "the smooth muscle of the stomach, duodenum, small bowel, and colon" (Gvoke label, DailyMed). But that's an IV diagnostic use, and reta is weaker than the body's own glucagon at that receptor (0.3 times as potent, per retatrutide vs tirzepatide). It's a reason to ask, not an answer.
I found no published study of how reta affects gut movement in people.
How common are reta diarrhea and constipation?
Common. Diarrhea ran higher than constipation in all four placebo-controlled trials. These are the 12 mg rates, matching is reta safe?, plus TRIUMPH-3.
Trial | Who | Diarrhea | Constipation |
|---|---|---|---|
TRIUMPH-1 | Obesity | 32.0% vs 13.5% | 26.1% vs 10.9% |
TRIUMPH-2 | Obesity + type 2 diabetes | 33.6% vs 13.2% | 16.8% vs 9.4% |
TRIUMPH-3 | Obesity + heart disease | 24.4% vs 8.7% | 15.7% vs 7.1% |
TRIUMPH-4 | Obesity + knee arthritis | 33.1% vs 13.4% | 25.0% vs 8.7% |
TRANSCEND-T2D-2 | Type 2 diabetes, vs Ozempic 2 mg | 32.7% vs 26.4% | 13.1% vs 19.8% |
Second number is placebo, except the last row, where it's Ozempic. Sources: Lilly, Dec 11, 2025, June 6, 2026, July 23, 2026, Oct 28, 2026.
The last row is the only same-trial comparison with another drug: more diarrhea, less constipation than Ozempic. It was open-label and isn't peer-reviewed.
Constipation also behaves differently with dose. TRIUMPH-1 by arm:
Side effect | 4 mg | 9 mg | 12 mg | Placebo |
|---|---|---|---|---|
Nausea | 28.6% | 38.4% | 42.4% | 14.8% |
Diarrhea | 25.2% | 34.1% | 32.0% | 13.5% |
Constipation | 23.8% | 25.9% | 26.1% | 10.9% |
Nausea climbs with dose. Constipation barely moves. Tirzepatide's label shows something similar: 17%, 14% and 11% from lowest dose to highest, vs 5% on placebo.
When do reta gut side effects start?
Usually during dose increases. Lilly said phase 2 gut side effects "usually occurred during the dose escalation period" (Lilly via BioSpace, June 2023). The phase 2 paper's abstract says they "were partially mitigated with a lower starting dose" (NEJM, 2023). Phase 3 stepped the dose up every four weeks. I found no week-by-week curve for reta. The full phase 3 papers are paywalled.
Approved drugs fill in the shape:
Tirzepatide: "The majority of nausea, vomiting, and/or diarrhea events occurred during dose escalation and decreased over time" (Zepbound label). Constipation isn't in that sentence.
Semaglutide 2.4 mg: nausea, diarrhea and vomiting peaked at about week 20. Constipation plateaued at about week 10 (Wharton).
One possible reason: tirzepatide's delay "is largest after the first dose and this effect diminishes over time." Whether each reta dose step restarts that is my reasoning, not a finding.
How long does reta diarrhea or constipation last?
For reta, nothing I could read gives a number. Lilly's phase 3 releases say "the majority resolved during treatment." That doesn't tell you whether it took days or months.
The best stand-in is a pooled analysis of semaglutide 2.4 mg in the STEP 1-3 trials (Wharton et al., 2022):
Side effect (semaglutide 2.4 mg) | Median length of one episode |
|---|---|
Vomiting | 2 days |
Diarrhea | 3 days |
Nausea | 8 days |
Constipation | 47 days (35 on placebo) |
It's a different drug, in a paper with Novo Nordisk employees as authors. A median means half of episodes ran longer. And rates stayed above placebo "throughout the 68-week treatment period."
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Why constipation then diarrhea on reta?
No trial I could read reports that sequence. Published rates count each side effect separately. These explanations hold up, but none has been tested on reta:
Two separate effects. Both are common, so some people will get both at different times.
Overflow. NIDDK says hard stool can weaken rectal muscles, which "let watery stools that build up behind the hard stool leak out" (NIDDK). Both labels list "severe constipation including fecal impaction" among post-approval reports.
Overcorrection. One OTC magnesium citrate label says it generally produces a bowel movement in 30 minutes to 6 hours (DailyMed). Loose stools after a laxative aren't a mystery. That's my reasoning.
When should you call a doctor about reta gut side effects?
When symptoms are severe, won't go away, or come with the signs below. This comes from approved drugs in the family and NIDDK, and it isn't a complete list.
What's happening | What the source says |
|---|---|
Gut symptoms that are severe or persistent | Contact a provider (Zepbound label) |
Nausea, vomiting or diarrhea that doesn't go away | "Tell your healthcare provider right away." Fluid loss may cause kidney problems (Wegovy Medication Guide) |
Severe stomach-area pain that won't go away, sometimes felt in the back | Pancreatitis warning: stop the drug and call right away (Wegovy guide) |
Upper stomach pain, fever, yellow skin or eyes, clay-colored stools | Gallbladder warning: call a provider (Wegovy guide) |
Constipation plus rectal bleeding, blood in stool, constant belly pain, inability to pass gas, vomiting or fever | See a doctor right away (NIDDK) |
Diarrhea lasting more than 2 days, six or more loose stools a day, high fever, black or bloody stools, or dehydration signs | Talk with a doctor right away (NIDDK) |
Zepbound's label reports acute kidney injury, "in some cases requiring hemodialysis," mostly in people whose gut side effects led to dehydration. Severe gut events ran 3.1% on Zepbound 15 mg vs 1% on placebo, and 4.1% vs 0.9% on Wegovy injection. I found no severe-GI rate for reta's phase 3 trials.
Tell the doctor exactly what you took and where it came from. These trial numbers come from Lilly's own drug, and they don't tell you what's in a vial from anywhere else.
What do labels and guidelines suggest for GI side effects?
Labels say little beyond fluids and telling your provider. The detail comes from NIDDK, written for gut symptoms in general, and the consensus, which was funded by Novo Nordisk and is opinion, not trial evidence. I found no test of any of it on reta.
Fluids. Wegovy's guide says "it is important for you to drink fluids." Zepbound's label says to "take precautions to avoid fluid depletion."
Antidiarrheals: NIDDK lists loperamide for acute diarrhea, but doctors typically advise against these medicines with bloody stools or fever. If you were constipated first, ask before using one, because of overflow. That caution is mine.
With constipation: the consensus suggests fiber, physical activity, water and considering stool softeners.
Dose changes are a prescriber's call. Wegovy's label says "consider delaying dosage escalation." Zepbound's says "consider a lower maintenance dosage." The TRIUMPH trials allowed a permanent dose reduction for gut events that hadn't improved with other steps (TRIUMPH design paper, 2025). Those instructions are for clinicians, not a plan to run on your own.
Magnesium citrate vs glycinate for reta constipation: is there an answer?
No. The AGA-ACG constipation guideline says: "Only MgO has been evaluated in RCTs; the bioavailability and clinical efficacy of other formulations of magnesium (e.g., citrate, glycinate, lactate, malate, sulfate) for CIC are unknown" (AGA-ACG guideline, 2023). MgO is magnesium oxide. CIC is chronic idiopathic constipation, which isn't drug-related constipation.
Product type | What the guideline says |
|---|---|
Polyethylene glycol (PEG) | Strong recommendation, moderate certainty |
Fiber supplements | Conditional, low certainty. "Only psyllium appears to be effective," with adequate hydration |
Magnesium oxide | Conditional, very low certainty. Two trials, 47 people on it |
Magnesium citrate, glycinate | Not evaluated in randomized trials. OTC citrate liquid is labeled "for relief of occasional constipation" |
Two cautions fit this drug family. The guideline says to avoid magnesium oxide with renal insufficiency, and the labels tie dehydration to kidney injury. The OTC citrate label says to ask a doctor first if you have abdominal pain, nausea or vomiting. For magnesium more broadly, see magnesium is infrastructure, not a hack.
Reta constipation and diarrhea FAQ
Is diarrhea for a week on reta normal?
Nobody has published what's normal for reta. On semaglutide 2.4 mg the median episode lasted 3 days, and NIDDK lists diarrhea past 2 days among reasons to talk with a doctor right away.
Does reta constipation go away?
Lilly says most side effects "resolved during treatment," without constipation timing. On semaglutide 2.4 mg the median episode was 47 days, vs 35 on placebo.
Can you have constipation and diarrhea on reta at the same time?
Published rates don't show overlap. NIDDK describes watery stool leaking past hard stool, which is a reason to ask a clinician before using an antidiarrheal.
Does reta cause more constipation than tirzepatide?
In separate trials, top-dose rates were 26.1% in TRIUMPH-1 and 11% in tirzepatide's label pool. The head-to-head, TRIUMPH-5, hasn't reported.
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If you've had gut side effects on a GLP-1 drug, reply and tell me when they hit and how long they lasted. If Lilly publishes real timing data, I'll put it next to what readers reported.