The question keeps coming up in every GLP-1 discussion: does retatrutide cause more anhedonia than tirzepatide?

Anhedonia — not depression. The volume knob on enjoyment being turned down. Food registers as fine but not good. Music doesn't land. Things you normally enjoy feel like chores. Nothing is wrong and nothing is particularly good.

It's a real phenomenon that some people experience on GLP-1 class compounds. But the explanation most people reach for — glucagon acts on dopamine, that's what separates the two drugs — doesn't hold up when you actually look at the receptor pharmacology.

Here's what the evidence actually shows. And here's what's missing from it.

First: The Correction Most Discussions Get Wrong

The common claim is that anhedonia was never measured in these trials. That's not quite right — and the real situation is more interesting.

Lilly's obesity trial protocols for both retatrutide and tirzepatide schedule the PHQ-9 alongside the C-SSRS suicide-risk assessment throughout the study. The retatrutide phase 2 protocol states it directly: participants "will be monitored for depression and suicidal ideation or behavior through AE collection and by using the C-SSRS and the PHQ-9 questionnaires," with scores "reviewed by the investigator at the time of each visit." Tirzepatide's SURMOUNT-1 schedule of activities lists the PHQ-9 at 11 scheduled visits and the C-SSRS at 19.

PHQ-9 question 1: "Little interest or pleasure in doing things."

That is an anhedonia item. It was asked repeatedly across thousands of trial participants in both programs.

So why is there still no published answer to the anhedonia question?

Three reasons:

1. It was used as a safety screen, not a measurement. The purpose was flagging anyone sliding toward serious depression so investigators could respond. The total score was reviewed. Nobody isolated question one, tracked it by dose group, and published it.

2. No dedicated anhedonia scale was used. Validated instruments exist and are standard in psychiatry research. None appear in these protocols.

3. The trials excluded the relevant population by design. Lilly's enrollment criteria screen out participants at higher risk of severe depression or suicidality before enrollment. The studied population was pre-filtered to remove the people most likely to display a mood effect.

The accurate framing: Lilly collected one anhedonia item as a safety check on a pre-screened population, and the item-level data has never been published by dose group.

That data exists inside Lilly. It is not public.

What the Exit Interview Data Shows

One published dataset on subjective experience deserves attention — and it argues against the flatness account.

Researchers conducted exit interviews with 40 participants leaving the Phase 2 retatrutide trial, covering eating, emotions, and lifestyle. Results:

25 described feeling happy
32 reported feeling good about themselves
24 reported improved energy
17 reported improvement in leisure activities

Two caveats: small sample, and the study was Lilly-sponsored under the title "Perceived benefits" — it wasn't designed to hunt for problems.

Meanwhile, a 2026 MedRxiv preprint that coded self-reported effects from gray-market retatrutide users did capture anhedonia reports.

Trial participants: improvement. Gray-market reports: flatness. Same molecule.

That split matters. The mechanism below may explain it.

Why GLP-1 Signaling Can Blunt Reward

This part is well-established. GLP-1 receptors are expressed in the brain's reward system — specifically in structures that determine what registers as enjoyable and worth repeating. GLP-1 activation there dampens dopamine signaling in that circuit.

This is not a side effect. It's the mechanism behind the effect everyone wants.

"Food noise" — that constant background pull toward eating — quiets because the reward system stopped tagging food as urgent. This is also why there's serious research interest in this class for alcohol and nicotine craving.

The problem: that circuit doesn't process food exclusively. It processes everything rewarding. Dampen it and food gets quieter, but so does music, anticipation, social engagement, and motivation generally.

The volume knob doesn't have a food-only setting.

GIP Is Shared — It Cannot Explain the Difference

This is where the usual comparison goes wrong.

Three receptors are in play: GLP-1, GIP, and glucagon.

Tirzepatide: GIP + GLP-1. Deliberately lopsided toward GIP. Research calculating actual receptor occupancy at clinical doses found considerably greater GIP engagement than GLP-1, and tirzepatide engages the GLP-1 receptor more weakly and differently than native GLP-1 does. (Willard et al. JCI Insight, 2020)

Retatrutide: GIP + GLP-1 + glucagon. Built on a similar chassis — heavy on GIP, lighter on GLP-1 relative to native hormones. (Urva, Coskun et al. Lancet, 2022)

GIP matters here. A 2021 study by Borner et al. tested GIP receptor activation against GLP-1-induced nausea across three species, including musk shrews (which can vomit, unlike rodents). GIP receptor activation blocked emesis and reduced illness behavior while preserving appetite suppression, weight loss, and glucose benefits. Single-nuclei RNA sequencing localized GIPR predominantly to inhibitory (GABAergic) neurons in the area postrema — the brain's poison detector — adjacent to the excitatory neurons GLP-1 activates. GIP engages local brakes and quiets the aversion circuit.

Source: Borner T, Geisler CE, et al. "GIP Receptor Agonism Attenuates GLP-1 Receptor Agonist-Induced Nausea and Emesis in Preclinical Models." Diabetes. 2021;70(11):2545-2553. PMC8564411. Cited 224+ times.

The critical point: both retatrutide and tirzepatide carry this GIP buffer. Whatever protection GIP provides, both compounds get it.

GIP therefore cannot be what separates them on anhedonia. Glucagon is the only real variable.

The Glucagon Connection — And Why It's Not What People Think

Here's where the discussion usually goes wrong again.

There is no good evidence that glucagon acts directly on the reward system. Glucagon receptors are not concentrated in dopamine circuitry the way GLP-1 receptors are. Anyone claiming glucagon agonism directly protects against anhedonia should be asked for the paper. The evidence isn't there.

The plausible connection is more interesting — and has nothing to do with neurochemistry.

It's arithmetic.

GLP-1 and GIP act on the intake side. They reduce food consumption.

Glucagon acts on the output side. It increases energy expenditure and fat oxidation.

Here's why that distinction matters for anhedonia:

Sustained caloric deficit produces the flat, unmotivated, nothing-registers state on its own — in people taking nothing whatsoever. Anyone who has watched someone go through serious contest prep without pharmaceuticals has seen this. Dramatically restricted intake for months reliably produces low mood, low drive, low libido, and blunted enjoyment.

Consider two people who both lose 15% of body weight:

The first: dramatically reduced intake. Deep deficit, sustained for months.

The second: moderately reduced intake plus elevated energy expenditure via glucagon activation. Same scale result, shallower deficit, less cumulative time in an underfed state.

If glucagon shifts part of the work from the intake side to the expenditure side, the same endpoint is reached with less exposure to the thing that produces flatness in the first place. Not because glucagon protects dopamine. Because you're spending less time starving.

This is mechanistically coherent. It is also entirely untested against anhedonia. That study has not been run.

Why the Trial vs. Gray Market Split Fits This Hypothesis

Trial participants titrate slowly under supervision toward moderate, controlled deficits.

Unsupervised gray-market use trends toward aggressive dosing, minimal monitoring, and minimal food intake. If someone is getting the drug from an online vendor and combining it with extreme restriction, they're not just on a GLP-1 — they're starving.

If flatness is driven by deficit depth rather than receptor pharmacology, you'd predict exactly the pattern observed: trial participants reporting improved mood and leisure activities; gray-market users reporting anhedonia on the same molecule.

The Second Glucagon Route (Speculative but Real)

One more pathway worth knowing — less established, but real enough to note.

Glucagon receptor activation causes the liver to release FGF21 (fibroblast growth factor 21). This isn't incidental — it's a necessary component of how glucagon drives weight loss. Mice engineered without liver FGF21 were partly resistant to glucagon's anti-obesity effects. (Presedo et al. FASEB J, 2020)

FGF21 also acts on the brain. Its obligate co-receptor is expressed in the hypothalamus and hindbrain, and blocking central signaling reduced glucagon's weight loss effect. So a meaningful portion of glucagon's action routes through the brain rather than the liver alone.

Separately — from an unrelated field — FGF21 has documented neuroprotective effects, and psychiatric mood stabilizers markedly elevate FGF21 in neurons. (Molecular Psychiatry, 2015)

This is not a claim that retatrutide is an antidepressant. The last connection comes from a different research context. It's included because it's real, not because relevance is established. But a compound driving a brain-active hormone with neuroprotective properties and an unusual psychiatric overlap is a thread worth following.

The Case Against All of This

The mechanism story is seductive. It shouldn't be oversold.

Retatrutide is still a serious GLP-1 agonist. It didn't remove GLP-1 activity — it added a receptor. Whatever reward dampening GLP-1 signaling produces, retatrutide still produces.

Larger results can mean larger deficits. Retatrutide produces very substantial weight loss. If flatness is chiefly deficit-driven, a more potent compound could generate more of it, not less, regardless of what glucagon does to expenditure.

Order is confounded in nearly every anecdotal comparison. Most reports involve one compound run before the other — not a fair between-molecule comparison, but a comparison between a first heavy exposure to this drug class and a later one at different body weight, different deficit, different expectations.

No head-to-head data exists. No study has compared these two compounds against each other on enjoyment or motivation outcomes.

What Would Actually Settle This

The cheapest version costs nothing new. Lilly could publish PHQ-9 question one scores by dose group from trials already completed. That data exists. It has never been released. Someone should formally request it.

The better version: a validated anhedonia scale administered across both compounds, with caloric deficit tracked as an independent variable.

That last part is the whole ballgame. Until someone separates the receptor effect from the effect of eating 1,100 calories a day — which would flatten anybody — this remains an argument about anecdotes rather than a research finding.

Why Tracking This Matters

This is exactly the kind of effect that daily tracking catches early.

If you're monitoring mood, motivation, energy, and enjoyment as part of your daily check-in — alongside body composition and protocol version — you see the shift in the data before you consciously recognize it as a problem. And crucially, you can see when it started relative to a dose change, a protocol adjustment, or a change in your caloric intake.

Without that data, the question "is this my drug or my deficit?" is unanswerable. With it, you have the timeline to at least form a hypothesis.

→ Track your protocol at PeptidesGPT.com

Sources:

Borner T, Geisler CE, Fortin SM, et al. "GIP Receptor Agonism Attenuates GLP-1 Receptor Agonist-Induced Nausea and Emesis in Preclinical Models." Diabetes. 2021;70(11):2545-2553. PMC8564411. Cited 224+ times.
Willard FS, et al. "Tirzepatide is an imbalanced and biased dual GIP and GLP-1 receptor agonist." JCI Insight. 2020.
Urva S, Coskun T, et al. "LY3437943, a novel triple GIP, GLP-1, and glucagon receptor agonist: phase 1b multiple-ascending dose trial." Lancet. 2022;400(10366):1869-1881.
Presedo N, et al. "Glucagon Regulates Energy Balance via FGF21 Signaling in the Brain." FASEB J. 2020;34(S1) (abstract). Full study: "Glucagon receptor signaling regulates weight loss via central KLB receptor complexes." JCI Insight. 2021.
PHQ-9/C-SSRS protocol documentation: ClinicalTrials.gov NCT04881760 (retatrutide phase 2, attached study protocol §8.3.2.13; exclusion: PHQ-9 ≥ 15 at Visit 1) and NCT04184622 (SURMOUNT-1, tirzepatide, attached study protocol §10.3.1.13 and schedule of activities).
Lilly Phase 2 retatrutide exit interviews: "Perceived benefits of treatment for obesity with retatrutide: a qualitative study of patients in a phase 2 clinical trial." N=40. Lilly-sponsored.
Self-Reported Side Effects Among Reddit Users Taking Unapproved Retatrutide. medRxiv preprint, May 2026.
Leng Y, et al. "FGF-21, a novel metabolic regulator, has a robust neuroprotective role and is markedly elevated in neurons by mood stabilizers." Mol Psychiatry. 2015;20(2):215–223 (online 2014). PMID 24468826.
Derek Pruski. "The Glucagon Receptor and Anhedonia." Research Radar. August 31, 2026.

PeptidesGPT is an educational platform. Retatrutide is an investigational compound not approved by the FDA. This content is for informational purposes only and does not constitute medical advice. Always consult a licensed healthcare provider before making decisions about your health or protocol.