GLP-1 Family Decision Tree: Sema vs Tirz vs Reta — When Each Wins

Six months on sema. Weight moved in the first sixteen weeks. Then the plateau. You start reading about tirzepatide — the SURMOUNT numbers, the body-composition signal, the GIP axis. Then you find the NEJM 2023 retatrutide phase 2. Twenty-four percent weight loss at 48 weeks. Now you are on three Discords arguing about glucagon catabolism.

The question is not which compound is best. The question is which compound, for you, at which point in the protocol, given your endpoint. Nobody in those Discords is framing it that way. This issue does.

Issue 3 worked through the half-life math that underlies all three compounds — steady state at five half-lives, AUC, accumulation dynamics. That framework is the substrate here. Issue 5 was the first application-arc issue; this is the second. The pattern holds: mechanism first, half-life second, titration third, decision tree last.

The three compounds sit on a spectrum of receptor engagement. That spectrum is the first decision node.

Semaglutide is a GLP-1 receptor agonist. Single axis. GLP-1 receptors mediate appetite suppression, gastric emptying, and insulin secretion. The STEP 1 trial (Wilding et al., NEJM 2021) documented 14.9% weight loss over 68 weeks at 2.4mg/week in roughly 2,000 participants — the cleanest body-composition evidence base in the category, with a substudy on lean-mass dynamics published in PMC8089287.

Tirzepatide adds GIP. Dual axis. The GIP receptor sits on adipocytes and muscle cells in addition to pancreatic beta cells, and researchers believe the GIP signal partially explains why SURMOUNT-1 (Jastreboff et al., NEJM 2022) achieved up to 22.5% weight loss at 72 weeks — outcomes that exceeded the STEP trials at matched follow-up. There is also a body-composition substudy argument that the GIP axis exerts an anabolic lean-mass signal not present in the sema-alone mechanism; the data are not fully resolved, but the signal is real enough to drive the clinical hypothesis.

Retatrutide adds glucagon. Triple axis. The glucagon receptor drives hepatic glucose production and thermogenesis — that is the metabolic-rate argument for reta. The NEJM 2023 phase 2 trial (Jastreboff et al.) documented 24.2% weight loss at 48 weeks at the highest dose tested. The theoretical tradeoff on the glucagon axis is muscle-catabolism pressure. Phase 2, not phase 3. The frontier caveat is load-bearing.

Single axis, dual axis, triple axis. The receptor map is the mechanism story; the titration ladder is the timing story.

Issue 3 was the half-life framework: half-life, steady state at five half-lives, AUC, receptor-behavior implications. All three GLP-1 family members are long-half-life compounds. Run each through that framework.

Semaglutide: approximately 7-day half-life (injectable; the Rybelsus oral formulation is substantially shorter due to absorption kinetics — a different pharmacokinetic problem). Weekly dosing in the STEP trials reflects a molecule that takes roughly five weeks to reach steady-state blood levels. The accumulation pattern Issue 3 described applies directly.

Tirzepatide: approximately 5-day half-life. Weekly dosing in SURMOUNT. Steady state at roughly five weeks on a weekly schedule.

Retatrutide: approximately 6-day half-life. Weekly dosing in the phase 2 trial, with some biweekly arms. Same steady-state window.

The practical implication: titration speed determines how fast you reach the blood-level plateau where the compound is actually doing its job. A rapid ramp accumulates faster to peak exposure. A slow ramp extends the time before steady-state is reached but attenuates the GI side-effect load on the way up. That is the titration ladder.

The titration ladder below is a description of how the phase 2 and phase 3 trials ramped each compound, and what the side-effect profile looked like at each increment. It is not a protocol recommendation.

Semaglutide. Per the published trial methods, STEP 1 used a 16-week ramp: 0.25mg/week for 4 weeks, 0.5mg for 4 weeks, 1.0mg for 4 weeks, 1.7mg for 4 weeks, then 2.4mg/week for the remaining 52 weeks. The front-loaded nausea/vomiting signal was concentrated in weeks 1–8 — the period of fastest receptor engagement relative to the participant's baseline exposure. STEP 1's body-composition substudy documented lean-mass losses alongside fat-mass losses, which is the read that drove the SURMOUNT-comparison argument. The gentlest mechanical profile of the three. The deepest human safety data.

Tirzepatide. SURMOUNT-1 used a 20-week ramp across the 5, 10, and 15mg arms — generally 4-week increments at 2.5mg steps. The GIP co-agonism appears to moderate the nausea burden relative to GLP-1-only mechanisms at matched GLP-1-receptor engagement — a hypothesis consistent with the SURMOUNT-1 tolerability data, though not a controlled comparison. The 22.5% weight loss at max dose came from the 15mg arm at 72 weeks. The SURMOUNT-2 trial compared tirzepatide to semaglutide head-to-head; tirzepatide outperformed at 40 weeks on the primary weight-loss endpoint.

Retatrutide. The NEJM 2023 phase 2 used a 24-week dose-escalation structure. The bloat-complaint cluster visible in community forums is a titration-speed artifact — it clusters in participants and field reporters who are ascending the dose range faster than the trial's ramp schedule. The trial's gradual protocol was specifically designed to attenuate that signal. At 48 weeks, the highest dose arm reported 24.2% weight loss — the top published number for any injectable compound in this class. Triple-axis mechanism. Phase 2 evidence base, not phase 3. The frontier caveat is not decorative.

The community complaint pattern by compound: sema — "it stopped working." Tirz — "the nausea was worse at first." Reta — "the bloat at 2mg." All three are titration-speed artifacts or plateau dynamics. The literature's gradual ramp schedules were designed to address them.

Three nodes. Operator-grade framing. Not a protocol.

Node 1: What is your entry point?

For a researcher entering the category from a GLP-1-naive baseline, semaglutide is the most-evidenced first compound — best-studied, lowest mechanical complexity, single-axis mechanism, deepest human safety record. It is the compound a researcher working from the broadest evidence base would land on first.

For a researcher already running sema with a plateau, the dual-axis GIP signal is the next mechanistic handle in the literature. SURMOUNT-2 showed tirzepatide outperformed semaglutide at 40 weeks in head-to-head comparison. The body-composition argument for the GIP axis is the secondary descriptor. Tirzepatide is the most-evidenced second-line compound in the literature.

For a researcher running tirzepatide with a plateau, or optimizing specifically for maximum body-composition or weight-loss outcome, retatrutide is the frontier compound in the class — triple-axis, phase 2 data, the highest published weight-loss number. The risk-tolerance bar associated with the compound is appropriately higher than the prior two.

Node 2: What is your primary endpoint?

For a researcher whose endpoint is metabolic only — insulin sensitivity, blood sugar dynamics, glycemic stability — semaglutide is the cleanest match: most human safety data, narrowest mechanism, deepest understanding of the receptor behavior.

For body composition with lean-mass preservation as a priority, tirzepatide is the most-evidenced fit. The GIP anabolic signal and the SURMOUNT body-composition substudy data are the operationally relevant descriptors.

For maximum weight loss plus metabolic optimization, accepting the frontier caveat, retatrutide carries the highest published number. The 24.2% weight-loss figure is real. Phase 2 is the evidence-base qualifier that goes with it.

Node 3: What does your GI tolerance profile look like?

For GI-sensitive researchers, semaglutide titrated at the STEP 1 ramp pace or slower is the gentlest option described in the literature for the class.

For GI-tolerant researchers, tirzepatide or retatrutide, titrated at the literature's described pace, are the more-aggressive options the trial protocols cover.

That is the tree. Three nodes; the leaves are yours.

Three papers are the intellectual floor. Everything else — Discord debate, clinic blogs, subreddit reports — is Tier 4 community aggregation sitting on top of this Tier 1 foundation.

STEP 1 — Wilding et al., NEJM 2021. Semaglutide 2.4mg/week, 68 weeks, ~2,000 participants. 14.9% mean weight loss. Body-composition substudy in PMC8089287. The anchor paper for semaglutide at the weight-management dose.

SURMOUNT-1 — Jastreboff et al., NEJM 2022. Tirzepatide 5/10/15mg/week, 72 weeks, ~2,500 participants. Up to 22.5% weight loss at the 15mg arm. The paper that established the dual-axis hypothesis as a clinical argument, not just a mechanistic one.

Retatrutide phase 2 — Jastreboff et al., NEJM 2023. Triple agonist, 48 weeks, dose-escalation design. 24.2% weight loss at the highest dose. The only published phase 2 data for retatrutide at scale. Read the methods section for the titration structure before reading the results.

These three papers are short. The STEP 1 methods section alone answers most of the Discord arguments about sema plateau dynamics. Read the anchor papers before reading the community consensus that claims to summarize them.

The GLP-1 family is not interchangeable. Receptor coverage, half-life, titration pace, and endpoint all determine the right sequence. The decision tree above is how a serious operator frames the choice — not "which one is best" but "which one, at which stage, for which endpoint." The three anchor papers are the floor. Start there.

Issue 07 — The Growth Stack: CJC-1295 + Ipamorelin vs MK-677 — Pulse vs Plateau. The IGF-1 response curve, the sleep impact, and why MK-677 water retention is the real selection criterion between two approaches with almost identical GH-axis endpoints on paper.

Colby

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