GLP-1 Peptides Compared: Semaglutide vs Tirzepatide vs Retatrutide

GLP-1 receptor agonists mimic the incretin hormone glucagon-like peptide-1, released from intestinal L-cells after eating. The core mechanism has three components: glucose-dependent insulin secretion from pancreatic beta-cells (potentiated only when glucose is elevated, which is why hypoglycaemia risk is low in monotherapy), delayed gastric emptying (prolonging satiety signalling), and appetite and reward-pathway suppression via hypothalamic and mesolimbic GLP-1 receptor populations, reducing caloric intake independent of gut-level effects. Newer molecules extend the same incretin-mimetic logic to additional receptors — GIP and the glucagon receptor — compounding the metabolic effect. Semaglutide, Tirzepatide, and Retatrutide represent three successive points on this receptor-expansion trajectory.

The table below summarises the defining research parameters for each compound. Doses reflect ranges used in the published trial literature discussed below, not a specific recommendation.

Semaglutide is a GLP-1 receptor mono-agonist with the deepest bench of long-term human trial data here. It is a modified GLP-1 analogue with a fatty-acid side chain conferring albumin binding, extending its half-life to roughly seven days. The STEP trial programme is the primary evidence base: in STEP-1, participants without diabetes lost a mean of 14.9% of body weight at 68 weeks on the 2.4mg dose versus 2.4% on placebo (Wilding et al., NEJM 2021). Across STEP 1, 3, 4, and 8, mean weight loss ranged from roughly 14.9% to 17.4%. Research dosing follows a slow escalation — 0.25mg weekly for four weeks, then through 0.5mg, 1mg, 1.7mg, up to 2.4mg — to manage tolerability. Dominant adverse events were gastrointestinal: nausea, diarrhoea, and vomiting, generally mild-to-moderate and transient. Semaglutide's evidence base functions as the reference standard against which newer multi-receptor agonists are measured.

Tirzepatide adds a second receptor target — GIP — to the GLP-1 backbone, a single peptide activating both receptors simultaneously. The rationale: GIP activation, though a weaker standalone signal than GLP-1, produces a synergistic effect on insulin sensitivity and adipocyte biology when combined with GLP-1 agonism. The SURMOUNT trial family is the primary evidence base: SURMOUNT-1 reported mean weight reductions of approximately 15% (5mg), 21.4% (10mg), and 22.5% (15mg) at 72 weeks versus 2.4% with placebo (Jastreboff et al., NEJM 2022). Head-to-head against semaglutide 1mg, SURPASS-2 found tirzepatide superior on glycaemic control and weight reduction at every dose (Frías et al., NEJM 2021). Research dosing escalates from 2.5mg weekly through 5mg, 7.5mg, 10mg, 12.5mg, to a 15mg maintenance dose. A notable secondary finding is improved lean-mass preservation relative to pure GLP-1 mono-agonism — an actively researched distinction between the two mechanisms.

Retatrutide extends the mechanism further, adding glucagon receptor agonism to the GLP-1/GIP backbone, making it a true triple agonist. Glucagon receptor activation is mechanistically distinct — rather than acting on insulin secretion or appetite, it increases hepatic glucose output and, more relevantly, resting energy expenditure and hepatic fat oxidation. Combined with GLP-1/GIP-driven appetite suppression, this is the proposed mechanism behind retatrutide's larger effect size. The pivotal evidence is Eli Lilly's published Phase 2 randomised, placebo-controlled trial: at the 12mg dose, participants lost a mean of approximately 24.2% of body weight at 48 weeks, with 100% on the 8mg and 12mg doses achieving at least 5% weight loss (Jastreboff et al., NEJM 2023). Trial dosing escalated from a 2mg or 4mg starting dose up to the 12mg weekly maintenance dose, mirroring the slow-titration approach used with the other two. Because this is Phase 2 data — large and well-controlled, but without completed Phase 3 outcomes — retatrutide is rated Tier A rather than Tier S: the efficacy signal is strong, but the evidence base is one phase behind.

It is important to be direct about what is and is not comparable here. No published RCT has tested all three head-to-head in the same population — the comparisons below are cross-trial, so differences in baseline characteristics, duration, and geography can confound a strict ranking. With that caveat:

The clearest statement the literature supports is a magnitude gradient tracking receptor count: each added pathway (GLP-1 → GLP-1+GIP → GLP-1+GIP+glucagon) correlates with a larger mean weight-loss effect in its respective trial. Glycaemic and cardiometabolic markers improve favourably across all three, though evidence depth narrows from semaglutide (Tier S) to retatrutide (Tier A, one large Phase 2 trial). GI tolerability is broadly similar, consistent with the shared delayed-gastric-emptying mechanism.

All three compounds are supplied as lyophilized (freeze-dried) powder and require reconstitution with bacteriostatic (BAC) water before use in a laboratory protocol. Standard practice is to add BAC water slowly down the inside wall of the vial, then gently swirl — never shake — until fully dissolved. Once reconstituted, all three should be stored refrigerated at 2–8°C, protected from light, with typical stability windows of 4–8 weeks post-reconstitution depending on concentration and formulation buffer. None should be frozen after reconstitution, and all should be visually checked before each use for particulate matter or discolouration.

Research protocols involving any GLP-1 class compound should be paired with a baseline and follow-up bloodwork panel. Priority markers: fasting glucose and HbA1c (core glycaemic control, given the insulin-secretion mechanism), fasting insulin (insulin sensitivity context), a full lipid panel including ApoB (a more direct measure of atherogenic particle number than LDL-C alone), thyroid function (TSH) — included because GLP-1 receptor agonists produced thyroid C-cell hyperplasia in rodent studies, a signal not replicated in human epidemiological data but still worth monitoring — amylase and lipase as a pancreatitis safety signal, and body composition via DEXA to separate lean-mass from fat-mass change, given the lean-mass differences discussed above. For full reference ranges and testing cadence, see the Bioactive Compounds bloodwork guide.

Framed honestly, the choice tracks the specific research question rather than a simple "best overall" ranking. Semaglutide is the appropriate reference compound for research characterising well-established, single-pathway GLP-1 mono-agonism — the deepest and longest-running evidence base of the three. Tirzepatide is the natural choice for research into GIP and GLP-1 receptor synergy, particularly the differential body-composition and lean-mass effects that distinguish dual agonism from GLP-1 alone. Retatrutide is the compound of interest for research targeting maximum weight-loss magnitude and triple-agonist mechanisms, including the energy-expenditure contribution from glucagon receptor activation — with the caveat that its evidence base remains one phase behind pending Phase 3 completion.

All three GLP-1 class research compounds are available through Bioactive Compounds. Retatrutide is available as a 30mg lyophilized vial from £120, or as the pre-filled Go-RETA pen at £140. Semaglutide (5mg vial) is listed from £90 and Tirzepatide (10mg vial) from £120 — both provisional pending final catalogue confirmation. There is no online checkout: every order is confirmed and quoted directly via WhatsApp, where our team can advise on BAC water volumes and reconstitution for your specific research protocol.