Tirzepatide

Tirzepatide is a synthetic 39-amino-acid peptide that acts as a dual agonist at both the glucose-dependent insulinotropic polypeptide (GIP) receptor and the glucagon-like peptide-1 (GLP-1) receptor. It received FDA approval as Mounjaro for type 2 diabetes in 2022 and as Zepbound for chronic weight management in 2023. It is the first approved dual incretin agonist, and its efficacy has reset the benchmark for metabolic pharmacotherapy — head-to-head trials have shown superiority over semaglutide on both glycaemic and weight endpoints.

The mechanism differs from GLP-1-only drugs in ways that matter clinically. Tirzepatide binds the native GIP receptor with affinity comparable to endogenous GIP, but binds the GLP-1 receptor with roughly 5x lower affinity than GLP-1 itself. The resulting signalling is "imbalanced" — more GIP-like — and on the GLP-1 side it shows biased signalling that prioritises cAMP production over β-arrestin recruitment, which reduces receptor internalisation and sustains downstream effect. The GIP component adds adipose-tissue and energy-expenditure effects that GLP-1-only agonists do not produce, while the GLP-1 arm still contributes satiety, gastric-emptying delay, glucose-dependent insulin secretion and glucagon suppression. A C20 fatty diacid side chain binds albumin and extends half-life to roughly 5 days, supporting weekly subcutaneous dosing.

Clinical evidence across the SURPASS (diabetes) and SURMOUNT (obesity) programmes is consistent and dose-dependent. SURPASS trials showed HbA1c reductions of 1.8-2.4% and weight loss of 7-12 kg across comparator arms. The head-to-head SURPASS-2 study at 40 weeks demonstrated tirzepatide superiority over semaglutide 1 mg (HbA1c -2.3% vs -1.9%; weight loss -11.2 kg vs -7.7 kg). SURMOUNT-1, the pivotal obesity trial, reported mean weight reductions of 15-22.5% at 72 weeks — 20-22% at the 15 mg dose — with 91% of the active arm achieving at least 5% weight loss compared with 14% on placebo. SURMOUNT-OSA (2024) showed 25-28% reductions in apnea-hypopnea index in obese obstructive sleep apnea patients at the 15 mg dose, supporting a subsequent regulatory pathway for OSA as an additional indication. SURMOUNT-MMO is the ongoing cardiovascular outcomes trial; interim 2025 data suggest a >20% MACE risk reduction signal in high-risk type 2 diabetes and obesity cohorts, with full results pending. Extension data to 104 weeks continue to show sustained 18-23% weight loss in responders.

Typical dosing follows a standard titration. Both Mounjaro and Zepbound start at 2.5 mg once weekly subcutaneously, with 4-week intervals between escalations: 5 mg, 7.5 mg, 10 mg, 12.5 mg, and 15 mg maximum. Injections are delivered via prefilled auto-injector pens at any time of day, with or without meals. The 15 mg maximum applies to both diabetes and obesity indications. The FDA declared the tirzepatide shortage resolved in February 2025, ending the 503B enforcement discretion that had allowed bulk compounding. As of 2026, 503A pharmacies may still compound tirzepatide for specific patient-identified need under valid prescription, but mass-produced compounded tirzepatide sits outside the current enforcement envelope. Microdosing protocols (1-4 mg) for tolerance management have been explored in off-label 2026 contexts, without trial-derived validation.

Safety considerations mirror the GLP-1 class with GIP-specific additions. The FDA black box warning for thyroid C-cell tumours applies — tirzepatide is contraindicated in patients with personal or family history of medullary thyroid carcinoma or MEN-2. Gastrointestinal adverse effects are the most common tolerability issue: nausea in 20-30% of users, vomiting, diarrhoea, with up to 10% of patients discontinuing during titration. These peak during dose escalation and resolve in about 80% of patients who persist. Gallbladder disease (cholecystitis at 1-2% above baseline), acute pancreatitis (<1% incidence in SURMOUNT/SURPASS), and dehydration-linked acute kidney injury are documented lower-frequency events. Hypoglycaemia is a concern when tirzepatide is combined with insulin or sulfonylureas — those patients require dose adjustment of the concomitant agent. 2024-2025 pharmacovigilance identified a rare ileus signal (FDA alert 2024) and an NAION (optic neuropathy) signal with odds ratios around 7.6 in post-marketing analyses, though absolute incidence remains very low.

Relative positioning against semaglutide is the most common question, and the trial data are unusually direct. In SURMOUNT-1 vs STEP-1, tirzepatide produced roughly 5-7% greater mean weight loss at comparable timepoints. In SURPASS-2 head-to-head, tirzepatide beat semaglutide 1 mg on HbA1c and weight by 4 kg. Gastrointestinal tolerability is broadly similar between the two, though individual experience varies. For obese non-diabetics specifically, the data favour tirzepatide.

Costs in 2026 track close to semaglutide. Brand Zepbound or Mounjaro runs $1,000-1,300/month cash, with manufacturer savings cards reducing out-of-pocket to $25-$550 for eligible patients depending on insurance status. 503A compounded tirzepatide is available in the $200-500/month range with prescription, at the cost of the usual compounding variability. Generic tirzepatide is not expected until patent expiry around 2036. As with semaglutide, the duration-of-use question looms over the pricing question — weight regain patterns after discontinuation mirror semaglutide's, and continuity of therapy (or robust lifestyle replacement) is the determinant of long-term outcome.

Long-term outcomes data beyond the initial 72-week SURMOUNT endpoints remains an open question, with real-world persistence rates and post-discontinuation trajectories only beginning to accumulate.