Tirzepatide

Tirzepatide is built on the GIP backbone with an Aib substitution and a C20 fatty-diacid chain, and it is deliberately imbalanced - roughly five-fold more potent at GIPR than GLPR relative to native hormones. GLP-1 agonism supplies the familiar satiety, delayed gastric emptying and glucose-dependent insulin release. The GIP arm adds improved adipocyte lipid buffering and insulin sensitivity, and central GIPR signalling in the area postrema appears to blunt the nausea that limits pure GLP-1 dosing, which is why higher effective exposures are tolerable. Whether the GIP contribution is agonism or functional receptor desensitisation is still debated in the literature.

Targets: GIP receptor, GLP-1 receptor

• · 2500 mcg weekly for 4 weeks, then 5000 mcg for 4 weeks, then increase by 2500 mcg every 4 weeks as needed to a maximum of 15000 mcg. Many people get most of the benefit at 5000-10000 mcg and never need the top dose.
• · Maintenance doses of 5, 10 or 15 mg; HbA1c reductions of 2.0-2.4% are typical.

Work out your exact syringe units →

• very commonNausea— Reported by roughly a quarter to a third of users, concentrated around dose increases.
• very commonDiarrhoea— Slightly more common than with semaglutide.
• very commonLoss of lean mass— Same problem as all incretins - protect it with resistance training and high protein, not hope.
• commonConstipation
• commonInjection-site reaction— More frequent with grey-market reconstitutions than with pens.
• uncommonGallbladder disease— Rate rises with the speed of weight loss.
• uncommonHypoglycaemia when combined with insulin or sulfonylureas— Tirzepatide alone almost never causes it.
• uncommonDelayed gastric emptying and aspiration risk under anaesthesia— Same class risk as semaglutide. Tell any anaesthetist or endoscopist you are on it; most centres want weekly GLP-1 agonists held for about a week before an elective procedure.
• rarePancreatitis— Rare but real; severe epigastric pain means stop.

• Personal or family history of medullary thyroid carcinoma or MEN2 - boxed warning.
• History of pancreatitis.
• Pregnancy; tirzepatide also reduces the effectiveness of oral contraceptives, so switch to a non-oral method for four weeks after starting and after each dose increase.
• Severe gastroparesis.

• redundantsemaglutide — Overlapping GLP-1 agonism; no reason to run both.
• cautioninsulin-analogues — Cut mealtime insulin substantially at initiation to avoid hypoglycaemia.
• synergycagrilintide — Amylin plus incretin is the current frontier combination; formally studied for semaglutide, used anecdotally with tirzepatide.
• conflictipamorelin — Ghrelin-receptor agonists drive appetite in the opposite direction and will partly undo the point of the drug.

• · Weight and waist weekly.
• · HbA1c every 3-6 months where relevant.
• · Body composition every 3-4 months.
• · Resting heart rate and blood pressure.
• · Contraception method if you are on the pill and could become pregnant.

FDA- and EMA-approved as Mounjaro (diabetes) and Zepbound (obesity). Grey-market 'research' tirzepatide is sold widely and is not legal for human use.

• Jastreboff et al. 2022, SURMOUNT-1, NEJM - 20.9% weight loss at 72 weeks on 15 mg (trial)
• Aronne et al. 2024, SURMOUNT-4, JAMA - regain after withdrawal (trial)
• Aronne et al. 2025, SURMOUNT-5, NEJM - head-to-head vs semaglutide (trial)
• Zepbound and Mounjaro US prescribing information (label)

Tirzepatide is not a GLP-1 drug with GIP bolted on; it is a GIP peptide engineered to also hit GLP1R, and the asymmetry matters. Relative to the native hormones it is roughly balanced at GIPR but substantially weaker at GLP1R, and it is strongly cAMP-biased at GLP1R with markedly reduced beta-arrestin recruitment. Reduced beta-arrestin means less receptor internalisation and desensitisation, so a molecule that binds GLP1R less tightly can still deliver more sustained downstream signal than a stronger, more balanced agonist - this is the pharmacological trick, and it is why potency measured at the binding site badly predicts clinical effect here. The GIP arm does three things that the GLP-1 arm does not. In white adipose tissue GIPR activation increases blood flow and lipoprotein lipase activity, improving the tissue's ability to buffer a postprandial lipid load rather than spilling it into liver and muscle - this is the most credible explanation for why tirzepatide improves insulin sensitivity more than weight loss alone predicts. In the central nervous system GIPR is expressed in the area postrema and hypothalamus, and central GIPR agonism reduces emesis in animal models, which is the leading explanation for why tirzepatide is better tolerated than a GLP-1 agonist of equivalent efficacy. And GIPR signalling in bone and in the vasculature produces effects nobody has fully characterised yet. There is a genuine open controversy here worth stating plainly: GIPR antagonism also produces weight loss in humans (that is exactly what MariTide does), so the field does not agree on whether tirzepatide's GIP arm works through sustained agonism or through functional receptor desensitisation that ends up looking like antagonism. Both camps have data. Nobody has resolved it.

The characteristic tirzepatide failure is the 12.5 and 15 mg steps. Tolerability is genuinely good up to 10 mg and then falls off a cliff for a lot of people, and because the drug is so effective they keep climbing anyway. Second, insulin-treated users who do not cut their prandial insulin at initiation - a 2.0-2.4% HbA1c reduction arriving over eight weeks will cause hypoglycaemia if the background regimen is untouched. Third, sulfur burps, reflux and constipation, which are mundane and are nonetheless the reason a lot of people quit. Fourth, the lean-mass problem is larger than with semaglutide purely because the total weight loss is larger. Fifth, the oral contraceptive interaction produces unplanned pregnancies and it is written on the label precisely because it happened. And sixth, grey-market reconstitution error: a 30 mg vial in 1.5 mL versus 1 mL versus 2 mL gives 1000, 1500 or 750 mcg per 5 units, and people who copy someone else's unit count from a forum without recomputing for their own vial routinely take double or half what they intend.

1. 2.5 mgWeeks 1-4 — Starting dose, not a therapeutic one. Its job is to let the gut adapt.
2. 5 mgWeeks 5-8 — The first properly effective dose. A large fraction of people would get most of their result by simply staying here for a year.
3. 7.5 mgWeeks 9-12 — Optional step. Only climb if weight loss has genuinely stalled for four weeks and side effects are mild.
4. 10 mgWeeks 13-16 — The common practical ceiling. SURMOUNT-1 gave 19.5% at this dose versus 20.9% at 15 mg - a small return for the extra nausea.
5. 12.5 mgWeeks 17-20 — Where most people hit their gastrointestinal limit.
6. 15 mgWeek 21 onward — Maximum approved dose. Diminishing returns are real above 10 mg.

• GIP receptor (GIPR) — Comparable to native GIP; the frequently quoted Kd is around 0.1-0.2 nM from Lilly's discovery work, not re-verified here

  Enhanced glucose-dependent insulin secretion, improved adipose lipid buffering and insulin sensitivity, and central signalling in the area postrema that appears to blunt nausea.

• GLP-1 receptor (GLP1R) — Roughly five-fold weaker than native GLP-1 at the binding site, but cAMP-biased with reduced beta-arrestin recruitment

  Satiety, delayed gastric emptying, glucagon suppression and glucose-dependent insulin release - the same clinical picture as semaglutide, produced by a weaker binder with better signalling persistence.

• Albumin — 99% bound

  C20 di-acid depot binding, giving the 5-day half-life.

• SURMOUNT-1 Phase 3 · n=2539 · 72 weeks · 2022

  Mean weight reduction of 15.0%, 19.5% and 20.9% at 5, 10 and 15 mg weekly versus 3.1% for placebo.

• SURMOUNT-4 Phase 3 withdrawal · n=670 · 88 weeks · 2024

  Continued tirzepatide gave a further 5.5% loss while switching to placebo produced 14.0% regain - the definitive demonstration that this is maintenance therapy, not a course.

• SURMOUNT-5 Phase 3 head-to-head · n=751 · 72 weeks · 2025

  20.2% weight loss with tirzepatide versus 13.7% with semaglutide 2.4 mg.

• SYNERGY-NASH Phase 2 · n=190 · 52 weeks · 2024

  Resolution of metabolic dysfunction-associated steatohepatitis without worsening of fibrosis in a substantially higher proportion than placebo.

Days 1-4: appetite drops, often more abruptly than with semaglutide. Weeks 1-4: gastrointestinal adaptation; nausea peaks 24-72 hours after each dose step. Week 4: steady state. Weeks 4-12: the fastest phase of weight loss, and where early rapid responders separate from slow responders - a SURMOUNT-5 post hoc analysis found early rapid loss predicted the final result. Weeks 20-52: steady accrual, roughly 1-2% of body weight per month. Weeks 72-88: still falling in SURMOUNT-1 and SURMOUNT-4, which is unusual - most obesity drugs have plateaued by then. Stopping: SURMOUNT-4 showed 14% regain over 88 weeks off drug.

Cagrilintide is the only addition with a mechanistic argument that does not overlap - amylin acts through calcitonin and RAMP-based receptors, not incretin receptors, and appears to spare lean mass. It has been formally studied with semaglutide, not tirzepatide, so combining it with tirzepatide is an extrapolation people make routinely and no trial supports. Anything that is already a GLP-1 or GIP agonist is redundant by definition, including retatrutide, semaglutide and VK2735. Ghrelin-receptor agonists (ipamorelin, MK-677, hexarelin) directly oppose the drug's purpose and MK-677 additionally worsens insulin sensitivity. The useful non-peptide additions are the same as for semaglutide: heavy resistance training, 1.6-2.2 g/kg protein, creatine, magnesium, adequate fibre, and enough sodium and water to survive the gastrointestinal losses. One tirzepatide-specific interaction that people miss: it reduces the effectiveness of oral contraceptives through delayed gastric emptying, so a non-oral method is needed for four weeks after starting and after each dose increase.

Against semaglutide: tirzepatide won SURMOUNT-5 decisively (20.2% versus 13.7%) and is generally better tolerated per unit of weight lost. Semaglutide's remaining advantages are the SELECT cardiovascular outcome data in non-diabetics, a longer record, and an oral form. Against retatrutide: retatrutide's phase 3 numbers are higher, but it adds glucagon-driven tachycardia and transient hyperglycaemia and is not approved anywhere, so you are trading a licensed product with a supply chain for a research chemical. Against CagriSema: REDEFINE-4 failed to show CagriSema was non-inferior to tirzepatide, which is a quiet but important result - the amylin combination did not beat the dual agonist. Against MariTide: MariTide dosing is monthly and regain after stopping appears much slower, but it is phase 3 and unavailable.

$350–$1100/month. Manufacturer single-dose vial programmes sit around 350-500 per month for lower doses; US list price for Zepbound or Mounjaro without coverage is roughly 1,000-1,100. Grey-market research vials are far cheaper but tirzepatide is among the most counterfeited peptides on the market. Market observation from mid-2026, not verified pricing.

• ?Whether the GIP component acts as a sustained agonist or produces functional receptor desensitisation is genuinely unresolved in the literature, and the fact that GIPR antagonism also causes weight loss is the reason.
• ?Receptor affinity figures are from Lilly's published discovery work and were not independently resolved in this session.
• ?The amino-acid sequence is not reproduced here because it was not verified against a primary source; the listed modifications are from the published molecular description.
• ?Long-term GIPR agonism effects on bone metabolism are not characterised in humans.
• ?No cardiovascular outcomes trial has read out for tirzepatide in the way SELECT has for semaglutide, so cardiovascular benefit is currently inferred from risk factors rather than demonstrated on events.
• ?Cost figures are market observations, not verified pricing.

• Tirzepatide Once Weekly for the Treatment of Obesity Jastreboff AM et al., N Engl J Med, 2022 · PMID 35658024

  SURMOUNT-1, the trial that established tirzepatide as the most effective approved obesity drug.

• Continued Treatment With Tirzepatide for Maintenance of Weight Reduction in Adults With Obesity: The SURMOUNT-4 Randomized Clinical Trial Aronne LJ et al., JAMA, 2024 · PMID 38078870

  What happens when you stop. Read this before planning a cycle.

• Tirzepatide as Compared with Semaglutide for the Treatment of Obesity Aronne LJ et al., N Engl J Med, 2025 · PMID 40353578

  SURMOUNT-5. The only randomised head-to-head against semaglutide.

• Tirzepatide for Metabolic Dysfunction-Associated Steatohepatitis with Liver Fibrosis Loomba R et al., N Engl J Med, 2024 · PMID 38856224

  SYNERGY-NASH - biopsy-confirmed liver outcomes, not just imaging.

• Improved health-related quality of life with tirzepatide versus semaglutide in adults with obesity or overweight from the SURMOUNT-5 trial Shukla AP et al., Diabetes Obes Metab, 2026 · PMID 41187971

  Patient-reported outcomes from the head-to-head, which are usually where a tolerability difference actually shows up.

• MOUNJARO (tirzepatide) injection - US prescribing information, section 12.3 Eli Lilly and Company, DailyMed

  Source for 80% bioavailability, 10.3 L volume of distribution, 0.061 L/h clearance, 99% albumin binding and the 8-72 hour tmax.