GLP-1 and metabolic medicine

Mounjaro

Also called: tirzepatide injection, Mounjaro pen, Zepbound

Mounjaro is the brand name Eli Lilly uses for tirzepatide injections, taken once a week. It is a dual agonist that activates both GLP-1 and GIP receptors. In the UK it is licensed for type 2 diabetes and, separately, for weight management. In trials it produces slightly more weight loss than semaglutide (Wegovy), with similar gut side effects.

What Mounjaro is

Mounjaro is tirzepatide, a single molecule that binds to two different gut hormone receptors at once: GLP-1 (the same target as semaglutide) and GIP. Both hormones are released by the small intestine after meals. Activating both is thought to amplify the effects on insulin release, gastric emptying, and appetite. In the US the same drug is sold for weight loss under the brand Zepbound.

How it compares to Wegovy

  • Average weight loss in the SURMOUNT-1 trial: 22.5 percent at 72 weeks on the 15 mg dose (NEJM headline; NICE cites 20.9 percent under a stricter treatment-regimen estimand). The 2025 SURMOUNT-5 head-to-head trial showed tirzepatide 20.2 percent vs semaglutide 13.7 percent at 72 weeks.
  • Side effect profile is similar: nausea, constipation, reflux, fatigue.
  • Dose schedule is similar: a slow titration over 4 to 5 months.
  • Both are weekly injections. The pens look almost identical.
  • In the UK, NICE recommends Mounjaro for weight management in eligible adults via specialist services.

The dose ladder

  1. Week 1 to 4: 2.5 mg weekly. Starter dose, tolerance only.
  2. Week 5 to 8: 5 mg weekly. First meaningful dose.
  3. Week 9 to 12: 7.5 mg weekly.
  4. Week 13 to 16: 10 mg weekly.
  5. Week 17 to 20: 12.5 mg weekly.
  6. Week 21 onwards: 15 mg weekly maintenance dose.

Side effects

  • Nausea, peaking after each dose increase and usually settling in 1 to 2 weeks.
  • Constipation. Slowed gut transit is part of the mechanism. SURMOUNT-1 reported around 12 to 17 percent of users across the dose range. Hydration and fibre matter.
  • Diarrhoea in a minority, usually early on.
  • Reflux from food sitting longer in the stomach.
  • Injection-site reactions: small bumps or itching, almost always self-resolving.
  • Rare but serious: pancreatitis, gallstones, severe gastroparesis. Persistent severe abdominal pain warrants stopping.

What is still unknown

Long-term cardiovascular outcomes for tirzepatide are still being studied (SURPASS-CVOT). Long-term safety in people without diabetes who take it for weight management is less well characterised than for semaglutide. Both drugs are extremely new in the population history of medication, which is worth keeping in mind when reading anecdote-driven press coverage.

Common questions

Mounjaro vs Ozempic: which is better?
Different molecules. Tirzepatide (Mounjaro) tends to produce slightly more weight loss in head-to-head trials. Semaglutide (Ozempic, Wegovy) has more years of post-launch data. The right choice depends on what the drug is being prescribed for, supply availability in the UK, and how each person tolerates side effects.
Will Mounjaro be on the NHS?
Yes, in principle. NICE TA1026 recommends tirzepatide for adults with BMI 35 or over plus a weight-related health condition, accessed through specialist Tier 3 services. Rollout is being phased in over several years because of capacity, supply and cost constraints.
Why does Mounjaro cause constipation?
GLP-1 and GIP both slow gut motility. That keeps food in the stomach and small intestine longer, which improves blood sugar but dries out stools by the time they reach the colon. Drinking more water, walking after meals, and eating more soluble fibre (oats, chia, kiwi) usually helps.
Can I switch from Wegovy to Mounjaro?
Yes, with medical supervision. Most clinicians use a short washout window (usually a few days), restart at a low dose of the new drug, and titrate up. Direct dose conversions do not exist because the molecules are different. Switching for non-response is more common than switching for tolerance.

Sources