Two different mechanisms

Mounjaro contains tirzepatide, which acts on GIP and GLP-1 receptors. Wegovy contains semaglutide, which acts on GLP-1 receptors. The mechanism explains the science. It cannot predict how you will respond, and it gives no basis for converting one dose into the other milligram for milligram. Both start low and build up gradually. Never begin either one at a high dose because you tolerated another medicine.

Use head-to-head evidence

SURMOUNT-5 compared tirzepatide at the maximum tolerated dose directly with semaglutide in adults with obesity and no type 2 diabetes. Tirzepatide gave greater average weight loss in that trial. A direct trial is stronger evidence than lining up separate placebo-controlled studies. Even so, some people will lose more on semaglutide, tolerate it better or prefer its pen.

Know which Wegovy dose was studied

SURMOUNT-5 tested semaglutide at 1.7 mg or 2.4 mg weekly. It did not test the later 7.2 mg weight-management option or Wegovy tablets. A headline claiming "Mounjaro beats all Wegovy" goes too far. Each formulation and dose needs its own evidence. Check the date of any comparison, because product authorisations can change after a study ends.

Goals beyond weight

UK Wegovy injection information includes specific indications for cardiovascular risk and MASH as well as weight management. Mounjaro has its own indications and evidence, including type 2 diabetes. Your choice may rest on more than average weight loss. A health outcome proven for one product does not carry over to every medicine that affects appetite or glucose.

Compare the whole experience

Talk through side effects, pregnancy and contraception, your other medicines, how confident you feel injecting and whether supply is reliable. Tirzepatide carries specific UK advice for people using oral contraception. Look at the full ongoing cost and the support included, beyond the starter price. A cheap first month is no sustainable plan if later doses or follow-up become unaffordable.

If you are thinking of switching

Ask your prescriber to explain the reason, the last dose of your current medicine, the new starting dose and the review date. Do not overlap the two medicines or use online conversion tables. Bring a record of past benefits and side effects. That record tells your clinician far more than one disappointing week on the scales. You and your clinician make the choice together, based on the evidence for you.

Changing medicine or changing provider

If you are unhappy with access, cost or support, a different service may fix the problem with no change of medicine. A good provider cannot make an unsuitable medicine right for you either. Be clear about the problem. Too little benefit, side effects you cannot live with, supply gaps and poor communication each need a different fix. Naming the real issue makes the review more useful and avoids a needless medicine change. Ask for a written clinical reason if a provider wants to switch you only because it has its preferred product in stock.

Sources

Your own medicine leaflet and prescriber come first. A source from another country does not mean the same rule applies in the UK.

  1. Mounjaro: UK prescribing information medicines.org.uk
  2. Wegovy injection: UK prescribing information medicines.org.uk
  3. SURMOUNT-5: original trial publication nejm.org

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