The first month on Mounjaro is mostly about letting your body get used to the medicine — not about dramatic weight loss. Knowing that up front helps set realistic expectations. This is general information, not medical advice; always follow your own prescriber's instructions.
The starting dose is a 'settling-in' dose
Everyone starts on 2.5mg once a week for the first four weeks. This is a starting dose intended to help your body adjust and reduce stomach-related side effects — it isn't the dose that drives most of the weight loss. Tirzepatide is long-acting and takes roughly four weeks to build up to a steady level in your body.
After four weeks, the dose usually steps up to 5mg, and can then increase by 2.5mg at a time (no sooner than every four weeks) up to a maximum of 15mg. There's no race to the top — the right dose is the lowest one that works well for you, agreed with your prescriber. You can map the steps with the titration planner.
Side effects in week one and beyond
Side effects are usually most noticeable in the first few days after your first injection and after each later dose increase, then tend to ease as your body adjusts. The most common are digestive — nausea, diarrhoea, constipation, vomiting and tummy discomfort. Our guide on how long Mounjaro side effects last goes into the timeline and when to seek help.
Appetite changes
Many people notice reduced appetite and feeling full sooner during the first weeks, because the medicine slows stomach emptying and acts on appetite signals in the brain. How quickly and how strongly this happens varies a lot from person to person — some feel it early, others mainly once the dose increases.
Weight-loss expectations
Because month one is a low starter dose, weight changes in the first few weeks are often modest, and that's expected. Meaningful weight loss typically builds over the following months as the dose increases and appetite changes take hold. UK guidance reflects this: treatment is reviewed at around six months, with at least 5% weight loss used as a marker of whether it's working.
Results vary from person to person, and these are general patterns rather than a guarantee. Diet, activity and the support around the medicine all matter — discuss your own goals and progress with your prescriber.
A few practical reminders
- Pick a fixed weekly day and rotate your injection site — see the guide on injection sites.
- If you use the contraceptive pill, the MHRA advises a barrier method for the first 4 weeks and after each dose increase — see Mounjaro and contraception.
- Miss a dose? Take it within 4 days, otherwise skip it — see what to do if you miss a dose.