Week four on Mounjaro is the end of the first pen for most people.
For people following the standard dosing schedule, the fourth injection completes the initial 4-week period at 2.5 mg , and this is the point where a prescriber reviews your progress and decides whether you stay on the same dose or move up.
For adults in the UK using or considering Mounjaro for weight management, it is a genuine milestone. Your first pen is finished, you have a month of experience behind you.Some early side effects may have started to settle by this point and it is the natural point to take stock rather than compare your results to strangers on the internet.
This guide explains what tends to happen across the first four weeks, what to expect by week 4 on Mounjaro, how long does Mounjaro take to start showing appetite and weight changes, what early side effects are typical, how to judge progress beyond the scale, what the transition out of the first pen involves, what to discuss at your prescriber review, and how Mounjaro works at a high level. Mounjaro contains the active ingredient tirzepatide, a dual glucose-dependent insulinotropic polypeptide and GLP-1 receptor agonist, so the treatment acts on GIP and GLP-1 pathways that help regulate appetite, food intake, gastric emptying and blood sugar.
Mounjaro is a prescription-only medicine and should be used under the supervision of an appropriately qualified prescriber.
Where You Are at Week 4
Mounjaro is injected once weekly, and each pen contains four doses. Week four therefore means your fourth injection and the last dose in your first pen.
| Point | What it means |
|---|---|
| Injections given | Four |
| Time on treatment | One month |
| Pen status | First pen complete |
| Strength | 2.5 mg during the initial 4-week period |
| Next step | Follow the prescribed dosing plan and review when appropriate |
You have been at a single strength for the whole month, which is deliberate.The standard schedule is 2.5 mg once weekly for 4 weeks, then 5 mg once weekly. If needed, the dose can subsequently be increased by 2.5 mg increments after at least 4 weeks at the current dose. The maintenance doses for weight management are 5 mg, 10 mg or 15 mg.Further increases may be considered in 2.5 mg steps, depending on response and tolerability.
What the First Four Weeks Typically Involve
Individual experiences differ considerably, but a broad pattern is commonly reported.
Week 1. During the first week, some people notice changes in appetite or gastrointestinal side effects such as nausea, diarrhoea or constipation. Others notice little change initially. Individual responses vary
Week 2. Some early side effects may begin to settle as the body adjusts. Appetite changes tend to feel more established, although this varies between individuals.
Week 3. You may start to become more familiar with how your body responds to treatment.
Week 4. You may have a clearer idea of how your appetite and side effects are responding by this point.
Some people feel very little in the first month, and that does not mean treatment has failed. Others get significant side effects that settle, and that does not mean they cannot continue.
How Much Weight Do People Lose in 4 Weeks?
There is no reliable amount of weight that everyone should expect to lose in the first 4 weeks. Weight change varies depending on factors such as starting weight, food intake, activity, fluid balance and individual response to treatment.
Early weight change varies widely between individuals, and the reasons are worth understanding because they explain why comparison is unhelpful:
Starting weight matters. Larger starting weights tend to show larger early absolute changes. Two people responding equally well can see very different numbers.
Water weight distorts early readings. Changes in glycogen stores and fluid balance affect the first weeks substantially. Some early loss is water, and some people see almost nothing initially despite fat loss occurring.
The starting strength is the lowest. You are on the smallest dose, so appetite effects are at their most modest.
Individual response varies genuinely. Some people respond strongly to tirzepatide, some moderately, some not enough to continue. This is biology, not effort.
Everything else still applies. Diet, activity, sleep, stress and other medications all influence what happens.
What matters more than the four-week number is the direction of travel over months, which is how clinical response is assessed. A slower start does not necessarily mean the treatment will be ineffective. A slow first month followed by steady progress is a common and perfectly good pattern.
Longer clinical trials have shown substantial average weight loss with tirzepatide, although results vary by dose and individual response
Better Things to Track Than the Scale
If your appetite has reduced but your weight has not changed much yet, this does not necessarily mean treatment will not work. Discuss your progress with your prescriber
- Appetite change. Is hunger genuinely reduced compared with a month ago?
- Portion sizes. Are smaller meals easier to manage because digestion slows and you stay full for longer?
- Food noise. Is constant thinking about food quieter?
- Measurements. Waist circumference often changes when scale weight is static.
- How clothes fit. Frequently noticed before the scale moves.
- Energy and sleep. Both worth noting for the review conversation.
- Side effect trajectory. Improving, stable, or worsening.
If appetite has genuinely reduced but the scale has not moved much, treatment is doing what it does. The next step is using that shift to build healthy habits that support lasting weight loss.
What Happens When the First Pen Runs Out
This is the practical question week four actually raises, and it catches people out.
Your prescriber reviews how you have responded and tolerated the starting strength as part of your treatment plan, and decides one of the following:
Continue at the same strength. Common if side effects are still settling, or if response is good and there is no reason to increase. Some people stay on lower doses for longer if they are doing well or still need time to settle.
Increase to the next strength. Considered if tolerance has been good. Not automatic, and not something to assume. If needed, this kind of dose adjustment can include moving toward higher doses, and most patients reach maintenance doses of 5, 10, or 15 mg only if that fits their response and tolerance.
Pause or stop. Continue, increase or stop treatment depending on your response, tolerability and clinical assessment.
Your prescriber determines the appropriate strength and timing. If you miss a dose, it should usually be taken within four days, otherwise wait for the next dose. Details of the strengths available are on the Mounjaro treatment page.
If You Move Up a Strength
Worth knowing in advance: early side effects can return temporarily after an increase, in much the same pattern as the first weeks. Some gastrointestinal side effects can return temporarily after a dose increase and may settle as your body adjusts.
This is expected rather than a sign something is wrong, though significant or persistent effects should be raised with your prescriber.
On Comparing Yourself to Others
Week four is when people start searching for other people's results, and it is worth being direct about why that is a bad idea.
Before-and-after posts and one-month success stories are self-selected. People who lose a lot quickly post about it. People with modest early progress generally do not. What you see is not a representative sample, and it is not a benchmark.
They also tell you nothing about the variables that matter: starting weight, strength, diet and exercise, physical activity, other conditions, other medicines, whether someone followed an exercise routine, made lifestyle changes, or whether the person continued at all. In practice, medication and lifestyle changes work better together.Clinical trials have shown substantial average weight loss with tirzepatide when used alongside diet and physical activity, although individual results vary
The useful comparison is you now versus you a month ago. If that comparison is disappointing, the conversation to have is with your prescriber, who can look at your actual situation.
Things Worth Knowing
- Week 4 completes the first pen, the fourth injection is the last at your starting strength.
- Early weight change varies considerably between individuals, so there is no fixed amount you should expect to lose by week 4.
- Some early loss is water, and static weight does not mean nothing is happening on your weight loss journey.
- Appetite change is a more useful signal than the scale at this stage. Changes in appetite, hunger and portion sizes can be useful signs to discuss with your prescriber.
- A dose increase is a clinical decision, not a default.
- Side effects can return briefly after an increase.
If you are still comparing options, other prescription weight loss treatments are covered separately.