Mounjaro Week 4: What to Expect (October 2026)

01 · The core answer

What Mounjaro week 4 usually looks like

Week 4 on Mounjaro is the last stretch of the starting dose, and for most people it feels steadier than the first injections. Each pen contains four weekly doses, so your first pen is nearly finished. The 2.5mg starting weeks are about adjustment, not about maximum results.²

The 2.5mg starting weeks are about adjustment, and experiences differ widely: some people notice appetite change and early loss in week one, others feel little until a higher dose — both are normal.² So if a friend has lost a lot and you have not, that does not mean something is wrong.

Early loss also includes water weight, so the trend over weeks matters more than any single weigh-in. Weigh at the same time of day, once a week at most, and look at the direction over several weeks. If you want a deeper walk-through, our guide to what to expect in the fourth week goes day by day.

02 · Your decision

The decision you face this week: step up, hold or pause

The question at week 4 is whether you step up to 5mg when your next pen arrives. If the starting dose has been tolerable, most people move up, because most measurable progress builds from 5mg onward.² If you found 2.5mg hard going, tell your prescriber. If a new dose is hard to tolerate, prescribers can hold the current dose longer before stepping up.²

Repeat orders are reviewed rather than auto-shipped, so your suitability and side effects are checked before the next pen is dispensed. The 5mg pen is £218.99 at the standard price; every later order is £25 off, and a new customer's first order is £40 off with code FIRST40. Full dose-by-dose costs are on the Mounjaro pen page, and our explainer on what Mounjaro costs per week helps you plan. For wider support around diet and activity, see our weight loss clinic.

03 · Side effects at week 4

Mounjaro week 4 side effects

Side effects at this point are usually the same digestive ones as before, and most are easing. Like all medicines, Mounjaro can cause side effects, and most are digestive.¹ The commonest are nausea, diarrhoea, vomiting, constipation and abdominal discomfort. Common effects include indigestion, reduced appetite, dizziness, reflux, burping, hair shedding and injection-site reactions.¹

Most side effects follow a pattern: they appear in the first days after starting or increasing a dose, peak early, and settle as your body adapts.¹ That is one reason Mounjaro uses a gradual dose ladder. Expect effects to be most noticeable in the days after each injection, then fade before the next.

One thing to plan for: when you step up to 5mg, some effects can return briefly, because they are most likely when moving up a dose. That is not a sign the medicine has stopped working or that you did anything wrong. It is the same adjustment, starting again at a higher level.

If you have a new symptom that is not on the Mounjaro patient leaflet list, tell your prescriber about it and consider reporting it through the Yellow Card scheme.¹⁹

04 · Diarrhoea and constipation

Mounjaro diarrhoea in week 4

Diarrhoea is one of the commonest effects and usually eases over time, though it can flare after an injection. In the trials supporting Mounjaro's weight-management licence, diarrhoea was reported by 18.7% of people on 5mg, 20.8% on 10mg and 22.5% on 15mg, compared with 7.8% on placebo.¹ Nausea, vomiting and diarrhoea were more common while the dose was being increased and became less frequent over time.¹

Practical steps that help most people: eat smaller meals slowly, stop at the first feeling of fullness, favour plain lower-fat foods while adjusting, stay well hydrated, and avoid lying down straight after eating.¹ If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team — dehydration needs prompt attention.¹ If loose stools are lasting, our page on diarrhoea lasting a week on Mounjaro covers what to do.

The opposite problem is just as common. Constipation is very common on Mounjaro and may affect more than 1 in 10 people, because food moves through the gut more slowly and many people eat and drink less.¹ Fluids come first, then fibre from vegetables and whole grains, and daily movement. If it persists, a pharmacist can recommend a suitable laxative. Contact your clinical team for pain, bloating or more than a few days without a bowel movement.

05 · Hunger and appetite

Feeling hungry on Mounjaro in week 4

Some hunger at week 4 is normal and does not mean the medicine has failed. Tirzepatide activates two gut-hormone receptors, GIP and GLP-1, which help regulate appetite and slow how quickly the stomach empties.¹ In practice, most people feel fuller sooner and less hungry between meals, but the strength of the effect differs from person to person and from day to day.

Starting-dose experiences differ widely. Some people notice a big appetite change straight away, others feel little until a higher dose, and both are normal.² Slower early progress does not predict failure, because the dose you respond to may still be ahead of you.²

If you are hungry between meals, build each meal around protein and vegetables, and choose small, planned snacks rather than grazing. Good options are plain yoghurt, boiled eggs, a handful of fruit or raw vegetables with a dip. Keep portions small and eat slowly, stopping at the first feeling of fullness. Rich, fried or very sugary snacks tend to sit badly while your stomach is adjusting.

Do not respond to hunger by changing your dose or injection timing yourself. Doses should only be changed by your prescriber, and our guide to no weight loss at this stage covers what to review first.

06 · Weight loss at this stage

How much weight loss to expect by week 4

Expect modest loss, and be wary of any promise of dramatic early results. Weight loss on Mounjaro is gradual by design. The first weeks on 2.5mg are an adjustment phase and losses are usually modest; most measurable progress builds from 5mg onward, month by month.²

The headline trial figures come from much later. In SURMOUNT-1 of 2,539 adults, people taking tirzepatide alongside diet and exercise lost on average 15% of their body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg over 72 weeks, compared with 3.1% with lifestyle changes alone.² These are trial averages, individual results vary, and treatment works alongside a reduced-calorie diet and increased physical activity, not instead of them.²

So is it realistic to lose a large amount in the first month? Not on the starting dose. Trials measured results at 72 weeks, so think in months, not days, and judge progress by trend, not individual weigh-ins.² Three months in, most people are still on the lower rungs of the dose ladder, and a sensible expectation on a standard titration is mid single digits as a percentage of body weight, with wide individual variation.²

Your prescriber reviews response over time. If you have lost less than 5% of your starting weight after 6 months on the highest dose you can tolerate, your prescriber will discuss whether continuing or stopping is right for you.² Week 4 is far too early for that judgement.

07 · Tiredness, hair and skin

Tiredness, injection sites and other early effects

Fatigue is a recognised common side effect, most noticeable in the first weeks and after dose increases. It usually reflects a mix of the body adjusting and simply eating less, so keep meals balanced (protein especially), stay hydrated, and keep gentle activity going.¹ For most people energy returns to normal within a few weeks. Tell your clinical team if tiredness is severe or not settling.

Injection-site reactions are common: mild redness, itching, a small bruise or a tender lump normally settles within a few days. Rotating sites each week and letting the pen warm briefly out of the fridge reduces stinging. Contact your clinical team if a reaction is spreading, hot, very painful, or lasts more than a week.¹

Temporary hair shedding is a recognised side effect, linked to rapid weight loss itself rather than a direct drug effect. It is unlikely to be your main concern this early, but protein intake and overall nutrition matter, which is another reason the reduced-calorie diet should stay balanced.¹

08 · Taking a break

Can you take a break from Mounjaro?

If you are thinking about a pause at this point, speak to your prescriber first rather than simply skipping doses. A break is a clinical decision, and the right answer depends on why you want it, for example side effects, illness or travel. Our page on taking a break from Mounjaro goes through the options.

What happens in your body is worth knowing. Tirzepatide has a long half-life, so after a final dose it takes roughly four to five weeks to clear the system. There is no way to speed this up, and nothing needs doing while it clears.

If the real issue is that a dose feels too much, the answer is usually not a long break. Prescribers can hold the current dose for longer before stepping up, and that is often a better route than stopping.²

09 · What to avoid and combine

What to avoid and which medicines to flag

Food-wise, the main thing to avoid while your body adjusts is anything heavy. Favour plain lower-fat foods, avoid very large portions, and avoid lying down straight after eating.¹ Dehydration is the practical risk to watch, so keep drinking water through the day.

On medicines, tell your prescriber about everything you take. If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion); your prescriber may need to adjust those medicines.¹ Mounjaro slows stomach emptying, which can reduce how well oral contraceptive pills are absorbed.⁷ The SmPC advises switching to a non-oral method, or adding a barrier method (such as condoms), for 4 weeks after starting Mounjaro and for 4 weeks after each dose increase.¹

Never use Mounjaro from a seller offering to supply without a consultation or prescription. Mounjaro is a prescription-only medicine, supplied only after an online consultation reviewed by a GPhC-registered prescriber, and counterfeit alerts are published by the MHRA.⁷

10 · When to get help

When to get help during week 4

Serious side effects are uncommon but need urgent attention. Seek immediate medical help for severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis).¹ Seek immediate medical help for pain in the upper right abdomen, fever or yellowing of skin or eyes (possible gallbladder problems), or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing.¹

Any big change in weight, appetite and routine can affect how you feel. Regulators reviewed whether GLP-1 medicines are linked to low mood or suicidal thoughts and concluded the evidence does not support a causal link, but tell your prescriber about any history of depression, and seek help promptly if your mood drops or you have distressing thoughts during treatment.¹⁷

If you have thoughts of harming yourself or ending your life, get help straight away. Call 999 or go to A&E if you are in immediate danger. In England you can call NHS 111 and choose the mental health option, any time. You can also call Samaritans free on 116 123, day or night. Tell your prescriber about any change in your mood while taking Mounjaro.

For anything else, our team reviews side effects as part of ongoing monthly reviews, and support is available Monday to Friday, 9am to 5pm.