Mounjaro 5mg UK: Results, Side Effects & Cost

Four weeks on the starter dose and something is meant to happen. That's the story most people arrive at Mounjaro 5mg with. The 2.5mg pen was the warm-up, and now the medicine is supposed to start working properly.

There's truth in that. The 2.5mg dose exists to let your body meet tirzepatide gently, and 5mg is where the clinical effect properly begins. But a lot of what gets said about this dose online is either too rosy or quietly wrong. Some people are told 5mg is "just another stepping stone" when it's actually a licensed dose people stay on for years. Others expect the scales to move dramatically the week after they switch, then panic when they don't.

What Mounjaro 5mg actually is

Mounjaro is the brand name for tirzepatide, a once-weekly injection that acts on two gut hormone receptors, GIP and GLP-1, rather than the single GLP-1 receptor that Wegovy targets. The result is slower stomach emptying, steadier blood sugar and a marked drop in appetite.

Everyone starts at 2.5mg. According to the Mounjaro KwikPen Summary of Product Characteristics, the starting dose runs for four weeks and is then increased to 5mg once weekly. Further increases happen in 2.5mg steps, with a minimum of four weeks at each level.

That same document also lists the recommended maintenance doses for adults as 5mg, 10mg and 15mg. Five milligrams is a destination in its own right, not merely a corridor to somewhere else. Plenty of people stay there.

Each 5mg KwikPen holds four weekly doses of 0.6ml. If you used the older pens and remember squeezing out a fifth dose, that's no longer how the current pens are designed.

How much weight do people lose on Mounjaro 5mg?

This is the question that brings most people to a search bar, and there is a good answer to it.

SURMOUNT-1 was the trial that established tirzepatide for weight management. It randomised 2,539 adults with obesity, or overweight plus a weight-related condition, to 5mg, 10mg, 15mg or placebo for 72 weeks. Results were published in the New England Journal of Medicine in 2022.

On the treatment-regimen analysis, which counts everyone regardless of whether they stopped early, Eli Lilly reported these figures at 72 weeks:

Sit with the 5mg column for a moment. Fifteen per cent of body weight. For someone starting at 100kg, that's fifteen kilos. Nearly half the people on the lowest maintenance dose lost 15% or more, and almost a third crossed 20%.

The higher doses do more. Nobody is pretending otherwise. But the gap between 5mg and 15mg is roughly six percentage points, and the side effect burden climbs alongside the dose. If 5mg is working and you feel well on it, "working" is the word that matters.

One caveat on the timeline. Seventy-two weeks is around sixteen months. Nobody in that trial saw 15% in month two. Weight loss on tirzepatide is slow and cumulative, which is why the week-by-week picture looks less dramatic than before-and-after photos suggest.

How long should you stay on 5mg?

There's no fixed answer, and anyone who gives you one is guessing. The SmPC sets a floor of four weeks before any increase. It doesn't set a ceiling.

In practice, three situations come up again and again.

You're losing weight steadily and feel well

Stay. This is the outcome you wanted. Increasing the dose because a chart says you can is how people end up nauseated for no clinical gain. Your prescriber will usually be happy to hold you at 5mg for as long as it keeps delivering.

You're tolerating it fine but the scales have gone quiet

Give it a full four to six weeks at 5mg before drawing conclusions, because appetite suppression can plateau briefly while your body adjusts. If nothing shifts after that, a step up to 7.5mg is reasonable. Talk it through with your prescriber rather than deciding alone.

Side effects are still rough at week four

Hold. There's no obligation to climb on schedule, and moving up while you're still queasy usually makes things worse. Many people spend eight or twelve weeks at 5mg before they're ready for more.

The one hard checkpoint comes at six months. Both the SmPC and NICE guidance on tirzepatide say that if you haven't lost at least 5% of your starting weight after six months on your highest tolerated dose, you and your prescriber should decide whether continuing makes sense. That's a review point, not a threat.

If you do settle at 5mg long-term, our guide to Mounjaro maintenance dosing covers what that looks like over a year or more.

Mounjaro 5mg side effects: the real numbers

Vague reassurance helps nobody, so here are the figures from the pooled SURMOUNT-1 and SURMOUNT-2 weight management trials, as listed in the SmPC.

Read those side by side and the picture is clearer than "some people get nausea". Just over half of people had some gut symptom, roughly a quarter had nausea, and fewer than two in a hundred found it bad enough to stop.

Severity matters as much as frequency. The SmPC records that gastrointestinal reactions were mild in 61.0% of cases and moderate in 34.4%. The incidence was highest during dose escalation and fell over time. So the week you move from 2.5mg to 5mg is likely to be the worst week, and it usually improves from there.

Two others come up often. Hair loss was reported by 4.9% of people on tirzepatide across the SURMOUNT trials, against 1.0% on placebo, and most people recovered while still on treatment. Acute gallbladder problems affected 2.0% on tirzepatide versus 1.6% on placebo, a small gap that tracks with the speed of weight loss rather than the drug itself.

What usually settles

Nausea, constipation, burping and a sense of fullness that arrives suspiciously early in a meal. These respond to the boring interventions: smaller portions, eating slowly, cutting back on fat and fried food, and drinking more than feels necessary. If constipation is the main problem, we've written a practical guide to fixing it.

What needs a phone call

Persistent severe abdominal pain, especially if it bores through to your back, needs same-day medical attention because it can signal pancreatitis. So does vomiting you can't keep on top of, signs of dehydration, or pain under your right ribs with yellowing skin or eyes. None of these are common. All of them are worth acting on quickly.

There's one interaction that catches people out. If you take the combined oral contraceptive pill, the SmPC advises switching to a non-oral method or adding a barrier method for four weeks when you start tirzepatide and for four weeks after every dose increase. Moving to 5mg counts as a dose increase.

You can report any suspected side effect directly to the MHRA through the Yellow Card scheme. Mounjaro still carries a black triangle, which means safety data is being actively collected.

For the full picture across every dose, see our guide to Mounjaro side effects.

2.5mg versus 5mg: what actually changes

The dose doubles. What that means in practice is less obvious than the arithmetic suggests.

At 2.5mg, most people notice a mild dip in appetite and a bit of early fullness. It's a starter dose by design, and the SmPC is explicit that it isn't intended to produce weight loss. Some people lose a few pounds anyway, mostly from eating less without trying.

At 5mg, appetite suppression becomes obvious rather than subtle. Portions shrink noticeably. Food noise quietens, sometimes to the point that people forget to eat, which brings its own problems. Weight loss becomes consistent week to week instead of occasional. And the side effects that were mild at 2.5mg tend to reassert themselves for a fortnight before settling again.

The pen itself is identical in size and mechanism. Same injection technique, same weekly rhythm, same rotation between abdomen, thigh and upper arm. Only the strength on the label changes.

If you miss a dose, the SmPC says take it within four days. Beyond that, skip it and carry on with your next scheduled injection. You can also shift your injection day if you need to, as long as at least three days separate two doses.

How much does Mounjaro 5mg cost in the UK?

Mounjaro is available on the NHS, but the access criteria are tighter than the licence. NICE recommends tirzepatide for NHS weight management only for adults with a BMI of at least 35 plus at least one weight-related condition, with the threshold reduced by around 2.5 kg/m² for people of South Asian, Chinese, other Asian, Middle Eastern, Black African or African-Caribbean background. Rollout has been staged, so many people who meet the criteria are still waiting.

Privately, almost every UK online pharmacy prices Mounjaro by dose band. The 5mg pen costs more than 2.5mg, and less than 7.5mg and above. This trips people up: the month you titrate, your bill goes up, which is worth planning for rather than discovering.

Prices move often, so rather than quoting numbers that will be stale by next month, we keep two comparisons updated: the cheapest Mounjaro options in the UK and how the main providers compare on price and care. Read both before you commit, because the headline price rarely includes consultations, needles or clinical follow-up.

A word of caution on the very cheapest end. Tirzepatide is prescription-only in the UK, and any seller offering it without a proper medical assessment is operating outside the law. Cheap becomes expensive quickly when the product is unregulated.

Getting the most out of 5mg

The medication handles appetite. Everything else is still yours to manage, and what you do in these months determines how much of the weight stays off later.

Protein first, at every meal. When you're eating substantially less, the risk isn't just fat loss, it's muscle loss alongside it. Aim for a palm-sized portion of protein before you fill the rest of the plate, and add resistance work twice a week if you can. Our guide to holding on to muscle during GLP-1 treatment goes into the detail.

Water, more than you think. Reduced appetite dulls thirst too, and mild dehydration explains a surprising share of the headaches and fatigue people blame on the drug.

Eat, even when you don't want to. Some people on 5mg drop to 800 calories a day without noticing. That feels efficient and isn't. Very low intake accelerates muscle loss, tanks energy and makes the whole thing harder to sustain. Our Mounjaro diet guide has a seven-day starter plan built around this.

And keep an eye on the appetite itself. When food noise goes quiet, the habits underneath it become much easier to change. That window is when the durable changes get made, because the medication won't rebuild those habits for you and stopping treatment tends to bring the old appetite back.

Who shouldn't take Mounjaro 5mg

The licensed indication for weight management covers adults with a BMI of 30 or above, or 27 and above with at least one weight-related condition such as high blood pressure, high cholesterol, sleep apnoea, cardiovascular disease, prediabetes or type 2 diabetes.

Tirzepatide is contraindicated if you've had a hypersensitivity reaction to it. It should be used with caution if you have a history of pancreatitis or severe gastrointestinal disease including gastroparesis, and in people with diabetic retinopathy that needs monitoring.

It should not be used in pregnancy. If you're planning to conceive, the SmPC advises stopping at least one month beforehand because tirzepatide has a long half-life. It isn't recommended for anyone of childbearing potential who isn't using contraception.

If you're being considered for NHS treatment, our summary of NHS eligibility for weight loss injections sets out how the criteria are applied in practice.

Whichever route you take, this needs a proper medical assessment. That's how you find out whether tirzepatide is safe for you specifically, given your history and whatever else you take.

This article is for informational purposes only and does not constitute medical advice. Always consult with a qualified healthcare professional before starting any treatment.