Mounjaro weight loss in the first month: what the trial shows

Social media is full of first-month numbers. The trial behind Mounjaro gives a more useful picture: a slow start on purpose, then steady loss over 72 weeks.

By Sian Anderson. Published 5 November 2026. 6 minute read. Information, not medical advice.

Sian Anderson, Writer and editor, MyGLP. Sources checked 5 November 2026.

The main Mounjaro trial did not publish a weight figure for the first month, so any exact number you see is not from the trial. Everyone started on 2.5mg, a starting dose, and moved up 2.5mg every 4 weeks. By week 20 the average loss on tirzepatide was 12.8kg, against 2.7kg on placebo.

That week 20 figure covers five months and all three doses together. The rest of this guide explains why the first month is slow by design, what the trial showed at 72 weeks, and how to use your first four weeks well.

Why 2.5mg is a starting dose

The Mounjaro patient leaflet says you start on 2.5mg once a week for 4 weeks, then move to 5mg. A prescriber can then raise the dose in 2.5mg steps, up to 15mg, with each dose kept for at least 4 weeks before the next rise. The 2.5mg step exists to let your body get used to the medicine. Nausea, diarrhoea and vomiting were most common while the dose was going up in the trial, so a gentle start keeps those down.

In real life the ladder is not fixed. Your prescriber decides when to move up, and many people stay on a dose for longer than 4 weeks. Do not change your dose on your own.

The dose ladder in SURMOUNT-1

SURMOUNT-1 followed 2,539 adults for 72 weeks, and 67.5% of them were women. The average starting weight was 104.8kg. Everyone began on 2.5mg and went up by 2.5mg every 4 weeks until they reached their set dose of 5mg, 10mg or 15mg. The 15mg group reached full dose by week 20.

The paper shows weight by week in a chart but prints numbers only at week 20 and week 72. Source: Jastreboff et al, NEJM 2022.
Time on treatment	Dose in the 15mg group	Average weight change reported
Month 1 (weeks 1 to 4)	2.5mg	Not published as a figure
Month 2 (weeks 5 to 8)	5mg	Not published as a figure
Month 3 (weeks 9 to 12)	7.5mg	Not published as a figure
Month 4 (weeks 13 to 16)	10mg	Not published as a figure
Month 5 (weeks 17 to 20)	12.5mg	12.8kg lost by week 20, all doses pooled (placebo 2.7kg)
Month 6 (weeks 21 to 24)	15mg	Not published as a figure
Week 72	15mg	20.9% of body weight (10mg 19.5%, 5mg 15.0%, placebo 3.1%)

We have left the blank months blank on purpose. Figures for weeks 4, 8 or 12 that you find online come from reading the chart by eye or from other studies. None of them is a promise of what you will lose.

What 72 weeks looks like

For the trial's average starting weight of 104.8kg, the average loss among people who kept taking the medicine was 16.1kg on 5mg, 22.2kg on 10mg and 23.6kg on 15mg. On placebo it was 2.4kg. Everyone also had advice on eating less and moving more.

Results at 72 weeks. Source: Jastreboff et al, NEJM 2022.
Share of people who lost	5mg	10mg	15mg	Placebo
5% or more	85.1%	88.9%	90.9%	34.5%
15% or more	48.0%	66.6%	70.6%	8.8%
20% or more	30.0%	50.1%	56.7%	3.1%
25% or more	15.3%	32.3%	36.2%	1.5%

Read the spread as well as the average. Around 1 in 10 people on the top dose lost less than 5%, and around a third lost a quarter of their weight or more. Your result depends on your dose, your body and how long you stay on treatment. By week 72, weight had levelled off in the 5mg group and was close to levelling off on the higher doses.

Why the scales can mislead you early on

The number on the scales is more than fat. Diarrhoea and vomiting in the first weeks drain fluid, and the leaflet warns about dehydration when you start. A fall on the scales in week one may be partly fluid, and a rise later may be fluid coming back.

Some of the weight lost over time is lean tissue, which includes muscle. In a group of 160 people in SURMOUNT-1 who had body scans, fat mass fell by 33.9% on tirzepatide, and fat loss was around three times the lean loss. Protein at every meal and strength exercise twice a week protect muscle. Our keeping muscle guide and protein guide show how on a small appetite.

Using the first month well

Weigh yourself once a week, on the same day and at the same time.
Eat small meals and stop when you feel full. Our nausea-friendly meals guide has ideas.
Drink plenty of fluids, as the leaflet advises, more so if you have diarrhoea or vomiting.
If you take the pill, use condoms as well for 4 weeks from your first injection, as the NHS advises. Our contraception guide has the detail.
Expect your period to shift as your weight changes. See periods on weight loss injections.
Write down side effects and when they happen, so you can tell your prescriber before the next dose rise.

Questions for your first review

Am I ready to move up to 5mg, or should I stay on 2.5mg for longer?
Which side effects should make me call you before my next injection?
How will we track progress beyond the scales, such as waist size, blood pressure or blood sugar?
Which contraception is right for me over the next few dose rises?

Weight on the scales is one measure. In SURMOUNT-1 waist size fell by 14.0cm on 5mg and 18.5cm on 15mg over 72 weeks, and systolic blood pressure fell by 7.2 mm Hg across the doses. Those changes matter for heart and metabolic health, and a tape measure tracks waist size at home.

The NHS 6 month check

If you get Mounjaro on the NHS, NICE TA1026 sets a review point. If you have lost less than 5% of your starting weight after 6 months on the highest dose you can manage, you and your clinician decide together whether to carry on. Month one is far too early to judge. Private prescribers set their own reviews.

Get urgent medical help for severe stomach pain that does not go away, which may spread to your back. The MHRA lists this as the main sign of pancreatitis. Call 111 or your prescriber for vomiting or diarrhoea you cannot keep up with.

For the full picture of benefits and NHS access, read Mounjaro for women. For each side effect with trial figures, see Mounjaro side effects in women. Whether women lose more than men is covered in do women lose more weight on Mounjaro?. Current prices for every dose are on our Mounjaro price page.

Paying for your own medicine? The lowest first-order prices today, with delivery included: Mounjaro from £89.00, Wegovy from £44.00, Wegovy pill from £52.99, Foundayo from £49.00.

Compare every price Discount codes

Sources

Mounjaro KwikPen patient information leaflet (Eli Lilly, via the electronic medicines compendium)
Tirzepatide once weekly for the treatment of obesity (SURMOUNT-1) (Jastreboff et al, New England Journal of Medicine, 2022)
Tirzepatide (NHS)
Tirzepatide for managing overweight and obesity (TA1026), recommendations (NICE)
GLP-1 receptor agonists and dual GLP-1/GIP receptor agonists: strengthened warnings on acute pancreatitis (MHRA Drug Safety Update, January 2026)

Sian Anderson

Sian writes the health and nutrition guides on MyGLP. Every clinical claim links to the NHS, NICE, a medical royal college, a regulator or a peer-reviewed paper, and the pages carry no medical advice.

Questions

How much weight can you lose on 1 month of Mounjaro?

The main trial did not publish a one-month figure. The first month is a 2.5mg starting dose meant to help your body adjust. The first published figure is at week 20, when the average loss was 12.8kg on tirzepatide against 2.7kg on placebo.

Is 2.5mg of Mounjaro enough to lose weight?

The leaflet describes 2.5mg as the starting dose for 4 weeks before moving to 5mg. In the trial, 5mg was the lowest dose people stayed on, with an average loss of 15.0% at 72 weeks. Your prescriber decides when and whether to move up.

Why am I not losing weight on Mounjaro in the first month?

Month one is a low starting dose, and the scales swing with fluid. In the trial, loss built over many months and was still going at 72 weeks on the higher doses. Talk to your prescriber before changing anything.

How much weight do people lose on Mounjaro in 6 months?

SURMOUNT-1 printed no 6-month figure. At week 20, just under 5 months, the average loss was 12.8kg on tirzepatide against 2.7kg on placebo, from an average starting weight of 104.8kg.

How much weight can you lose on Mounjaro in a year?

The trial reports at 72 weeks, about 16 to 17 months. Average loss was 15.0% of body weight on 5mg, 19.5% on 10mg and 20.9% on 15mg, against 3.1% on placebo.