When do GLP-1 side effects peak?

The product information for both Mounjaro and Wegovy describes gastrointestinal side effects as occurring mainly during dose escalation and decreasing over time. In practice that means the days after a first dose, and the days after each dose increase, are when nausea, vomiting and diarrhoea are most commonly reported. If the dose stays the same, the pattern commonly eases over the following two to four weeks. That is a common pattern rather than a rule, and plenty of people report very little.

How common is nausea on Wegovy or Mounjaro?

In a pooled analysis of the STEP 1 to 3 trials of semaglutide 2.4 mg, nausea was reported by 43.9 percent of people on the medicine and 16.1 percent on placebo, diarrhoea by 29.7 percent versus 15.9 percent, vomiting by 24.5 percent versus 6.3 percent, and constipation by 24.2 percent versus 11.1 percent. In that analysis 98.1 percent of these events were mild to moderate and 4.3 percent of people stopped treatment permanently because of them. For tirzepatide, the pooled weight-management trials in the label report nausea in 25 to 29 percent of people depending on dose versus 8 percent on placebo, diarrhoea in 19 to 23 percent versus 8 percent, vomiting in 8 to 13 percent versus 2 percent, and constipation in 11 to 17 percent versus 5 percent.

Do GLP-1 side effects come back after every dose increase?

Commonly, yes. The product information for these medicines ties gut side effects to the dose-escalation period rather than to the medicine in general, so each step up can restart the pattern for a few days. People who had an easy first month are often surprised by this. If a dose increase is harder than the previous one, that is worth telling your prescriber before the next step is due. Whether to hold, slow down or change anything is their decision, not something to work out alone.

Which GLP-1 side effects need urgent medical help?

Severe stomach pain that will not go away, often spreading through to your back, is the warning sign for acute pancreatitis and the product information tells patients to seek immediate medical attention. So do vomiting you cannot stop or being unable to keep fluids down, signs of dehydration such as very little or very dark urine and dizziness on standing, severe pain under the ribs on the right side especially with fever or yellowing of the eyes or skin, and any swelling of the lips, face, tongue or throat or difficulty breathing. In the UK, contact your prescriber, call NHS 111 for urgent advice, or call 999 for difficulty breathing, swelling of the face or throat, collapse, or pain you cannot bear.

Why is my hair thinning on a GLP-1, and does it stop?

Hair loss is listed for both Mounjaro and Wegovy, at low single-digit percentages in the trials, and it is reported far more often by women than by men. It runs on a different clock to the gut symptoms: it is usually noticed a few months in rather than in the first weeks, and it does not restart at each dose step. The pattern described after rapid weight loss of any kind is telogen effluvium, where a batch of hairs shifts into the shedding phase at once and sheds diffusely rather than in patches. In telogen effluvium the shedding settles once the trigger settles and hair regrows over months, though that is the general description of the condition rather than a prediction about any individual. Bald patches with a clear edge, or a sore or scaly scalp, are a different pattern and need a GP to look at them.

When do GLP-1 side effects peak?

The product information for both Mounjaro and Wegovy describes gastrointestinal side effects as occurring mainly during dose escalation and decreasing over time. In practice that means the days after a first dose, and the days after each dose increase, are when nausea, vomiting and diarrhoea are most commonly reported. If the dose stays the same, the pattern commonly eases over the following two to four weeks. That is a common pattern rather than a rule, and plenty of people report very little.

How common is nausea on Wegovy or Mounjaro?

In a pooled analysis of the STEP 1 to 3 trials of semaglutide 2.4 mg, nausea was reported by 43.9 percent of people on the medicine and 16.1 percent on placebo, diarrhoea by 29.7 percent versus 15.9 percent, vomiting by 24.5 percent versus 6.3 percent, and constipation by 24.2 percent versus 11.1 percent. In that analysis 98.1 percent of these events were mild to moderate and 4.3 percent of people stopped treatment permanently because of them. For tirzepatide, the pooled weight-management trials in the label report nausea in 25 to 29 percent of people depending on dose versus 8 percent on placebo, diarrhoea in 19 to 23 percent versus 8 percent, vomiting in 8 to 13 percent versus 2 percent, and constipation in 11 to 17 percent versus 5 percent.

Do GLP-1 side effects come back after every dose increase?

Commonly, yes. The product information for these medicines ties gut side effects to the dose-escalation period rather than to the medicine in general, so each step up can restart the pattern for a few days. People who had an easy first month are often surprised by this. If a dose increase is harder than the previous one, that is worth telling your prescriber before the next step is due. Whether to hold, slow down or change anything is their decision, not something to work out alone.

Which GLP-1 side effects need urgent medical help?

Severe stomach pain that will not go away, often spreading through to your back, is the warning sign for acute pancreatitis and the product information tells patients to seek immediate medical attention. So do vomiting you cannot stop or being unable to keep fluids down, signs of dehydration such as very little or very dark urine and dizziness on standing, severe pain under the ribs on the right side especially with fever or yellowing of the eyes or skin, and any swelling of the lips, face, tongue or throat or difficulty breathing. In the UK, contact your prescriber, call NHS 111 for urgent advice, or call 999 for difficulty breathing, swelling of the face or throat, collapse, or pain you cannot bear.

Why is my hair thinning on a GLP-1, and does it stop?

Hair loss is listed for both Mounjaro and Wegovy, at low single-digit percentages in the trials, and it is reported far more often by women than by men. It runs on a different clock to the gut symptoms: it is usually noticed a few months in rather than in the first weeks, and it does not restart at each dose step. The pattern described after rapid weight loss of any kind is telogen effluvium, where a batch of hairs shifts into the shedding phase at once and sheds diffusely rather than in patches. In telogen effluvium the shedding settles once the trigger settles and hair regrows over months, though that is the general description of the condition rather than a prediction about any individual. Bald patches with a clear edge, or a sore or scaly scalp, are a different pattern and need a GP to look at them.

GLP-1 side effect timeline

Where you are in the arc, what is commonly reported at that point, and how long each symptom usually lasts. For Mounjaro and Wegovy in the UK. Free, no sign-up.

Information, not medical advice. Last reviewed 25 July 2026.

The short answer: the UK product information for Mounjaro and Wegovy says gut side effects happen mainly during dose escalation and decrease over time. So the days after your first dose, and the days after every dose increase, are when nausea, vomiting and diarrhoea are reported most. On an unchanged dose the pattern commonly eases over the next two to four weeks. Constipation, reflux and hair thinning run on their own clocks and do not follow that curve.

Stop and get urgent help for any of these

Severe stomach pain that will not go away
Vomiting you cannot stop, or nothing staying down
Signs of dehydration
Severe pain under your right ribs
Any sign of an allergic reaction
A swollen, painful belly with vomiting and no bowel movement

These are not part of the normal settling-in pattern. Contact your prescriber, or NHS 111 if you cannot reach them. Call 999 for difficulty breathing, swelling of the face or throat, collapse, or pain you cannot bear.

Which medicine

Count your weeks from

Which week are you in since your first dose?

Week 1

If you have had a dose increase since you started, the week since that increase is usually the more useful number.

What are you getting? Pick any that apply.

Where week 1 sits in the arc

Weeks 1 to 2(you are here)
Weeks 3 to 4
Weeks 5 to 8
Week 9 and beyond

Weeks 1 to 2 since your first dose, on Mounjaro

This is the part of the arc where gut symptoms are reported most often.

This is your first dose, and it is the lowest dose in the schedule. It is a starting dose rather than a treatment dose.

Mounjaro doses can step up every four weeks at the earliest, so that is roughly how often the arc can restart.

Commonly reported at this point

Nausea, usually strongest in the first two or three days after the injection and easing across the week.
Getting full much faster than you expect, and food losing its appeal partway through a meal.
Bowels changing in one direction or the other. Constipation and looser stools are both very commonly reported.
More burping than usual, and reflux if you eat a large meal or lie down soon after eating.
Feeling flat or tired, which often tracks how much less you are eating rather than the medicine on its own.

This describes what people in the trials reported, not what will happen to you. Plenty of people get very little of this, and some get a lot. Neither means anything about how well the medicine is working.

Pick a symptom above to see how long it usually lasts, what tends to help, and what is worth taking to your prescriber.

The numbers behind the arc, for Mounjaro

Percentage of people reporting each reaction over the two pooled weight-management trials, by maintenance dose. These are whole-trial figures, not week-by-week ones.

Reported	5 mg (n=630)	10 mg (n=948)	15 mg (n=941)	Placebo (n=958)
Nausea	25%	29%	28%	8%
Diarrhoea	19%	21%	23%	8%
Vomiting	8%	11%	13%	2%
Constipation	17%	14%	11%	5%
Abdominal pain	9%	9%	10%	5%
Indigestion	9%	9%	10%	4%
Injection site reactions	6%	8%	8%	2%
Fatigue	5%	6%	7%	3%
Burping	4%	5%	5%	1%
Hair loss	5%	4%	5%	1%
Reflux	4%	4%	5%	2%

The label only lists reactions reported by 2% or more of people and more often than on placebo, so anything missing from this table is either rarer than that or no more common than placebo.

Severity, and how often people stop

Across both trials, 4.8% to 6.7% of people on tirzepatide stopped treatment for any adverse reaction, against 3.4% on placebo, and the label notes most of those who stopped did so in the first few months because of gut symptoms.

Any gut side effect was reported by 56% at all three maintenance doses, against 30% on placebo. Gut symptoms led 1.9% to 4.3% across the three doses to stop, against 0.5% on placebo.

"The majority of nausea, vomiting, and/or diarrhea events occurred during dose escalation and decreased over time."

ZEPBOUND US prescribing information, section 6.1

The placebo column is there for a reason. A good chunk of what gets reported on these medicines gets reported without them too. Figures from different medicines in this tool come from different labels with different reporting cut-offs, so read each table on its own rather than treating them as a head-to-head.

How Mounjaro steps up

Starts at 2.5 mg once weekly for four weeks, then 5 mg, then steps up in 2.5 mg increments no sooner than every four weeks. The maintenance doses for weight management are 5 mg, 10 mg and 15 mg once weekly, and 15 mg is the maximum.

From tirzepatide product information. Your prescriber may use a slower schedule, and that is a clinical decision, not a deviation you need to fix.

Anything on the urgent list? Contact your prescriber, or NHS 111 if you cannot reach them. Call 999 in an emergency.

Get urgent medical help for any of these

These are not part of the normal settling-in pattern. Do not wait to see whether they pass, and do not wait for your next appointment.

Severe stomach pain that will not go away, often spreading through to your back
With or without vomiting. This is the warning sign for acute pancreatitis, and the product information for these medicines tells patients to seek immediate medical attention if it happens.
Vomiting that will not stop, or not being able to keep fluids down
If several hours have passed and nothing is staying down, get medical advice the same day rather than waiting to see if it passes.
Signs of dehydration
Passing very little urine or dark urine, dizziness when you stand, a dry mouth, or feeling confused. Dehydration from vomiting and diarrhoea can affect how your kidneys work, and older people are more vulnerable to it.
Severe pain under the ribs on your right side
Particularly with a fever, or with yellowing of the whites of your eyes or your skin. Gallstones and gallbladder inflammation are listed for all of these medicines.
Any sign of an allergic reaction
Swelling of the lips, face, tongue or throat, a spreading rash, wheezing, or any difficulty breathing or swallowing. Call 999 straight away, do not wait.
A swollen, painful belly with no bowel movement and vomiting
Bowel obstruction has been reported after these medicines were on the market. Constipation that turns into pain plus vomiting needs urgent assessment, not another laxative.

What to do

Contact your prescriber or the pharmacy that supplied your medicine.
For urgent advice when you cannot reach them, contact NHS 111.
Call 999 or go to A&E for difficulty breathing, swelling of the face or throat, collapse, or pain you cannot bear.

If you are not sure how serious it is, that is exactly what NHS 111 is for. Nobody will mind you getting in touch.

Why the arc restarts every time your dose goes up

The thing that catches most people out is that this is not one arc, it is a series of them. GLP-1 doses go up in steps, and the product information for tirzepatide states plainly that nausea, diarrhoea and vomiting "occurred primarily during dose escalation and decreased over time". The semaglutide table says the same thing with a footnote attached to almost every gut symptom: mainly seen in the dose-escalation period.

So two good weeks on 5 mg tells you very little about the two weeks after you move to 7.5 mg. That is not a sign anything has gone wrong. It is the same adaptation happening again at a higher level. Knowing that in advance is worth a surprising amount, because the version of this that makes people stop taking their medicine is usually the one they were not expecting.

What the numbers actually say

A pooled analysis of the STEP 1 to 3 trials looked at 2,117 people on semaglutide 2.4 mg and 1,262 on placebo over 68 weeks. Nausea was reported by 43.9% of people on the medicine and 16.1% on placebo. Diarrhoea, 29.7% against 15.9%. Vomiting, 24.5% against 6.3%. Constipation, 24.2% against 11.1% (Wharton et al., 2022). The Wegovy label reports the same trials at 44%, 30%, 24% and 24%, which is the same picture rounded to whole numbers.

Two things in that table get skipped over. The first is the placebo column, which is not zero. A good chunk of what gets blamed on the medicine gets reported by people who are not on it. The second is severity: in the same analysis, 99.5% of these events were non-serious, 98.1% were mild to moderate, and 4.3% of people stopped treatment permanently because of them. Common and severe are different questions, and the answers here point in different directions.

Tirzepatide has its own table, and it is in the label rather than in a journal abstract. Across the two pooled weight-management trials, nausea was reported by 25% to 29% of people depending on the dose against 8% on placebo, diarrhoea by 19% to 23% against 8%, vomiting by 8% to 13% against 2%, and constipation by 11% to 17% against 5%. Between 4.8% and 6.7% stopped for any adverse reaction, against 3.4% on placebo. The SURMOUNT-1 trial report says the same thing in words rather than percentages: the most common adverse events were gastrointestinal, mostly mild to moderate, and occurred primarily during dose escalation (Jastreboff et al., 2022).

What about the oral GLP-1 tablets?

You will have seen orforglipron in the news. It is a once-daily GLP-1 tablet, and the FDA approved it in the United States in April 2026. As of July 2026 it has no marketing authorisation from MHRA, so it cannot lawfully be prescribed or supplied in the UK yet. We will add it to this tool if and when that changes.

If a website offers to sell you an oral GLP-1 that is not licensed where you live, that is your signal to close the tab. An unlicensed supply chain is also an unregulated one, and there is nobody to report a side effect to.

The symptoms that do not follow the curve

Constipation is the obvious one. It tracks how little you are eating and drinking as much as it tracks the dose, so it does not fade the way nausea does and it can arrive weeks after everything else has settled. Reflux and burping behave similarly, because they relate to a slow stomach and to portion size rather than to the step you are on.

Hair thinning is on a different scale altogether. It is listed for all of these medicines at low single-digit percentages, and it is typically noticed months in rather than weeks in. The product information reports hair loss in 4.9% of people on tirzepatide across the pooled weight-management trials against 1.0% on placebo, and 2.5% against 1.0% for semaglutide. Both add that the events were mainly mild and that most people recovered while they carried on taking the medicine. It is also strongly skewed by sex, which the US tirzepatide label is the one to spell out: in its own pooled trials, hair loss was reported by 7.1% of women against 0.5% of men. That timing fits telogen effluvium, the diffuse shedding pattern seen after rapid weight loss from any cause, including surgery and illness. Dermatology researchers have started looking at this specifically in people on GLP-1 medicines (Burke et al., 2025). Iron, ferritin, vitamin D, B12 and thyroid are all common causes of shedding, all checkable, and all worth asking your GP about rather than assuming the medicine is the whole story. Getting enough protein is the piece you control: work out your daily range.

Gallbladder symptoms are a later pattern

Gallstones are listed for all of these medicines, and gallbladder inflammation with them. A meta-analysis of 76 randomised trials covering 103,371 people found GLP-1 medicines were associated with an increased risk of gallbladder or biliary disease, with a relative risk of 1.37 overall and higher in the weight-loss trials specifically (He et al., 2022). Rapid weight loss raises gallstone risk on its own, so this is hard to pull apart from the weight change itself.

The practical point is the timing. This one does not show up in week one. Severe pain under your right ribs, particularly with a fever or yellowing of your eyes or skin, needs urgent attention whenever it turns up, including months in when everything else has settled down.

What this tool will not do

It will not tell you whether to change, delay or stop a dose. That is your prescriber's call.
It will not predict your week. It shows what people reported at that point in the trials, which is a distribution, not a forecast.
It will not tell you whether your symptoms mean the medicine is working. In the pooled analysis, weight loss was similar in people who had gut side effects and people who did not.
It cannot examine you. If something feels wrong, the urgent list and a phone call beat any web page.

Sources

Every study below was checked against PubMed before it went on this page, and every percentage was read directly off a regulator-approved label. Where a figure appears in both a label and a journal, both are listed so you can see they agree.

Before you go

This page is information, not medical advice, and it cannot examine you. It does not replace your prescriber, your GP or the leaflet in your box. Mounjaro and Wegovy are prescription-only medicines that must only be taken under the supervision of a prescriber, and nothing here is a recommendation to start, change or stop any treatment. Never change, delay or stop a dose based on a web page.

Frequency figures are whole-trial percentages from the UK product information and from the published trials, not predictions about you. If you think you are having a side effect from a medicine, you can report it through the MHRA Yellow Card scheme.

Reported

5 mg (n=630)

10 mg (n=948)

15 mg (n=941)

Placebo (n=958)

Nausea

25%

29%

28%

Diarrhoea

19%

21%

23%

Vomiting

11%

13%

Constipation

17%

14%

11%

Abdominal pain

10%

Indigestion

10%

Injection site reactions

Fatigue

Burping

Hair loss

Reflux

Why the arc restarts every time your dose goes up

What the numbers actually say

What about the oral GLP-1 tablets?

The symptoms that do not follow the curve

Gallbladder symptoms are a later pattern