Weight Loss Injection Side Effects: What Is Common and When to Seek Help
Weight Loss Injection Side Effects: What Is Common and When to Seek Help
Weight loss injections can cause side effects, although not everyone experiences them.
Common reactions can include:
• Nausea
• Vomiting
• Diarrhoea
• Constipation
These are among the side effects most often reported with medicines commonly used for weight management in the UK.
Many digestive side effects are:
• Mild to moderate
• More noticeable when treatment starts
• More noticeable after a planned dose increase
• Likely to improve as the body adjusts
However, severe symptoms should never be treated as proof that the medicine is working.
Persistent vomiting can cause dangerous dehydration.
Severe abdominal pain may be a sign of pancreatitis or another condition requiring urgent assessment.
Seek urgent medical help for severe, persistent abdominal pain
Pain that:
• Does not go away
• May spread to the back
• May occur with nausea or vomiting
can be a sign of acute pancreatitis.
The MHRA strengthened its warning about this risk in January 2026.
Do not wait for a routine review.
The short answer
Common reactions include:
• Nausea
• Vomiting
• Diarrhoea
• Constipation
• Stomach discomfort
Some people may also experience:
• Tiredness
• Headache
• Hair loss
• Redness or itching where the injection is given
The exact list differs between medicines.
Mild nausea may improve with:
• Smaller meals
• Eating slowly
• Stopping when comfortably full
Repeated vomiting, inability to keep fluids down or signs of dehydration require prompt medical advice.
Seek urgent medical help for:
• Severe persistent abdominal pain
• Serious allergic symptoms
• Jaundice
• Severe bowel symptoms
• Sudden loss of vision
Do not:
• Increase the dose yourself
• Take an extra dose
• Switch injections without advice from the prescribing clinician
Always read the patient information leaflet supplied with your medicine because:
• Risks differ
• Instructions differ
• Side effect profiles differ
Why do weight loss injections cause digestive side effects?
Many current weight management injections act on signals involved in:
• Appetite
• Fullness
• Blood glucose
• Digestion
They can slow the movement of food through the stomach and reduce appetite.
These effects can support treatment, but they can also lead to:
• Nausea
• Fullness
• Reflux
• Burping
• Constipation
• Diarrhoea
Side effects are not a reliable measure of whether a treatment will be effective.
Someone can respond to treatment without feeling sick.
Another person may have side effects without achieving the hoped for result.
The exact frequency and type of side effect depend on:
• The medicine
• Dose
• Other medicines
• Health conditions
• The individual
A general article cannot replace:
• The patient leaflet
• Advice from the prescriber who knows your medical history
What side effects are common?
Nausea
Feeling sick is one of the best known side effects.
It may be more noticeable:
• After starting treatment
• After moving to a higher dose
A large, rich or high fat meal may make it worse for some people.
Mild nausea may improve if you:
• Eat smaller meals
• Eat slowly
• Stop when comfortably full
• Choose simpler, lower fat food while symptoms are present
• Avoid lying down immediately after eating
• Take regular small drinks if larger amounts feel difficult
• Avoid alcohol if it worsens the symptoms
Contact your prescriber or pharmacist if nausea:
• Is severe
• Keeps returning
• Prevents normal eating
• Does not improve
Vomiting
Vomiting can occur, particularly during the adjustment period.
One isolated episode may not be serious if you:
• Otherwise feel well
• Can drink normally
Repeated vomiting is different.
It can:
• Cause dehydration
• Affect other medicines
Seek prompt medical advice if you:
• Cannot keep fluids down
• Are vomiting repeatedly
• Have signs of dehydration
Vomiting with severe persistent abdominal pain requires urgent assessment.
Diarrhoea
Diarrhoea may be mild and short lived.
Ongoing diarrhoea can cause dehydration.
Take regular fluids unless a healthcare professional has told you to restrict fluid intake.
A pharmacist can advise whether an oral rehydration product may be appropriate.
Seek advice if diarrhoea is:
• Severe
• Persistent
• Contains blood
• Occurs with a high temperature
• Accompanied by marked weakness
• Accompanied by dizziness
• Accompanied by very little urine
Constipation
Constipation can develop because:
• Food moves more slowly through the digestive system
• You may be eating less
• You may be drinking less
Helpful measures may include:
• Adequate fluid
• Gentle activity
• A gradual increase in fibre where suitable
Do not simply add a large amount of fibre if you have:
• Severe pain
• Vomiting
• A swollen abdomen
Contact a pharmacist or prescriber before using a laxative, particularly if you:
• Have another medical condition
• Take other medicines
Severe constipation with:
• Significant pain
• Vomiting
• Abdominal swelling
• Inability to pass wind
needs urgent medical assessment.
Stomach pain, bloating, reflux and burping
Mild:
• Cramping
• Fullness
• Indigestion
• Reflux
• Bloating
• Burping
can occur.
Smaller meals and avoiding food that clearly triggers symptoms may help.
The severity and pattern matter.
New pain that is:
• Severe
• Persistent
• Worsening
• Associated with vomiting
• Spreading to the back
should not be managed as ordinary indigestion.
Reduced appetite
Reduced appetite is an expected treatment effect for many people.
Being unable to eat or drink enough is not a goal.
Very low food intake can contribute to:
• Tiredness
• Constipation
• Nutrient shortfalls
• Loss of muscle
Tell your prescriber if you are:
• Regularly missing most meals
• Struggling to drink
• Feeling faint
• Losing weight more quickly than expected
A treatment plan should support:
• Adequate nutrition
• Safe weight change
Injection site reactions
Some injections can cause mild:
• Redness
• Itching
• Tenderness
• A small local reaction
Follow the instructions for:
• Site rotation
• Injection technique
supplied with the product.
Seek advice if a reaction is:
• Spreading
• Very painful
• Hot
• Producing discharge
• Accompanied by fever
Difficulty breathing or swelling of the:
• Face
• Tongue
• Throat
is an emergency rather than a routine injection site reaction.
Tiredness, headache and hair loss
Tiredness, headache and hair loss are listed with some weight management injections.
The side effect profile is not identical for every medicine.
Tiredness or headache may also be related to:
• Dehydration
• Low food intake
• Low blood sugar
• Poor sleep
• Another illness
Do not assume every new symptom is caused by the injection.
Contact a healthcare professional if symptoms are:
• Persistent
• Severe
• Concerning
When do side effects usually happen?
Digestive side effects are often more noticeable:
• After treatment begins
• During a planned dose increase
NICE guidance notes that:
• Nausea
• Vomiting
• Diarrhoea
tend to be more common during dose increases and may reduce over time.
That does not mean every symptom should simply be waited out.
Contact the prescribing service if side effects are:
• Troublesome
• Persistent
• Not settling
The clinician may decide to:
• Delay an increase
• Review the current dose
• Check for another cause
• Stop treatment
The correct decision depends on:
• The medicine
• Individual circumstances
Do not increase the dose while significant side effects are continuing
Follow the schedule given by your prescriber.
Contact the prescribing service before:
• Increasing the dose
• Restarting after a break
• Changing to another injection
Signs of dehydration
Nausea, vomiting and diarrhoea can sometimes lead to severe dehydration.
The MHRA has received reports of people requiring hospital treatment after digestive side effects.
Possible signs include:
• Very dark urine
• Passing much less urine than usual
• A very dry mouth
• Dizziness, particularly when standing
• Marked weakness
• Unusual sleepiness
• Confusion
• A fast heartbeat
• Inability to keep fluids down
Seek urgent advice if these symptoms are present.
People with:
• Kidney conditions
• Heart conditions
• Liver conditions
• A prescribed fluid restriction
should ask their clinician how to manage fluid intake rather than following a generic target.
When should you seek urgent medical help?
Severe persistent abdominal pain
Seek urgent medical attention if you develop severe pain in the abdomen that:
• Does not go away
• May spread to the back
• May be accompanied by nausea or vomiting
This can be a sign of acute pancreatitis.
Pancreatitis is uncommon.
However, rare severe and fatal cases have been reported.
The symptoms can initially be mistaken for ordinary digestive side effects.
The severity and persistence of the pain are important.
Possible gallbladder problems
Gallstones or inflammation of the gallbladder are possible serious reactions with some weight management injections.
Seek prompt medical assessment for severe upper abdominal pain, particularly if it occurs with:
• Fever
• Vomiting
• Pale stools
• Yellowing of the skin
• Yellowing of the whites of the eyes
Yellowing can be less noticeable on some skin tones.
Serious allergic reaction
Call 999 for:
• Difficulty breathing
• Swelling of the face
• Swelling of the lips
• Swelling of the tongue
• Swelling of the throat
• Severe wheezing
• Collapse
• Another suspected serious allergic reaction
Do not drive yourself to hospital.
Very low blood sugar
Weight management injections do not usually cause low blood sugar on their own.
The risk can increase when some are used with:
• Insulin
• Certain diabetes medicines
Possible symptoms include:
• Sweating
• Shaking
• Dizziness
• Hunger
• Blurred vision
• Weakness
• Confusion
• A fast heartbeat
Follow the personalised hypo plan given by your diabetes team.
Call 999 if the person is:
• Unconscious
• Having a seizure
• Unable to swallow safely
Do not give food or drink to someone who cannot swallow.
Sudden or rapidly worsening vision
The MHRA advises urgent assessment for:
• Sudden blindness
• Rapid deterioration in eyesight
because a very rare serious eye condition has been linked with one medicine in this group.
Sudden vision loss is urgent regardless of the suspected cause.
Contact:
• An urgent eye service
• Your prescriber
• Emergency care
as appropriate.
Severe bowel symptoms
Seek urgent assessment for:
• Severe constipation
• Significant abdominal swelling
• Repeated vomiting
• Inability to pass wind
• Rapidly worsening abdominal pain
Do not keep treating these symptoms at home as ordinary constipation.
How to access urgent help across the UK
For urgent symptoms that are not immediately life threatening, contact:
• Your prescribing service
• Your GP
• Your local urgent care service
In England, Scotland and Wales:
• Call 111
In Northern Ireland:
• Contact your GP out of hours service when your practice is closed
Call 999 or attend an Emergency Department for a medical emergency.
Do not drive yourself if you have:
• Severe pain
• Faintness
• Confusion
• Breathing difficulty
• Serious visual symptoms
When should you contact your prescriber?
Contact the prescribing service promptly if:
• Side effects are troublesome
• Side effects are persistent
• Side effects are getting worse
• You are vomiting repeatedly
• You are struggling to drink
• Constipation is not improving with suitable basic measures
• You are unable to eat enough
• You feel faint or weak
• An injection site reaction is spreading
• An injection site reaction is becoming painful
• You develop a new symptom and are unsure whether it is related
• You have used more than the prescribed amount
• You missed several doses
• You had a treatment break and want to restart
• You are due to increase the dose but still have significant side effects
• You think you may be pregnant
Do not wait for a routine appointment if symptoms match one of the urgent warning signs above.
What if you use too much?
Using more than the prescribed amount can be dangerous.
It may cause:
• Severe nausea
• Vomiting
• Low blood sugar
depending on the medicine and other treatment being used.
Seek medical advice promptly.
Do not:
• Try to correct the mistake with another injection
• Skip future treatment without advice
• Wait for symptoms to appear before asking for help
Keep the following available:
• Medicine
• Package
• Patient leaflet
so the healthcare professional can identify exactly what was used.
Should you stop treatment because of side effects?
Do not make a routine change without speaking to the prescriber.
If treatment is also being used for type 2 diabetes, suddenly stopping may affect blood glucose.
Urgent symptoms are different.
If pancreatitis or another serious reaction is suspected:
• Seek urgent medical help
• Do not take another dose unless the assessing clinician advises it is safe
Confirmed pancreatitis means medicines in this group should not be restarted unless specialist advice specifically establishes an appropriate alternative plan.
Pregnancy, contraception and side effects
Weight management injections in this group should not be used during pregnancy.
Contact a healthcare professional straight away if you think you may be pregnant.
Vomiting and severe diarrhoea can also affect how well an oral contraceptive is absorbed.
One particular weight management medicine has additional advice about using a barrier method:
• After starting
• After each dose increase
Check the leaflet supplied with your exact medicine.
Discuss contraception with:
• The prescriber
• Pharmacist
The time required between stopping treatment and trying to become pregnant differs between medicines.
Do not use a generic timescale.
What should you tell a healthcare professional?
When asking for help, provide:
• The exact name of the injection
• The exact strength
• When the last dose was used
• Whether the dose recently changed
• When symptoms began
• Whether symptoms are worsening
• Whether you can eat
• Whether you can drink
• Whether you can urinate
• Whether you can open your bowels
• Other medicines
• Diabetes treatment in particular
• Relevant medical conditions
• Any history of pancreatitis
• Any history of gallbladder problems
• Where the medicine was obtained
Privately prescribed treatment may not appear automatically in every connected health record.
Tell an emergency clinician that you use a weight management injection even if it is not shown on their system.
How can side effects be reported?
Patients, carers and healthcare professionals can report a suspected medicine side effect through the MHRA Yellow Card scheme.
A report can be useful even when it is not certain that the medicine caused the reaction.
Reporting a side effect is not a substitute for getting medical help.
Contact an appropriate healthcare service first if you are unwell.
How Keltoi handles side effect concerns
Keltoi’s website and questionnaire do not decide whether treatment should:
• Continue
• Change
• Stop
For Keltoi weight management patients, Dr Waqar Ahmed, Clinical Lead, makes the clinical decision after:
• Reviewing the relevant information
• Requesting any further assessment needed
Patients should follow the contact and urgent care instructions supplied with their treatment.
An online message should never be used instead of emergency care for:
• Severe pain
• Breathing difficulty
• Collapse
• Another serious symptom
Frequently asked questions
How long do weight loss injection side effects last?
Mild digestive effects may improve as the body adjusts.
Some are more noticeable:
• After starting treatment
• After increasing treatment
There is no guaranteed timescale.
Contact the prescriber if symptoms are:
• Troublesome
• Persistent
• Worsening
Is nausea a sign that the injection is working?
No.
Nausea is a side effect, not a measure of effectiveness.
Treatment can work without making someone feel sick.
Should I increase my dose if side effects have not settled?
Do not increase without advice.
Contact the prescribing service before the planned increase.
The clinician may need to:
• Delay the increase
• Change the plan
What stomach pain is concerning?
Severe persistent abdominal pain that:
• Does not go away
• May spread to the back
• Occurs with nausea and vomiting
requires urgent medical assessment.
Severe upper abdominal pain with:
• Fever
• Jaundice
may indicate a gallbladder problem.
When is constipation an emergency?
Seek urgent assessment for severe constipation with:
• Significant pain
• A swollen abdomen
• Repeated vomiting
• Inability to pass wind
These symptoms should not be managed as routine constipation.
Can weight loss injections cause low blood sugar?
The risk is generally low when the injection is used alone.
It can increase when treatment is combined with:
• Insulin
• Certain diabetes medicines
Follow the blood glucose and hypo advice given by your diabetes team.
Can I take medicine for nausea, diarrhoea or constipation?
Ask a:
• Pharmacist
• Prescriber
first.
The best option depends on:
• The symptom
• Other medicines
• Health conditions
Do not use symptom relief to delay assessment of:
• Severe pain
• Dehydration
• Possible bowel obstruction
Should I tell an anaesthetist or dentist about the injection?
Yes.
Tell the clinical team before:
• Surgery
• A procedure involving anaesthesia
• A procedure involving sedation
Do not stop treatment independently.
The correct plan depends on:
• The procedure
• The medicine
• The reason for treatment
Where can I find the full side effect list?
Read the patient information leaflet supplied with the medicine.
You can also ask:
• The dispensing pharmacy
• The prescriber
for the current leaflet.
The MHRA advises patients to use information for the exact product because:
• Side effects differ
• Instructions differ
What to remember
Nausea, diarrhoea, constipation and vomiting are common weight loss injection side effects.
However, they should remain manageable.
Severe symptoms are not a necessary part of treatment.
Seek urgent help for:
• Severe persistent abdominal pain
• Serious allergic symptoms
• Sudden loss of vision
• Significant dehydration
• Jaundice
• Severe bowel symptoms
Contact the prescriber before:
• Increasing the dose
• Restarting after a break
• Trying to manage persistent symptoms alone
Sources
• MHRA: GLP 1 medicines for weight loss and diabetes
• MHRA: Strengthened warnings on acute pancreatitis
• NICE: Overweight and obesity management
• MHRA Yellow Card scheme
• nidirect: GP out of hours service
Last updated: 19 August 2026
Medical disclaimer: This article provides general information and is not a diagnosis, individual treatment plan or substitute for medical advice. Read the leaflet supplied with your medicine and follow the instructions from your prescribing clinician.
Call 999 for a medical emergency. For urgent advice that is not immediately life threatening, contact an appropriate urgent care service in your part of the UK.