PATIENT SAFETY GUIDE
Weight loss injection side effects: what is normal and when should you get help?
The most common side effects of weight-loss injections affect the digestive system. Nausea, diarrhoea, vomiting, constipation, indigestion and stomach discomfort can occur, particularly when treatment starts or the dose changes.
Many symptoms are mild and settle with time, but persistent vomiting, dehydration, severe abdominal pain, breathing difficulty or sudden changes in vision need prompt medical attention. This guide explains the difference.
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Clinically reviewed on 20 June 2026. This article provides general information and does not replace individual medical advice.
The quick answer
Mild nausea, reduced appetite, a feeling of fullness, constipation, diarrhoea, indigestion, burping and tiredness are recognised effects of medicines such as semaglutide and tirzepatide. They are often most noticeable after starting treatment or increasing the dose.
Symptoms should remain manageable. You should not feel that you must simply “push through” repeated vomiting, marked dehydration, severe pain or symptoms that are getting worse.
Severe, persistent abdominal pain that may spread to the back is not a normal side effect. Seek urgent medical help because it can be a sign of acute pancreatitis.
How urgently should you respond?
USUALLY MONITOR
Mild and manageable symptoms
Mild nausea, early fullness, burping, heartburn, temporary diarrhoea or constipation, a mild headache, tiredness or a small injection-site reaction can often be monitored and discussed at your next review.
CONTACT A CLINICIAN
Persistent or worsening symptoms
Contact your pharmacy, prescriber, GP or NHS 111 if vomiting or diarrhoea continues, you cannot drink normally, constipation is painful, dizziness is significant, urine becomes dark or infrequent, or symptoms worsen after a dose increase.
URGENT MEDICAL HELP
Potential red-flag symptoms
Seek urgent help for severe persistent abdominal pain, facial or throat swelling, breathing difficulty, collapse, sudden or rapidly worsening vision, or severe constipation with marked bloating, pain and vomiting.
What are the most common weight loss injection side effects?
Digestive symptoms make up most reported side effects. The exact pattern varies between people and between medicines, and not everyone develops them.
Low blood sugar is mainly a concern for people with diabetes who also use insulin or a sulphonylurea. Warning signs include sweating, shaking, hunger, weakness, confusion, palpitations or drowsiness. Follow your diabetes treatment plan and seek advice if episodes occur.
When are side effects most likely?
Side effects are often most noticeable during the first few weeks of treatment and after a dose increase. This is one reason doses are normally increased gradually rather than moving immediately to a higher strength.
For many people, mild nausea, vomiting or bowel changes improve as the body adjusts. However, the next dose increase should not be treated as automatic. If you are still experiencing significant symptoms, contact the prescribing service before progressing.
Symptoms that continue for several weeks, and particularly symptoms lasting four weeks or more, deserve clinical review even if they are not severe. A review may consider hydration, food intake, other medicines, the current dose, an alternative treatment or a different diagnosis.
Practical ways to manage mild side effects
For nausea or indigestion
Choose smaller meals, eat slowly and stop when comfortably full. Plain or lower-fat foods may be easier to tolerate. Avoid large meals, rich or greasy food, lying down immediately after eating and excessive alcohol.
For vomiting or diarrhoea
Take frequent small sips of fluid rather than drinking a large amount at once. Oral rehydration products may be appropriate, but ask a pharmacist first if you have kidney disease, heart failure or a fluid restriction.
For constipation
Maintain fluid intake, keep physically active within your ability and increase fibre gradually where suitable. Increasing fibre without adequate fluid can make constipation worse.
For tiredness or headache
Check that you are drinking enough and still eating regular, nutritionally balanced food. Very low calorie intake can contribute to fatigue, dizziness, headaches and loss of muscle.
Ask before adding another medicine. A pharmacist can advise whether a laxative, oral rehydration product, antacid or other treatment is suitable. Do not repeatedly self-treat persistent symptoms without discussing the cause.
Watch carefully for dehydration
Nausea, vomiting and diarrhoea can lead to fluid and electrolyte loss. Significant dehydration can affect kidney function and may require hospital treatment.
Seek same-day advice if you cannot keep fluids down, are passing very little urine, feel faint or confused, or have an existing kidney condition and develop significant vomiting or diarrhoea.
Red flags: when side effects may be serious
Severe persistent abdominal pain
Pain that does not go away, particularly upper abdominal pain that spreads to the back, with or without vomiting, can indicate acute pancreatitis. Stop the medicine and seek urgent medical assessment.
Breathing difficulty or rapid swelling
Swelling of the face, lips, tongue or throat, wheezing, difficulty swallowing, pale clammy skin or collapse may indicate a severe allergic reaction. Call 999.
Severe constipation with pain and vomiting
Marked abdominal swelling, worsening pain, vomiting and inability to pass stool or wind can indicate bowel obstruction and need urgent assessment.
Sudden or rapidly worsening eyesight
Anyone taking semaglutide who develops sudden vision loss or a rapid deterioration in vision should seek urgent medical or eye assessment.
Possible gallbladder symptoms
Severe pain under the right ribs or in the upper abdomen, especially with fever, vomiting or yellowing of the skin or eyes, requires prompt medical assessment.
Do not wait for a routine weight-management review if you have a red-flag symptom. Use NHS 111 or urgent medical services as appropriate; call 999 for breathing difficulty, collapse or another immediate emergency.
Do Mounjaro and Wegovy cause the same side effects?
There is substantial overlap. Both tirzepatide and semaglutide can cause nausea, vomiting, diarrhoea, constipation, abdominal discomfort, indigestion, reflux, tiredness and injection-site reactions.
It is not possible to guarantee that one medicine will cause fewer side effects for a particular person. Previous response, medical history, other medicines, dose, rate of escalation, food intake and individual sensitivity all matter.
There are also treatment-specific precautions. Tirzepatide may reduce the reliability of oral contraception after treatment starts and after each dose increase. Semaglutide has a very rare recognised association with a serious eye condition that can cause sudden or rapidly worsening vision. These issues should be discussed during the clinical assessment.
Important situations to tell your healthcare team about
Local weight-management support in Treherbert and the Rhondda
Bute Clinic by Treherbert Pharmacy provides face-to-face assessment and ongoing review. Patients can discuss side effects, tolerability, dose changes, medicine interactions and whether further treatment remains appropriate.
The clinic is accessible to patients from Treherbert, Treorchy, Cwmparc, Ton Pentre, Ystrad Rhondda, Llwynypia, Tonypandy, Trealaw, Porth, Rhigos, Hirwaun and Aberdare.
Book a free private weight management consultation
Call Treherbert Pharmacy on 01443 776133
A consultation does not guarantee that treatment will be prescribed or supplied.
Frequently asked questions
How long do weight loss injection side effects last?
Mild digestive side effects often improve over several days or weeks and may recur temporarily after a dose increase. Persistent, severe or worsening symptoms need review rather than waiting indefinitely for them to settle.
Should I stop my injection if I feel sick?
Mild nausea does not always require treatment to stop. Contact the prescriber before the next dose if symptoms are significant. Stop and seek urgent help for severe persistent abdominal pain, a serious allergic reaction or another red-flag symptom.
Which weight loss injection has the least side effects?
There is no option that reliably causes the fewest side effects for everyone. The safest choice depends on your health, medicines, previous response and ability to tolerate treatment. This is a clinical decision rather than a simple product comparison.
Can I take a laxative or anti-sickness medicine?
Sometimes, but the most suitable option depends on your symptoms, medical history and other medicines. Ask a pharmacist or prescriber before starting regular treatment, especially if symptoms are persistent.
Can I increase my dose if I have not lost much weight?
Do not increase the dose early or outside the agreed schedule. The initial and intermediate doses allow the body to adjust. Increasing while side effects are unresolved can make them worse.
Can Treherbert Pharmacy advise me if another provider supplied the treatment?
The pharmacy can provide general advice and assess urgent concerns. Continuing or changing treatment normally requires a full assessment and evidence of the medicine, dose and last administration. Bring the pen or packaging where possible.
Official patient information and reporting
Read the patient information leaflet supplied with your medicine. The following UK sources provide further safety information.
Wegovy patient leafletOfficial patient information for semaglutide injection.
Mounjaro patient leafletOfficial patient information for tirzepatide injection.
MHRA Yellow CardPatients and healthcare professionals can report suspected medicine side effects.
Treherbert Pharmacy and Bute Clinic
167 Bute Street, Treherbert, Treorchy, CF42 5PE
Telephone: 01443 776133
Article information
Written by: Jonathan James, Independent Prescribing Pharmacist, GPhC number 2066318
Clinically reviewed by: Thomas Harries, Superintendent Pharmacist, GPhC number 2210031
Published: 20/06/2026
Last clinically reviewed: 20/06/2026