Weight-loss injection side effects - what is normal and when should you get help?

Nausea, diarrhoea, constipation and vomiting are common with weight-loss injections, particularly when starting treatment or increasing the dose. These symptoms often settle, but severe or persistent symptoms need medical advice. Here is how to tell the difference.

Which side effects can occur?

Weight-loss injections work partly by reducing appetite and slowing the movement of food through the stomach. Digestive symptoms are therefore among the most frequently reported side effects.

Depending on the medicine, these may include:

• feeling sick or being sick
• diarrhoea or constipation
• abdominal discomfort, bloating or indigestion
• heartburn and burping
• feeling unusually full after a small amount of food
• tiredness or headache
• mild redness or itching where you inject

The MHRA’s guidance on GLP-1 medicines says most side effects are mild to moderate or short-lived. Digestive symptoms are more likely when treatment is started or the dose is being increased. The precise side-effect profile varies between medicines, so also check the patient leaflet supplied with your own prescription.

A symptom being common does not necessarily mean it is acceptable for you. Speak to your prescriber, GP or pharmacist if it is troublesome, getting worse or not settling.

What can help with mild digestive symptoms?

If you can still eat, drink and carry out your usual activities, a few adjustments may make mild symptoms easier to manage:

• Eat smaller amounts more frequently instead of having a large meal.
• Eat slowly and stop when you feel comfortably full.
• Choose plain, lower-fat foods if nausea is a problem.
• Avoid lying down soon after eating if you have heartburn or reflux.
• Sip fluids regularly, especially if you have diarrhoea or have been sick.
• For constipation, maintain your fluid intake and gradually include fibre and gentle activity if you can.

The dietary guidance from Guy’s and St Thomas’ NHS Foundation Trust includes practical suggestions for nausea, vomiting, diarrhoea and constipation. Its advice is intended for mild symptoms; it is not a substitute for assessment when you cannot keep fluids down or feel significantly unwell.

Do not automatically move up to the next dose while side effects are still difficult to tolerate. For tirzepatide (Mounjaro), NICE’s practical prescribing guide advises delaying a dose increase until adverse effects have resolved or become tolerable. Any decision to reduce, pause or stop a prescribed medicine should be made with a clinician, particularly if it is also treating type 2 diabetes.

A pharmacist or doctor can advise whether a short course of treatment for nausea or constipation is appropriate. Check before taking over-the-counter remedies if you have other medical conditions or use regular medication.

When should you contact your prescriber or a GP?

Arrange medical advice promptly if:

• side effects are worsening rather than settling
• you are repeatedly vomiting
• you cannot keep enough fluid down
• your urine has become unusually dark or you feel light-headed
• constipation is accompanied by significant pain or bloating
• symptoms began or became much worse after a dose increase
• you are eating so little that you are concerned about weakness or poor nutrition
• you are having symptoms of low blood sugar, such as shaking, sweating, dizziness, palpitations or confusion

Vomiting and diarrhoea can occasionally cause severe dehydration requiring hospital care. The risk matters more if you are older, have kidney problems or take medicines affected by fluid loss.

Low blood sugar is particularly a concern when these medicines are used alongside insulin or other diabetes medicines such as sulfonylureas, although it can also occur in people without diabetes. Follow your personal diabetes treatment plan and seek medical help if symptoms do not improve. NHS information on Semaglutide (Wegovy) explains the symptoms and other serious effects associated with that medicine.

Which symptoms need urgent help?

Severe, persistent abdominal pain

Seek urgent medical assessment if you develop severe abdominal pain that does not go away, especially if it spreads to your back or is accompanied by nausea or vomiting. This may be a sign of acute pancreatitis, a known but infrequent adverse effect of GLP-1 and GLP-1/GIP receptor agonists. If pancreatitis is suspected, stop taking the medicine and seek urgent medical attention.

The MHRA strengthened its pancreatitis warnings in January 2026 after rare severe cases were reported. If the pain is severe, persistent, worsening, or you feel very unwell, attend A&E or call 999. If pancreatitis is confirmed, the medicine should not be restarted.

Abdominal pain, particularly if accompanied by fever, jaundice (yellowing of the skin or eyes) or pain under the right ribs, may indicate gallstones or another gallbladder problem and should be assessed promptly by a healthcare professional.

Sudden or rapidly worsening eyesight

Sudden vision loss, including partial vision loss, or rapidly worsening eyesight needs urgent same-day assessment. Semaglutide (Wegovy) has very rarely been linked with non-arteritic anterior ischaemic optic neuropathy (NAION), a serious condition affecting blood flow to the optic nerve.

If you develop sudden loss of vision or rapidly worsening eyesight while taking semaglutide, urgently attend eye casualty if available in your area, or go to A&E. Do not wait for a routine medication review.

A severe allergic reaction

Call 999 if you develop difficulty breathing, swelling of the tongue or throat, faintness, or a widespread raised and itchy rash alongside breathing or swelling symptoms. These can be signs of anaphylaxis.

What happens at a medication review?

A useful review is not simply a decision about whether to continue the same dose. The clinician may:

• confirm the exact medicine, dose and timing of your symptoms
• check whether symptoms started after a dose increase
• assess hydration and examine you where necessary
• review diabetes, blood-pressure and other regular medicines
• consider whether tests are clinically indicated
• discuss delaying an increase, reducing the dose, pausing treatment or considering another approach
• check that you are receiving the medicine from a regulated pharmacy

Tell the clinician if your injection was prescribed privately. Private prescriptions may not automatically appear in your NHS record, so bring the pen or packaging, or have a photograph of the label and details of your current dose available. Never switch between products or doses without advice: the MHRA warns that unsupervised switching can increase side effects or make treatment less effective.

If you are being treated through DocTap’s GP-led medical weight-loss service, side effects can be considered as part of your ongoing GP review. If the medicine was prescribed elsewhere, a general GP appointment may still help assess your symptoms and decide what care you need. An in-person appointment is usually preferable if abdominal symptoms require examination. DocTap repeat reviews also ask about side effects and significant changes in health, rather than treating repeat supply as automatic.

A private appointment is not the right route for a medical emergency. Use NHS 111, an urgent treatment service, A&E or 999 according to the severity of your symptoms.

Finally, you can report a suspected medicine reaction through the MHRA Yellow Card scheme. Reporting does not replace medical care, but it helps regulators monitor medicine safety.