GLP-1 side effects: what is common, and when to get help

Adult information for readers in Ireland · Sources checked 14 September 2026. This guide does not diagnose symptoms or replace the leaflet for your exact medicine.

“That can happen on these injections” is sometimes useful reassurance. It is not enough when someone cannot keep fluids down, has persistent severe pain or notices a sudden change in their sight.

Good information about side effects has to do two jobs: put common symptoms into perspective and make the exceptions easy to recognise. A long undifferentiated list does neither particularly well.

This guide covers the medicines commonly discussed together as GLP-1 treatments, including semaglutide and the dual GIP/GLP-1 medicine tirzepatide. The figures below come from a specific semaglutide trial programme; they should not be treated as identical rates for every drug or dose.

Symptoms that should not wait

If you are currently unwell, start here rather than reading the whole article.

Severe abdominal pain that persists, particularly if it spreads to the back, needs immediate medical assessment. It can be a sign of pancreatitis. Do not assume it is ordinary nausea or wait until the next routine appointment. Wegovy patient leaflet

Sudden loss of vision or rapidly worsening eyesight also needs urgent assessment. Seek help immediately; a website cannot establish the cause. The EMA has identified NAION, an optic-nerve condition, as a very rare semaglutide side effect, potentially affecting up to 1 in 10,000 users. That finding concerns semaglutide and should not automatically be applied to every medicine in this group. EMA safety update

Repeated vomiting, inability to keep fluids down or worsening symptoms warrant prompt clinical advice. Use your GP, prescribing service or out-of-hours service; use emergency care for severe illness. The categories here are not a complete checklist of every possible emergency.

How common are digestive symptoms?

An analysis pooling three 68-week STEP trials compared 2,117 adults assigned semaglutide 2.4 mg with 1,262 assigned placebo. Digestive symptoms were more frequent with semaglutide. Wharton and colleagues

Participants reporting each symptom during STEP 1–3. Semaglutide 2.4 mg; not a comparison between brands.

Symptom	Semaglutide	Placebo
Nausea	43.9%	16.1%
Diarrhoea	29.7%	15.9%
Vomiting	24.5%	6.3%
Constipation	24.2%	11.1%

Source: Wharton et al. A participant could report more than one symptom; the percentages should not be added. These are study-period proportions, not weekly risks.

Most digestive events were mild or moderate, and often occurred during or shortly after dose escalation. Nevertheless, 4.3% of semaglutide participants permanently discontinued treatment because of digestive adverse events. The analysis was funded by Novo Nordisk.

Its weight-loss analysis also suggests an important distinction: the treatment effect was largely independent of these symptoms. Feeling sick is not a requirement for benefit. Symptoms should not become a badge of successful treatment, nor should feeling well be a reason to assume a dose is ineffective.

The placebo column is useful because symptoms also occur without the active medicine. Timing can raise suspicion, but a new complaint still needs assessment on its own merits.

What may help with mild symptoms

For mild nausea or uncomfortable fullness, a multidisciplinary expert consensus recommends smaller portions, slower eating and stopping when comfortably full. It also suggests avoiding lying down after meals and choosing easier-to-digest, lower-fat foods when symptoms are troublesome. These are expert recommendations, not proof that a particular food prevents side effects. Gorgojo-Martínez and colleagues

Think of these as modest adjustments to discuss, not a demand to eat as little as possible. If symptoms are repeatedly preventing normal meals, bring that to the prescriber rather than continually narrowing what you eat.

For vomiting or diarrhoea, hydration deserves attention. The HSE suggests small sips if drinking is difficult, and a pharmacist can advise on oral rehydration sachets. Dark urine, reduced urination, thirst and dizziness can signal dehydration. Seek medical advice if it is not improving; follow any individual fluid restrictions you have been given. HSE: dehydration

Constipation may call for a review of fluids, food and activity, with a pharmacist or clinician advising on suitable treatment. Do not use that advice to explain away severe pain or vomiting. The expert consensus also describes clinician-led adjustments to dose escalation when symptoms persist; it is not a reason to change the dose yourself. Expert consensus

Other medicines and procedures change the conversation

Insulin and sulphonylureas matter. Tirzepatide’s product information warns that combining it with these medicines increases hypoglycaemia risk. A prescriber may need to review doses. Do not make those changes independently. EMA: Mounjaro

Shaking, sweating and confusion can be symptoms of low blood glucose. If you have diabetes, follow your agreed hypo plan and check glucose as directed. If someone is unconscious, call emergency services and do not give food or drink by mouth. HSE hypo guidance

Tell the team before anaesthesia or deep sedation. Delayed stomach emptying can leave residual stomach contents and increase aspiration risk. Mounjaro’s product information highlights this. Give the team the exact product, dose and last-dose date, and follow their instructions; there is no universal stop interval offered in this article. EMA: Mounjaro

Check the leaflet, not just the category name. Wegovy’s information also lists risks including gallstones and intestinal obstruction. This guide does not reproduce the full list or determine which explanation fits a symptom. EMA: Wegovy

Make the next clinical conversation more useful

“I feel awful” is a valid reason to ask for help. A few details can make that conversation easier: the medicine and dose, the date of the last injection or dose increase, when symptoms started, whether they are worsening, and whether you can eat, drink and pass urine normally.

Mention other medicines, too. A symptom diary can be brief: dates and a few sentences are enough. There is no need to track every mouthful or turn recovery into another job.

If treatment needs to change, ask what happens next and who will follow up. Our guide to stopping GLP-1 treatment covers the withdrawal evidence and planning questions. If the first question is why these medicines are being used at all, the Irish GLP-1 overview provides that background.

Reporting a suspected side effect in Ireland

Patients and carers can report directly to the HPRA, including when they suspect a medicine but are not certain. A useful report identifies the medicine and describes what happened. These reports contribute to safety monitoring; submitting one does not prove causation. HPRA: patient reporting

Reporting is separate from getting care. The HPRA does not provide clinical advice, so contact a health professional about symptoms and use emergency services when needed. You do not have to decide whether a reaction is officially attributable to the medicine before asking for help.

More on GLP-1 medicines in Ireland

• GLP-1 medicines in Ireland: evidence, benefits and access — Start with the class-wide Irish overview.
• How to get Ozempic in Ireland — The regulated access pathway.
• Wegovy, Mounjaro and Ozempic costs in Ireland — Private price and funding context.
• GLP-1s and pregnancy — Current pregnancy, preconception and breastfeeding evidence.
• What happens after stopping a GLP-1? — Weight regain and long-term treatment evidence.