What Are the Side Effects of GLP-1 Drugs? (October 2026)

01 · The core answer

Which side effects should you expect?

Like all medicines, Mounjaro can cause side effects, and most are digestive.¹ The most common — affecting more than 1 in 10 people — are nausea, diarrhoea, vomiting, constipation and abdominal discomfort.¹ Common effects (up to 1 in 10) include indigestion, reduced appetite, dizziness, reflux, burping, hair shedding and injection-site reactions.¹

For a wider overview, see our guide to side effects of GLP-1 meds.

02 · Your next step

Is this relevant to you, and what next?

If you are considering treatment, the question is whether the likely effects are acceptable for you. A GPhC-registered prescriber reviews your health history in a free online consultation, and we provide ongoing monthly reviews and side-effect support.

Related guides: Mounjaro sode effects and Glp1 shot side effects.

03 · Stomach side-effect rates

How common are the stomach effects?

In the trials supporting Mounjaro's weight-management licence, nausea was reported by 24.6% of people on 5mg, 29.0% on 10mg and 28.0% on 15mg, compared with 8.5% on placebo.¹ Diarrhoea was reported by 18.7% of people on 5mg, 20.8% on 10mg and 22.5% on 15mg, compared with 7.8% on placebo.¹ Nausea, vomiting and diarrhoea were more common while the dose was being increased and became less frequent over time.¹

Most people carry on without trouble. In the SURMOUNT-1 trial, 4.3% of people on 5mg, 7.1% on 10mg and 6.2% on 15mg stopped treatment because of side effects, compared with 2.6% on placebo.²

04 · Other common effects

Hair, energy, skin and injection sites

Hair shedding, tiredness and injection-site reactions are the effects people ask about most after the stomach ones. Hair loss was reported by 4.9% of people taking tirzepatide in the weight-management trials, compared with 1.0% on placebo.¹ It is linked to rapid weight loss itself rather than a direct drug effect, and for most people it settles as weight stabilises.

Fatigue is most noticeable in the first weeks and after dose increases. It usually reflects the body adjusting and simply eating less, so keep meals balanced, protein especially, and stay hydrated. Tell your clinical team if tiredness is severe or not settling.

Mild redness, itching, a small bruise or a tender lump at the injection site is common and normally settles within a few days. Rotating sites each week reduces problems. Contact your clinical team if a reaction is spreading, hot or very painful.

"Ozempic face" is not a drug effect. It is what losing a lot of weight quickly can do to facial volume, on any method, and the same applies to loose skin. Adequate protein and resistance exercise help protect lean mass. Not every effect is unwelcome: see Mounjaro good side effects.

05 · Managing symptoms

How to manage nausea, constipation and timing

Most side effects appear in the first days after starting or increasing a dose, peak early and settle as your body adapts.¹

Practical steps that help most people: eat smaller meals slowly, stop at the first feeling of fullness, favour plain lower-fat foods while adjusting, stay well hydrated, and avoid lying down straight after eating.¹³ If constipation persists, a pharmacist can recommend a suitable laxative. Contact your clinical team for pain, bloating or more than a few days without a bowel movement.

If a new dose is hard to tolerate, prescribers can hold the current dose longer before stepping up. Speak to your clinical team rather than pushing through severe symptoms.

06 · Serious side effects

Serious side effects and mood

Serious side effects are uncommon but need urgent attention. Seek immediate medical help for severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis). Also seek immediate medical help for pain in the upper right abdomen, fever or yellowing of skin or eyes (possible gallbladder problems), or signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing.¹

If you take insulin or a sulfonylurea alongside Mounjaro, watch for low blood sugar (shakiness, sweating, confusion). Your prescriber may need to adjust those medicines.¹ If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team, because dehydration needs medical help straight away.

Regulators have looked specifically at low mood and suicidal thoughts. The EMA's safety committee reviewed the evidence and concluded it does not support a causal link, and the MHRA keeps this under continuous review.¹⁷ Even so, tell your prescriber about any history of depression, and seek help promptly if your mood drops or you have distressing thoughts during treatment. You can report any suspected side effect through the Yellow Card scheme.¹⁹ Eye-related questions are covered in Mounjaro ocular side effects.

If you have thoughts of harming yourself or ending your life, get help straight away. Call 999 or go to A&E if you are in immediate danger. In England you can call NHS 111 and choose the mental health option, any time. You can also call Samaritans free on 116 123, day or night. Tell your prescriber about any change in your mood while taking Mounjaro.