What Are the Side Effects of Taking Weight Loss Injections? (October 2026)

01 · The core answer

Common side effects of weight loss injections

Like all medicines, Mounjaro can cause side effects, and most are digestive. 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

02 · Your next step

Is this relevant to you, and what next?

If you are considering treatment, the sensible question is whether you can accept a few weeks of possible digestive upset in return for a licensed medicine. Every order is reviewed by a GPhC-registered prescriber, who checks suitability before anything is dispensed.

Our ongoing monthly reviews include side-effect support.

Related guides: Weight Loss Injections Register Online and Weight loss injections side effects nhs and Fat loss injections side effects.

03 · Trial side-effect rates

What the trials found

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 Hair loss was reported by 4.9% of people taking tirzepatide in the weight-management trials, compared with 1.0% on placebo.¹ Injection-site reactions were reported by 8.0% of people taking tirzepatide in the weight-management trials, compared with 1.8% on placebo.¹

04 · Managing side effects

Managing the everyday ones

Most people can manage digestive effects with a few habits: 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

For constipation, start with fluids, add fibre from vegetables and whole grains, and keep moving daily. If it 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.¹³

For injection sites, rotate the site each week and let the pen warm briefly out of the fridge to reduce stinging. Contact your clinical team if a reaction is spreading, hot, very painful, or lasts more than a week.¹³

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.¹

05 · Serious side effects

Serious side effects: when to get help

Serious side effects are uncommon but need urgent attention.¹³ Seek immediate medical help for:

• severe, persistent stomach pain that may spread to your back, with or without vomiting (possible pancreatitis);
• pain in the upper right abdomen, fever or yellowing of skin or eyes (possible gallbladder problems);
• signs of a severe allergic reaction such as swelling of the face or throat and difficulty breathing.

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.¹³

If nausea or vomiting is persistent or you cannot keep fluids down, contact your clinical team — dehydration needs prompt attention. You can report any suspected side effect through the Yellow Card scheme.¹⁹

06 · Face, hair and mood

Hair, face, skin and mood

Temporary hair shedding is a recognised side effect, linked to rapid weight loss itself rather than a direct drug effect. For most people it settles as weight stabilises, and balanced nutrition with enough protein matters.¹

"Ozempic face" is not a drug effect. It is what losing a significant amount of weight quickly can do to facial volume, whatever the method. The same applies to loose skin. Adequate protein, resistance exercise and a pace guided by your prescriber help protect lean mass.¹

On mood, the EMA's safety committee reviewed the evidence and concluded in 2024 that it does not support a causal link with suicidal thoughts, 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. For pregnancy questions, see pregnancy while on weight loss injections.

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.