Mounjaro side effects: what’s common, what’s normal, and when to get help
Most Mounjaro side effects are stomach and digestion related and are most noticeable when you start or increase the dose. Many people find symptoms settle as the weeks go on, but it helps to know what’s expected and what needs medical advice.
Common Mounjaro side effects
- Nausea
- Reduced appetite / feeling full quickly
- Burping, bloating or wind
- Indigestion or reflux (heartburn)
- Constipation or diarrhoea
- Stomach discomfort or cramps
- Vomiting (less common, but can happen)
- Headache
- Tiredness or fatigue
Less common side effects
- Dizziness
- Injection-site irritation (redness, itching, tenderness)
- Feeling “off food” or changes in taste
Good to know: Side effects are more likely if you eat quickly, eat large portions, or have very fatty or rich meals—especially around injection day.
When to contact a doctor urgently
Seek urgent medical advice the same day if you have any of the following:
- Severe or persistent upper stomach/abdominal pain, especially if it spreads to your back or comes with vomiting
- Repeated vomiting or you can’t keep fluids down
- Signs of dehydration (very dark urine, dizziness, fainting, not peeing much)
- Black stools or vomiting blood
- Severe allergic reaction (swelling of face/lips/tongue, breathing difficulty, widespread rash)
Quick overview: side effects by stage
| Stage | What you might notice | What often helps |
|---|---|---|
| Day 1–7 | Nausea, burping, reduced appetite, reflux, constipation or looser stools | Smaller meals, eat slowly, sip fluids, avoid greasy/spicy foods near injection day |
| Weeks 2–4 | Symptoms often ease, but constipation/reflux can linger | Keep hydration steady, add fibre gradually, keep evening meals lighter |
| After dose increases | Short flare of nausea/reflux/burping or bowel changes | Keep meals plain for a few days, smaller portions, prioritise protein, steady fluids |
Tip: If symptoms reliably flare after each increase, note the pattern and discuss whether staying longer on the current dose is a better plan.
How long do Mounjaro side effects last?
Many people notice side effects most in the first few days after starting and for a few days after dose increases. Symptoms often improve within days to a couple of weeks as your body adjusts.
- Nausea / reflux: often improves as you learn which foods trigger symptoms and as your dose settles.
- Constipation: may persist if fluids and fibre drop—manage early with steady hydration and gradual fibre.
- Diarrhoea: is often short-lived but needs attention to hydration.
If side effects are severe, persistent, or worsening, or you cannot keep fluids down, seek medical advice promptly.
How to order Mounjaro with an online doctor
Step 1: complete a short medical questionnaire with an online doctor in Ireland, such as Zava.
Step 2: if prescribing Mounjaro is appropriate, a prescription is issued and sent to your chosen pharmacy via Healthmail. If you choose Healthwave, the prescription can be addressed to healthwave.dundrum@healthmail.ie.
Step 3: the pharmacy confirms availability, explains pen use and storage. If you choose Healthwave, we pack discreetly and deliver typically within 1–2 working days, or arrange convenient collection.
Prefer to speak directly with a doctor?
Easily book a video consultation with a registered doctor via Eirdoc.
Prefer to see a doctor in person?
For medicine specific details, see our page on Mounjaro.
Week 1 to 4 on low dose Mounjaro: settling in and nausea management
The first dose is usually low so you can adapt. Nausea, burping, reduced appetite, constipation or diarrhoea are common at the start. Eat slowly, keep water intake steady and choose smaller meals. Many people find soup, yoghurt, porridge, eggs, beans, fish or chicken with vegetables easier than heavy meals.
Spacing coffee, spicy foods and alcohol away from the injection day can help. If reflux or constipation appears, ask a pharmacist about suitable over the counter options. If vomiting, dehydration or severe tummy pain occurs, contact a doctor promptly.
Week 5 to 8: increasing the dose and preventing reflux
As your dose rises, reflux or a feeling of fullness can return. Keep portions small, avoid very high fat meals and try to eat earlier in the evening. Consider planning the next day’s meals so choices stay gentle while you adjust.
Bring questions to your review and share patterns you have noticed. If symptoms flare with each increase, ask whether pausing at the current dose for longer is a safer plan before moving up.
Week 9 to 12: listening to appetite and protecting hydration
Appetite usually falls further. Focus on protein, fibre and fluids rather than very low calories. Add beans, lentils, eggs, fish or chicken and plenty of vegetables. If you are losing weight quickly or feel lightheaded, raise this with your doctor.
Note any medicines that can irritate the stomach and take them with food if appropriate. If you develop persistent severe tummy pain, black stools or repeated vomiting, stop treatment and seek urgent care.
Beyond week 12: maintenance dose and long term comfort
Once you find a comfortable dose, side effects often settle. Continue gentle activity, steady sleep routines and a simple meal plan. Book regular reviews to check comfort, goals and any other medicines that might need dose changes.
Practical tips to reduce Mounjaro side effects
- Eat slowly, stop before you feel very full and avoid heavy late meals.
- Choose lean protein, whole grains, fruit and vegetables; keep water intake steady.
- Plan injection day meals to be simple and easy to digest.
- Keep a note of symptoms and bring it to reviews.
- Rest and choose light foods if you feel unwell. Seek medical advice if symptoms are severe or persistent.
When to contact a doctor during treatment
Speak to a doctor if you have severe or persistent tummy pain, repeated vomiting, signs of dehydration or black stools. Discuss planned surgery or deep sedation in advance due to delayed stomach emptying on GLP-1 treatment.
Supports available in Ireland
- HPRA, authorised medicine record: Mounjaro
- HSE, medicines for obesity
- HSE, obesity treatment overview
FAQs
When do Mounjaro side effects start?
Some people notice symptoms after the first injection, while others mainly notice side effects after the first dose increase. Side effects are usually most noticeable in the first few days of a new dose.
What are the most common Mounjaro side effects?
The most common side effects are nausea, reduced appetite, burping/bloating, reflux, constipation or diarrhoea, and stomach discomfort. Headache and tiredness can also happen, especially early on.
What helps with Mounjaro nausea?
Eat smaller meals, eat slowly, avoid very fatty or rich foods, and sip fluids regularly. Plain foods (e.g., yoghurt, porridge, soup, eggs, crackers, bananas) are often easier on nausea days.
Do side effects get worse after dose increases?
They can. A dose increase often causes a short-term flare of nausea/reflux/burping or bowel changes. For many people, this settles within days to a couple of weeks.
How long do Mounjaro side effects last?
They usually settle within days to weeks at each dose. If symptoms remain or are severe, contact a doctor to review dose pacing or supportive treatment.
Can I drink alcohol on Mounjaro?
Alcohol can worsen nausea and reflux. If you choose to drink, keep amounts small and avoid drinking near your injection day while you adjust.
What should I eat if I feel nauseous?
Light choices such as soup, yoghurt, porridge, crackers, bananas or eggs are usually easier than fried or spicy meals.
Where can I see current prices of Mounjaro in Ireland?
Visit Mounjaro for our up to date pricing.
If you would like to check suitability, you can fill in a quick questionnaire with our online doctor partner, Zava.
Need pharmacist advice today? Ask a Pharmacist.
