Mounjaro Risks: What You Need to Know Before Starting
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Mounjaro (tirzepatide) is approved by the MHRA in the UK for weight management and has strong clinical evidence behind it. At the maximum 15mg maintenance dose over 72 weeks, the average weight loss was 22.5% in the SURMOUNT-1 trial. But like all medications, it carries risks. Here is an honest breakdown of what they are and how significant each one is.
Key Facts
- The most common side effects are gastrointestinal and affect more than 1 in 10 people — but usually improve over time
- Serious side effects including pancreatitis and gallbladder problems are less common but require awareness
- Mounjaro is contraindicated in people with a personal or family history of medullary thyroid carcinoma or MEN 2
- Nutritional deficiencies are one of the most common and most manageable risks during treatment
Common Side Effects: What Most People Experience
The most frequently reported side effects of Mounjaro are gastrointestinal. They are most pronounced when starting treatment or after a dose increase, and tend to improve as the body adjusts.
| Side effect | How common | When it tends to occur |
|---|---|---|
| Nausea | More than 1 in 10 | Early treatment, dose increases |
| Vomiting | More than 1 in 10 | Early treatment, dose increases |
| Diarrhoea | More than 1 in 10 | Early treatment, dose increases |
| Constipation | More than 1 in 10 | Throughout treatment |
| Decreased appetite | More than 1 in 10 | Throughout treatment |
| Hair loss | Up to 1 in 10 | 2–4 months after weight loss begins |
| Fatigue | Up to 1 in 10 | Throughout treatment |
Serious Risks: Less Common but Worth Knowing
⚠️ Pancreatitis
Inflammation of the pancreas has been reported. In clinical trials pancreatitis occurred in approximately 0.32–0.39% of patients across all doses, comparable to placebo. Seek immediate medical attention if you experience severe, persistent abdominal pain.
⚠️ Gallbladder problems
Gallstones have been reported and may occur more often during rapid weight loss. Most are asymptomatic but sudden severe abdominal pain lasting more than a few hours should be assessed by a doctor.
⚠️ Kidney problems
Not a direct effect of tirzepatide on the kidneys, but dehydration caused by vomiting and diarrhoea can put strain on kidney function. Staying hydrated during treatment is important.
⚠️ Allergic reactions
Severe allergic reactions are rare but possible. Symptoms include rapid swelling of the lips, tongue or throat, breathing difficulties and a fast heartbeat. Seek emergency medical help immediately if these occur.
Thyroid Risk: What the Evidence Actually Says
You may have seen references to thyroid cancer risk in relation to Mounjaro. This requires some context.
In pre-clinical studies conducted in rats, tirzepatide caused thyroid C-cell tumours. However, there have been no confirmed cases of thyroid cancer linked to Mounjaro in humans. The human relevance of the rodent findings has not been established.
Mounjaro is contraindicated in people with a personal or family history of medullary thyroid carcinoma (MTC) or Multiple Endocrine Neoplasia syndrome type 2 (MEN 2). If you notice a lump or swelling in your neck, hoarseness, difficulty swallowing or shortness of breath, contact your GP promptly.
The Nutritional Risk: Often Overlooked
One of the most consistent and underreported risks of Mounjaro use is nutritional deficiency. Because the medication significantly suppresses appetite, food intake can drop by close to 40%. When calorie intake falls that sharply, so does the intake of essential vitamins and minerals.
13.6%
of GLP-1 users developed vitamin D deficiency after 12 months of treatment — across a review of 480,825 adults
B12, magnesium and calcium deficiencies are also commonly reported. This nutritional risk is one of the most manageable, and targeted supplementation can address it directly.
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Who Should Not Take Mounjaro?
- People with a personal or family history of medullary thyroid carcinoma (MTC)
- People with Multiple Endocrine Neoplasia syndrome type 2 (MEN 2)
- People who are pregnant or breastfeeding
- Children under 10 years of age
- People with a known allergy to tirzepatide or any ingredient in the formulation
Always discuss your full medical history with your prescribing clinician before starting Mounjaro.
What Happens When You Stop?
This is one of the least discussed but most important considerations. Mounjaro manages appetite and metabolic function while you take it. When you stop, appetite typically returns and weight regain is common. Clinical data shows that people who discontinue GLP-1 medications tend to regain a significant portion of lost weight within 12 months.
This does not make Mounjaro a bad option. It means it should be understood as an ongoing treatment for a chronic condition rather than a short-term fix. The decision to start should factor in long-term use.
Frequently Asked Questions
- Is Mounjaro safe to take long term?
- Mounjaro is approved by the MHRA for use in the UK and has undergone rigorous clinical testing. The longest trials ran to 72 weeks. Long-term data beyond that is still being collected. It should always be used under medical supervision.
- Who should not take Mounjaro?
- Mounjaro is contraindicated in people with a personal or family history of medullary thyroid carcinoma or Multiple Endocrine Neoplasia syndrome type 2. It is also not recommended during pregnancy or breastfeeding.
- What are the most common side effects of Mounjaro?
- The most common side effects, affecting more than 1 in 10 people, are nausea, vomiting, diarrhoea and constipation. These are most pronounced when starting treatment or increasing the dose and tend to improve over time.
- Does Mounjaro cause nutritional deficiencies?
- Yes, reduced food intake on Mounjaro can lead to deficiencies in vitamin D, B12, magnesium and other key nutrients. This is one of the more manageable risks and can be addressed with targeted supplementation.
- What happens when you stop taking Mounjaro?
- Clinical data shows that people who stop GLP-1 medications tend to regain a significant portion of lost weight within 12 months. This suggests the medication manages the condition rather than curing it, similar to other chronic disease treatments.
Sources
- Superdrug Online Doctor. Is Mounjaro Safe? Side Effects and Long-Term Use Risks. 2026.
- Drugs.com. Mounjaro Side Effects: Common, Severe, Long Term. 2026.
- PMC. Pancreatitis in Patients Receiving Tirzepatide: A One-Year Audit in a UK District General Hospital. 2025.
- Urbina et al. (2026). Micronutrient and Nutritional Deficiencies Associated With GLP-1 Receptor Agonist Therapy. Clinical Obesity.
- MHRA. Mounjaro (tirzepatide) authorised for weight management in the UK.