Mounjaro (tirzepatide) is a dual GIP and GLP-1 receptor agonist licensed in the UK for type 2 diabetes and weight management. Whilst gastrointestinal side effects such as nausea and diarrhoea are well-documented, questions occasionally arise about whether Mounjaro causes anaemia. This article examines the current evidence linking tirzepatide to anaemia, explores why such concerns may emerge during weight loss treatment, and provides guidance on recognising symptoms and seeking appropriate medical advice. Understanding the established safety profile of Mounjaro helps patients and clinicians distinguish between medication-related effects and coincidental health changes.

Understanding Mounjaro and Its Common Side Effects

Mounjaro (tirzepatide) is a prescription medication licensed in the UK for the treatment of type 2 diabetes mellitus and, more recently, for weight management in adults with obesity (BMI ≥30 kg/m²) or overweight (BMI ≥27 kg/m²) with at least one weight-related comorbidity. It is prescribed as an adjunct to a reduced-calorie diet and increased physical activity. Mounjaro belongs to a novel class of medicines known as dual glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonists. By mimicking the action of these naturally occurring incretin hormones, Mounjaro enhances insulin secretion when blood glucose levels are elevated, suppresses glucagon release, slows gastric emptying, and reduces appetite—collectively leading to improved glycaemic control and significant weight loss.

As with all medications, Mounjaro is associated with a range of side effects, the majority of which are gastrointestinal in nature. The most commonly reported adverse effects according to the UK Summary of Product Characteristics (SmPC) include:

• Nausea and vomiting

• Diarrhoea or constipation

• Abdominal pain and discomfort

• Reduced appetite

• Dyspepsia (indigestion)

• Injection site reactions

These gastrointestinal symptoms are typically mild to moderate in severity and tend to diminish over time as the body adjusts to the medication. Dose escalation is gradual (starting at 2.5mg weekly and increasing in 2.5mg increments) to help minimise these effects. Less common but more serious adverse reactions can include pancreatitis, gallbladder disease (including cholelithiasis), hypoglycaemia (particularly when used with other glucose-lowering agents), dehydration potentially leading to acute kidney injury, and allergic reactions.

The Medicines and Healthcare products Regulatory Agency (MHRA) and the European Medicines Agency (EMA) continue to monitor the safety profile of tirzepatide as real-world data accumulates. Patients who suspect they may be experiencing side effects from Mounjaro should report them via the MHRA Yellow Card Scheme (yellowcard.mhra.gov.uk or the Yellow Card app).

Understanding the established side effect profile of Mounjaro is essential when evaluating whether new or unexpected symptoms—such as anaemia—might be related to the medication or arise from other causes.

The Link Between Mounjaro and Anaemia: What the Evidence Shows

There is currently no official, established link between Mounjaro (tirzepatide) and the development of anaemia. Anaemia—defined as a reduction in haemoglobin concentration or red blood cell mass—has not been identified as a recognised adverse effect in the UK Summary of Product Characteristics (SmPC) for Mounjaro, nor has it been highlighted as a safety signal in the European Medicines Agency's European Public Assessment Report (EPAR).

The pivotal clinical trials for tirzepatide, including the SURPASS and SURMOUNT programmes, involved thousands of participants and monitored haematological parameters. Based on available regulatory reviews, these studies did not identify anaemia as an adverse reaction attributable to tirzepatide. The published trial data and regulatory assessments do not suggest that tirzepatide treatment is associated with clinically significant changes in haemoglobin levels or red blood cell indices.

It is important to recognise that the absence of evidence is not absolute proof that a relationship cannot exist in rare cases or specific subpopulations. Pharmacovigilance systems continue to collect data on all reported adverse events, and regulatory authorities remain vigilant for emerging safety concerns. However, based on current evidence, if anaemia develops in a patient taking Mounjaro, it is more likely to be coincidental or related to other factors rather than a direct pharmacological effect of tirzepatide.

Patients and healthcare professionals should be reassured that anaemia is not a known or expected consequence of Mounjaro therapy. Any new diagnosis of anaemia warrants thorough investigation to identify the underlying cause, which may include nutritional deficiencies, chronic disease, gastrointestinal blood loss, or other medical conditions unrelated to the medication.

Why Anaemia Concerns Arise with Weight Loss Medications

Despite the lack of direct evidence linking Mounjaro to anaemia, concerns about this condition can arise in the context of weight loss medications for several indirect reasons. Understanding these mechanisms helps clarify why patients or clinicians might question a possible association.

Nutritional deficiencies are a primary concern with any intervention that results in significant weight loss or reduced food intake. Mounjaro's appetite-suppressing effects and gastrointestinal side effects—particularly nausea, vomiting, and early satiety—can lead to substantially decreased caloric and nutrient intake. If dietary intake becomes inadequate or unbalanced, patients may develop deficiencies in:

• Iron: Essential for haemoglobin synthesis; deficiency leads to iron-deficiency anaemia

• Vitamin B12: Required for red blood cell maturation; deficiency causes megaloblastic anaemia

• Folate: Also necessary for DNA synthesis in red blood cells

• Vitamin C: Enhances iron absorption; deficiency can indirectly contribute to anaemia

Patients who experience prolonged gastrointestinal symptoms or adopt restrictive eating patterns while taking Mounjaro may benefit from dietary assessment and potentially dietitian support. Additionally, rapid weight loss itself—regardless of the method—can sometimes unmask pre-existing nutritional inadequacies or chronic conditions that were previously compensated.

Other common causes of anaemia in this patient population should be considered. Many patients with type 2 diabetes take metformin, which can reduce vitamin B12 absorption with long-term use. Heavy menstrual bleeding is another frequent cause of iron deficiency anaemia in women of reproductive age. Gastrointestinal conditions such as peptic ulcer disease, inflammatory bowel disease, or malignancy can cause chronic blood loss leading to anaemia—these are not related to tirzepatide but may be part of the differential diagnosis.

Finally, patients taking weight loss medications often have multiple comorbidities—including type 2 diabetes, cardiovascular disease, and chronic kidney disease—that independently increase anaemia risk. Disentangling medication effects from disease-related factors requires careful clinical assessment.

Recognising Symptoms of Anaemia While Taking Mounjaro

Anaemia can develop gradually, and its symptoms may be subtle, particularly in the early stages. Patients taking Mounjaro should be aware of the signs and symptoms of anaemia so they can seek timely medical evaluation if concerns arise. Common symptoms of anaemia include:

• Fatigue and weakness: Persistent tiredness that does not improve with rest

• Pallor: Pale skin, mucous membranes, or conjunctivae

• Shortness of breath: Particularly on exertion or during activities previously well-tolerated

• Dizziness or light-headedness: Especially when standing up quickly

• Palpitations: Awareness of a rapid or irregular heartbeat

• Headaches: Often accompanied by difficulty concentrating

• Cold hands and feet: Due to reduced oxygen delivery to peripheral tissues

In more severe cases, patients may experience chest pain, particularly if they have underlying cardiovascular disease, as the heart works harder to compensate for reduced oxygen-carrying capacity.

It is important to note that many of these symptoms can overlap with common side effects of Mounjaro itself or with the metabolic changes associated with weight loss and improved glycaemic control. For example, fatigue might be attributed to caloric restriction, and dizziness could result from dehydration or postural hypotension. This overlap can make it challenging to identify anaemia based on symptoms alone.

Patients should not attempt to self-diagnose anaemia. If any of these symptoms develop or worsen while taking Mounjaro—particularly if they are persistent, progressive, or accompanied by other concerning features—it is essential to consult a healthcare professional. A simple blood test (full blood count) can quickly determine whether anaemia is present and guide further investigation into the underlying cause.

Seek urgent medical attention (call 999 or go to A&E) if you experience:

• Severe chest pain or shortness of breath at rest

• Fainting (syncope)

• Black, tarry stools (melena)

• Vomiting blood (haematemesis)

• Severe dizziness with inability to stand

When to Seek Medical Advice About Anaemia and Mounjaro

Patients taking Mounjaro should seek medical advice promptly if they experience symptoms suggestive of anaemia, particularly if these symptoms are new, persistent, or worsening. While anaemia is not a recognised side effect of tirzepatide, any unexplained change in health status warrants clinical evaluation to ensure appropriate diagnosis and management.

Specific situations that should trigger contact with a GP or healthcare provider include:

• Persistent or severe fatigue that interferes with daily activities

• Noticeable pallor or comments from others about looking pale or unwell

• Unexplained shortness of breath, particularly if it represents a change from baseline

• Dizziness, fainting, or near-fainting episodes

• Chest pain or palpitations, especially in patients with known cardiovascular disease

• Signs of gastrointestinal bleeding, such as black, tarry stools or visible blood in vomit or stool

Healthcare professionals will typically arrange a full blood count (FBC) to assess haemoglobin levels, red blood cell indices, and other haematological parameters. If anaemia is confirmed, further investigations will be guided by the type of anaemia identified. These may include:

• Iron studies: Serum ferritin, transferrin saturation, and total iron-binding capacity

• Vitamin B12 and folate levels: To identify megaloblastic anaemia

• Renal function tests: Chronic kidney disease is a common cause of anaemia in patients with diabetes

• Coeliac serology: Testing for coeliac disease is recommended in unexplained iron deficiency anaemia

• Gastrointestinal investigations: If iron deficiency anaemia is confirmed without an obvious cause, referral for endoscopy or other imaging may be required under the NICE suspected cancer referral pathway (NG12)

According to NICE guidance, the management of anaemia depends on identifying and treating the underlying cause. If nutritional deficiencies are identified, appropriate supplementation should be initiated alongside dietary advice. Patients should continue taking Mounjaro as prescribed unless specifically advised otherwise by their healthcare provider. Do not stop tirzepatide unless advised; clinicians will decide based on cause and severity. Urgent cessation may be required if serious adverse effects (such as suspected pancreatitis or severe dehydration) are suspected. Open communication between patients and healthcare teams ensures that any concerns are addressed promptly and that treatment remains safe and effective.

Remember to report any suspected side effects from Mounjaro via the MHRA Yellow Card Scheme (yellowcard.mhra.gov.uk or the Yellow Card app).

Frequently Asked Questions