Mounjaro (tirzepatide) is a dual GIP/GLP-1 receptor agonist licensed in the UK for type 2 diabetes mellitus, offering significant glycaemic control and weight reduction. Whilst generally well-tolerated, Mounjaro can cause adverse effects ranging from common gastrointestinal disturbances to rare serious complications such as acute pancreatitis. Accurate ICD-10 coding of Mounjaro adverse effects is essential for clinical documentation, pharmacovigilance, and patient safety monitoring within UK healthcare settings. This article provides comprehensive guidance on recognising, classifying, and coding Mounjaro-related adverse effects using WHO ICD-10 codes, alongside MHRA reporting requirements and clinical management considerations for UK practitioners.

Understanding Mounjaro (Tirzepatide) and Adverse Effects

Mounjaro (tirzepatide) is a novel glucose-dependent insulinotropic polypeptide (GIP) and glucagon-like peptide-1 (GLP-1) receptor agonist licensed in the UK for the treatment of type 2 diabetes mellitus in adults. Tirzepatide has been recommended by NICE (TA870) for adults with type 2 diabetes as part of a triple therapy regimen or as an alternative to insulin when specific HbA1c thresholds and BMI criteria are met. This dual-action therapy enhances glycaemic control and promotes weight reduction.

According to the UK Summary of Product Characteristics (SmPC), tirzepatide works by stimulating insulin secretion in a glucose-dependent manner, suppressing glucagon release, slowing gastric emptying, and reducing appetite through central nervous system pathways. These pharmacological effects contribute to improved HbA1c levels and significant weight loss, making Mounjaro a valuable option for patients with inadequate glycaemic control on conventional therapies.

However, like all medications, Mounjaro is associated with a spectrum of adverse effects ranging from mild gastrointestinal disturbances to rare but serious complications. Accurate documentation of adverse effects using ICD-10 codes is essential in UK clinical practice for several reasons: it facilitates systematic monitoring, supports pharmacovigilance activities, and contributes to national and international drug safety databases. In UK secondary care, the WHO ICD-10 4-character codes are used for diagnoses, while primary care typically uses SNOMED CT for clinical documentation.

Clinicians prescribing Mounjaro must be familiar with both the clinical presentation of adverse effects and their appropriate documentation. This knowledge ensures comprehensive patient care, appropriate risk stratification, and timely intervention when complications arise.

Common Adverse Effects of Mounjaro and Their ICD-10 Codes

The most frequently reported adverse effects of Mounjaro are gastrointestinal in nature, reflecting the drug's mechanism of slowing gastric emptying and its effects on the gastrointestinal tract. According to the UK SmPC, these effects are typically dose-dependent and often diminish over time as patients develop tolerance.

Nausea is the most common adverse effect, occurring in approximately 15–25% of patients during clinical trials. This can be coded as R11.0 (Nausea) or R11.2 (Nausea with vomiting) when both symptoms are present. When coding adverse drug reactions in the UK, the clinical manifestation should be followed by a Y-code to indicate the medication involved (e.g., Y42.3 for adverse effects of insulin and oral hypoglycaemic drugs).

Diarrhoea affects approximately 13–16% of patients and is typically coded as R19.7 (Diarrhoea, unspecified) plus Y42.3. Persistent or severe diarrhoea warrants assessment for dehydration and electrolyte disturbances, particularly in elderly patients or those with comorbidities.

Vomiting (coded as R11.1 for vomiting alone, plus Y42.3) occurs in 8–12% of patients and may necessitate temporary dose reduction or treatment interruption if severe. Constipation (K59.0 plus Y42.3) is reported in approximately 6–8% of patients, while dyspepsia (K30 plus Y42.3) and abdominal pain (R10.4 for unspecified abdominal pain, plus Y42.3) are also documented.

Decreased appetite (R63.0 plus Y42.3) is common and contributes to the weight loss effect but should be monitored to ensure adequate nutritional intake. Injection site reactions are generally mild, presenting as erythema, pruritus, or induration at the subcutaneous injection site. These reactions typically resolve spontaneously without intervention and should be coded according to the specific manifestation (e.g., L50.9 for urticaria if present) plus Y42.3.

Serious Adverse Effects: ICD-10 Classification and Clinical Recognition

While most adverse effects of Mounjaro are mild to moderate, clinicians must remain vigilant for serious complications that require immediate clinical attention and appropriate ICD-10 documentation.

Acute pancreatitis is a rare but potentially life-threatening adverse effect associated with GLP-1 receptor agonists, including tirzepatide. Patients presenting with severe, persistent abdominal pain radiating to the back, accompanied by nausea and vomiting, require urgent assessment. This condition is coded as K85.3 (Drug-induced acute pancreatitis) with Y42.3 to indicate an adverse effect of antidiabetic medication. Mounjaro should be discontinued immediately if pancreatitis is suspected, and patients should not be rechallenged with the medication.

Hypoglycaemia risk increases when Mounjaro is used in combination with insulin or sulfonylureas. Severe hypoglycaemia can be coded as E11.0 (Type 2 diabetes mellitus with coma) when hypoglycaemic coma occurs, or E16.0 (Drug-induced hypoglycaemia without coma) with Y42.3 to indicate the causative medication. Patients should be educated about hypoglycaemia symptoms and the importance of glucose monitoring, particularly during dose titration.

Acute kidney injury (N17.9 plus Y42.3) may occur secondary to severe dehydration from gastrointestinal adverse effects. According to the UK SmPC, no dose adjustment is required in renal impairment; however, renal function should be monitored if severe gastrointestinal adverse reactions lead to dehydration. Diabetic retinopathy may be transiently worsened with rapid glycaemic improvement; patients should continue with their routine diabetic eye screening programme in line with NICE guidance (NG28).

Hypersensitivity reactions, including anaphylaxis (T78.2 plus Y42.3) and angioedema (T78.3 plus Y42.3), though rare, constitute medical emergencies requiring immediate treatment discontinuation and appropriate emergency management. Cholelithiasis (K80.2 plus Y42.3) and cholecystitis (K81.9 plus Y42.3) may develop due to rapid weight loss and altered gallbladder motility.

ICD-10 Coding for Mounjaro-Related Complications in UK Clinical Practice

Accurate ICD-10 coding for Mounjaro-related adverse effects serves multiple purposes within the UK healthcare system, including clinical documentation, epidemiological surveillance, and pharmacovigilance. Proper coding requires understanding both the adverse effect itself and the context of drug therapy.

In UK clinical coding practice, adverse effects of medications in therapeutic use follow a specific pattern according to NHS England National Clinical Coding Standards (ICD-10). The primary diagnosis code should reflect the clinical manifestation (e.g., K85.3 for drug-induced pancreatitis), followed by an external cause code from the Y40–Y59 range to indicate the medication involved. For Mounjaro and other antidiabetic medications, Y42.3 (Adverse effects of insulin and oral hypoglycaemic drugs) is typically used.

For gastrointestinal complications, appropriate codes include:

• Unspecified diarrhoea: R19.7 + Y42.3

• Nausea: R11.0 + Y42.3

• Vomiting: R11.1 + Y42.3

• Nausea with vomiting: R11.2 + Y42.3

• Constipation: K59.0 + Y42.3

Dehydration (E86.0 + Y42.3) should be coded when present secondary to vomiting or diarrhoea. When weight loss is excessive or concerning, R63.4 (Abnormal weight loss) + Y42.3 should be documented alongside the primary diabetes code.

For more serious complications:

• Drug-induced acute pancreatitis: K85.3 + Y42.3

• Drug-induced hypoglycaemia without coma: E16.0 + Y42.3

• Type 2 diabetes with hypoglycaemic coma: E11.0 + Y42.3

• Acute kidney injury: N17.9 + Y42.3

Clinical coders should reference the NHS England National Clinical Coding Standards (ICD-10) and the WHO ICD-10 (UK version) to ensure compliance with national requirements and optimise clinical documentation quality.

Reporting and Monitoring Adverse Effects: Yellow Card Scheme and MHRA Guidance

The Yellow Card Scheme, operated by the MHRA and the Commission on Human Medicines (CHM), is the UK's system for collecting information on suspected adverse drug reactions. Healthcare professionals and patients are encouraged to report any suspected adverse effects associated with Mounjaro, particularly as it is a relatively new medication requiring enhanced pharmacovigilance.

All suspected adverse reactions to Mounjaro should be reported, especially:

• Serious reactions, regardless of whether they are well-recognised

• Reactions resulting in hospitalisation or prolonged hospitalisation

• Suspected reactions causing congenital abnormalities

• Any reaction not previously documented in the Summary of Product Characteristics

Reports can be submitted online at www.mhra.gov.uk/yellowcard, via the Yellow Card app, or by telephone. The scheme accepts reports from healthcare professionals, patients, and carers, recognising that patient-reported outcomes provide valuable real-world safety data.

MHRA guidance emphasises the importance of monitoring patients during the initial months of Mounjaro therapy, particularly during dose escalation. Clinicians should:

• Monitor renal function if severe gastrointestinal adverse effects lead to dehydration

• Educate patients about symptoms of pancreatitis and the need for immediate medical attention

• Review concomitant medications that may increase hypoglycaemia risk

• Be aware that rodent studies showed C-cell thyroid tumours, though human relevance is unknown; routine calcitonin or thyroid ultrasound screening is not recommended, but patients should report symptoms such as neck mass, dysphagia, or persistent hoarseness

According to the UK SmPC, Mounjaro is not indicated for use in children or adolescents under 18 years. It is not recommended during pregnancy or breastfeeding.

Patient safety advice should include clear instructions on recognising adverse effects, when to seek medical attention, and the importance of maintaining adequate hydration. Patients should be provided with written information and encouraged to report any concerns promptly. Regular follow-up appointments enable early detection of complications and optimisation of treatment regimens.

Frequently Asked Questions