Medication-related considerations in childhood obesity 

Childhood obesity is a major public health challenge in the UK, with important implications for medicines use and safety. In England, 12% of children aged 2–10 years and 19% of young people aged 11–15 years were classified as obese in 2022^​1​. The latest National Child Measurement Programme data, published in 2024, show obesity rates of 9.6% in reception-aged children (i.e. children aged four to five years) and 22.1% in Year 6 (i.e. children aged 10–11 years)^​2​. Children living in the most deprived areas in England are twice as likely to have obesity as those in the least deprived, with this gap widening over time^​3​.

Childhood obesity is defined using body mass index (BMI), adjusted for age and sex. BMI provides a practical estimate of body fat in children and young people; however, it is not a direct measure of adiposity and should be interpreted with caution^​4,5​. Assessing the BMI of children is more complicated than for adults, owing to changes to BMI as they grow and mature. In addition, there are different growth patterns seen in boys and girls^​5​. A BMI at or above the 91st centile indicates overweight, while a BMI at or above the 98th centile indicates obesity^​6​. The Royal College of Paediatrics and Child Health (RCPCH) BMI chart or World Health Organization growth charts can be used to determine centile placement^​6​.

Obesity alters drug absorption, distribution, metabolism and excretion^​7,8​. These physiological changes increase the risk of both subtherapeutic treatment and adverse drug reactions if standard weight-based dosing is used without appropriate adjustment. However, many commonly used references, such as British National Formulary for Children (BNFC)^​9​ and summary of product characteristics (SPCs), do not provide dosing guidance for weight extremes, contributing to potential variability in prescribing.

Pharmacists and other healthcare professionals have an important role in optimising medicines use in children with obesity. This article provides a practical overview of the evidence and considerations regarding safe prescribing, with examples of commonly affected medicines, guidance on weight-based calculations and strategies to reduce harm.

How obesity affects drug distribution, metabolism and clearance in children

Obesity causes important alterations in physiology, such as changes in tissue composition, increased circulating blood volume and cardiac output, altered regional flow distribution, and impaired liver and kidney function^​10​. All of these physiological alterations can affect pharmacokinetic (PK) parameters, including drug absorption, volume of distribution (Vd), metabolism and elimination^​11​. As a result, the physiochemical properties of a drug, such as lipid solubility or relative protein binding, might have different effects on drug PK in obese children compared with non-obese children^​10​.

Drug distribution

Factors that affect drug distribution include body composition, regional blood flow and plasma protein binding^​12​. Paediatric patients have different body compositions compared with adults (i.e. more water, less fat), making the impact of obesity more complex and age-dependent^​11​.

Lipophilic drugs accumulate in adipose tissue. As a result, obese children with higher volumes of adipose tissue have an increased Vd for lipophilic drugs (e.g. midazolam, diazepam and propofol)^​12​. This necessitates a larger loading dose but may also result in delayed clearance and a prolonged half-life. Conversely, hydrophilic drugs (e.g. aminoglycosides) scarcely distribute into fatty tissue^​13​. Using total body weight (TBW) as a metric for dosing can, therefore, predispose to an overestimation of Vd in hydrophilic drugs and a risk of overdosing. Loading doses are based on Vd, and the larger the Vd, the higher the initial dose required to achieve therapeutic plasma concentrations^​14​. However, this should be assessed on a case-by-case basis, as the relationship between a drug’s lipophilicity and its distribution to adipose tissue is not always consistent and predictable — especially for highly lipophilic drugs^​14​.

Metabolism

The impact of obesity on drug metabolism differs greatly depending on the metabolic pathway involved^​15​. Obesity may alter the activity of both phase I and phase II reactions; however, its effect on the expression and function of CYP enzymes is inconclusive, with the exception of CYP3A4 and CYP2E1^​6​. The activity of CYP3A4 — which is responsible for over 50% of phase I reactions — is reduced in patients who are obese^​15​. Expression of CYP2E1 has been reported to be increased in these patients^​6​. However, limited data exist about phase I and phase II metabolism in obese children^​16​. It is, therefore, necessary to exercise caution with dosing in children with obesity. 

Clearance

For drugs that are eliminated renally, the effect of obesity on glomerular filtration and renal clearance is important^​10​. Obesity increases the glomerular filtration rate by increasing renal blood flow^​10​, which has an impact on renally excreted drugs, leading to increased clearance. Obesity can also lead to an increase in hepatic blood flow. This can cause faster elimination and increased clearance for some drugs, leading to reduced efficacy^​17​. Clearance is the most important factor when calculating maintenance doses. 

It is important to acknowledge the limited availability of pharmacokinetic studies focusing on individual drugs in obese children, as this population is often underrepresented in clinical trials during drug development^​6​. As a result, accurately determining the true impact of childhood obesity on the pharmacokinetic parameters outlined above remains challenging.

Limitations of current dosing references in accounting for weight extremes

Overall, current evidence and resources lack the flexibility and precision required for safe dosing in children at extreme ends of the weight spectrum. As discussed above, for children with obesity, TBW-based dosing may result in inappropriately high doses for lipophilic drugs that accumulate in fat tissue and have prolonged clearance. Conversely, hydrophilic drugs may not distribute widely into fat. TBW-based calculations may overestimate the required dose, increasing the risk of toxicity^​13​. For common measures of weight used when dosing medicines in childhood obesity, see Table 1^​14​.

These dosing references also assume a linear relationship between weight and metabolic maturity, which may not apply to children with obesity or failure to thrive. Obese children may weigh the same as older children but have immature organ function, while underweight children may have more advanced metabolism than expected for their size^​18​. 

Moreover, there is not enough guidance on the use of alternative size descriptors, such as ideal body weight (IBW) or adjusted body weight (AdjBW), which are often more appropriate in these populations. The BNFC has made efforts to incorporate weight descriptors for children at weight extremes; however, the limited evidence base often results in continued reliance on TBW in clinical practice^​6​. Finally, other important physiological factors in obesity, such as increased glomerular filtration rate, are rarely considered, despite their potential to alter drug clearance^​18​.

Table 2 summarises the currently available data for ten medications commonly used in children; however, some of the information has been derived from adult obesity data^​14​. It is intended to generate discussion and for paediatric teams to agree on a consensus locally. For a more complete list, see this table, published by UK Medicines Information (UKMi) and Neonatal and Paediatric Pharmacists Group (NPPG) in 2021^​14​. The list is not exhaustive, and the absence of a medication does not imply that no dose adjustment is needed in children who are obese. The information does not apply to children aged under two years^​6​.

The role of pharmacists 

Pharmacists play an important role in ensuring dosing decisions for children with obesity are appropriate and evidence-based.

Pharmacists can support safe prescribing by:

• Reviewing prescriptions for medicines where dosing is affected by weight; 
• Advising on the most appropriate weight measure to use (e.g. AdjBW may be more suitable for certain medicines)^​19​;
• Highlighting when therapeutic drug monitoring is required and helping interpret results where pharmacokinetics may be altered^​20​;
• Encouraging clear documentation of weight, height and BMI in the patient’s medical record^​4​.

In complex cases, involving the local medicines information team or paediatric pharmacy specialists may be necessary to ensure safe and effective treatment.

Medicines that contribute to weight gain

Some medicines commonly prescribed in childhood are associated with weight gain. In children already living with obesity, certain medicines may worsen existing health problems and increase the risk of future complications, including insulin resistance and cardiovascular disease^​21​.

Medicines known to affect weight include:

• Corticosteroids: for the treatment of conditions such as asthma, nephrotic syndrome, and autoimmune diseases. Long-term use can lead to increased appetite, alter fat distribution and contribute to metabolic changes^​22​;
• Antipsychotics: second-generation antipsychotics (e.g. risperidone, olanzapine) are associated with rapid and significant weight gain, even after short-term use^​23​;
• Antidepressants: some also contribute to weight gain^​24​.

Pharmacists should support prescribers and families in balancing the benefits and risks of treatment. This includes:

• Identifying alternative medicines with lower risks; 
• Recommending baseline and follow-up monitoring of BMI, blood glucose and lipids, in line with national and local guidance; 
• Discussing non-pharmacological interventions, where clinically appropriate;
• Regular medication reviews, including deprescribing, where appropriate.

Language and weight stigma

Children and families affected by obesity may already face stigma or discrimination in daily life, so the way healthcare professionals talk about weight can have a real impact. When negative experiences occur in healthcare settings, they can reduce trust, increase anxiety and lead to disengagement from services^​25​.

Pharmacists should use sensitive, non-judgemental language, focusing on health rather than appearance. For example, using terms such as ‘a child living with obesity’ is more respectful than ‘an obese child’^​26​. Avoid implications of blame, and acknowledge the complex causes of obesity, including genetics, environment and mental health, to help build rapport. 

Where appropriate, pharmacists can also use consultations as opportunities for health promotion, offering advice and signposting to local or national lifestyle support services, such as weight management programmes or physical activity schemes. This should always be done in a respectful and child-centred way, tailored to the family’s readiness to engage.

In some cases, safeguarding concerns may arise. If a child is experiencing severe obesity alongside other signs of neglect, or if parents are unable or unwilling to engage with treatment plans, further assessment or referral to safeguarding teams may be necessary. Pharmacists should be aware of their responsibilities and should not hesitate to seek guidance when required.

Conclusion

Obesity in children complicates safe and effective medication use, owing to its association with changes in drug absorption, distribution, metabolism and clearance. Standard weight-based dosing can lead to under-dosing or over-dosing, increasing the risk of treatment failure or toxicity. Despite this, many dosing references do not offer clear guidance for children at weight extremes, contributing to variability in prescribing. Pharmacists have a vital role in ensuring doses are appropriate, advising on the correct weight measures to use, supporting monitoring (where needed) and promoting clear documentation of weight and BMI. Through close collaboration with prescribers and families, pharmacy professionals can help reduce medicines-related harm and improve outcomes for children living with obesity.