(neonate to 16 years old) should always be undertaken with care. There are significant differences between children and adults in terms of pharmacokinetics, pharmacodynamics and practical aspects of drug treatment. Children should not be regarded simply as small adults as dosing requires calculation based on weight, age, surface area and renal function. Prescribing for children comes with significant risks of errors and potential harm and therefore should not be carried out if inexperienced without senior medical or specialist pharmacy support.
Pharmacokinetics
A variety of factors influence pharmacokinetics in the young. Drug absorption may be affected by relatively high gastric pH in infants, and topical preparations are better absorbed because the skin is relatively thin. Low body fat content may reduce the volume of distribution of lipophilic drugs, and lower plasma albumin concentration reduces plasma protein binding. Immaturity of neonatal hepatic enzyme systems and renal excretion mechanisms may impair drug elimination, necessitating dosage reduction in the first months of life.
Pharmacodynamics
The differential effects of some drugs in children compared with adults are well recognised, an example being the sedative effect of stimulant amphetamine derivatives in children, a paradoxical effect used in the management of ADHD. Although effects such as this are recognised, the mechanisms are poorly understood.
Practical aspects of prescribing in children
Medicine has no benefit if the child refuses to take prescribed substances due to unpleasant taste, bulky formulation or lack of understanding of the necessity to take the prescription. Many children's medicines contain excipients, aiming to make them more palatable. However, less used medicines may not. Discussion with parents regarding reward style encouragement or offering advice as to food/drinks which the medicine can be mixed with to improve the taste may help.
Physical and developmental stages should also be considered. For example, the management of common childhood complaints like asthma can be complicated by the need to acquire an effective inhaler technique, which younger children find difficult.
For children, social aspects of taking medicine are also important. Being ‘different’ from classmates or friends may impact on adherence to medication, particularly prevalent in children with diabetes requiring insulin. Taking time to explore the child's concerns may prevent potentially life‐threatening complications.
Safety
Safety is paramount when prescribing for children as overdosing and adverse effects can be serious. Abbreviations such as mcg, mg, ng and mL should be avoided and the full instruction written (e.g. micrograms, nanograms or milligrams). This helps to avoid errors, as doses are as likely to be ng as they are mg depending on the weight of the child and can be easily misread. One vial of drug may contain enough active product to overdose a child by 10 or 100 fold. For example, a vial of morphine sulphate contains 10 mg (10 mL) of substance. In a neonate, the recommended dose is 5–20 μg/kg/h, which in a normal 3 kg baby would only equate to a 15–60 μg (0.15–0.6 mL) dose. Therefore, one vial contains enough solution to significantly overdose the child.
Failure to calculate maximum daily allowances of drugs in children is another common reason for errors. After calculating the individual dose for a child, the daily dose should be correlated against the maximum daily allowance to ensure it does not exceed the allowance for the child's age and weight.
Development and regulation of medicines for children
Young children lack capacity to provide informed consent making trials of medicines in this age group ethically not possible. The majority of medicines available to children are unlicenced or their use is ‘off label’ (i.e. used for a purpose not indicated by the manufacturing label). Most paediatric units have developed their own guidelines and monographs for dosing, taking into account the factors discussed above. Valuable resources such as the BNF for Children are validated against available evidence, and are also helpful in the safe and appropriate management of diseases in childhood.
Prescribing for the elderly
A previously well 78‐year‐old woman is admitted with confusion and urinary frequency. She has a past medical history of myocardial infarction and osteoarthritis. She takes enalapril, Simvastatin, aspirin, ibuprofen and omeprazole regularly. On examination, she appears dehydrated and confused but otherwise there is nothing unusual. Blood results showed a raised white cell count, c‐reactive protein, urea and creatinine (eGFR = 30). You are asked to complete her prescription chart taking into account the current clinical situation and to prescribe nitrofurantoin to treat the urinary tract infection.
Introduction
By 2033, it is estimated 20% of the UK population will be 65 or over. Currently 60% of prescriptions from community pharmacists in the UK are for the elderly, a trend that is likely to rise. Evidence‐based medicine has led to an increased prescription of medications intended for prognostic benefit. With an increased prevalence of chronic disease in the elderly, this has resulted in polypharmacy. This poses a new challenge for prescribers.
Drug absorption, distribution, metabolism, excretion and activity all change with age, but the presence of multiple pathologies frequently has a greater effect than ageing alone.
Adverse effects from medications are three times as likely in the elderly than in the general adult population. Unwanted side effects in this population are more often dose related than idiosyncratic and attributable to altered pharmacokinetics and pharmacodynamics. The highest incidence of side effects documented is caused by some of the most commonly prescribed drugs including sedatives, diuretics and NSAIDs.
Drug absorption
Ageing results in increased gastric pH, delayed gastric emptying, decreased intestinal motility and reduced splanchnic blood flow. However, the extent to which these factors alter bioavailability of a drug is thought to be negligible. For example, the rate of absorption of digoxin is slower in the elderly but the overall bioavailability remains the same.
Drug distribution
Age‐related changes in body composition, protein binding and organ blood flow can all have significant effects on drug disposition. Ageing increases the composition of body fat to water. Therefore, initial drug concentrations of water‐soluble drugs such as digoxin and cimetidine can be increased. Lipid‐soluble drugs such as diazepam will have an increased volume of distribution, lower plasma concentration and prolongation of the elimination half‐life resulting in a prolonged effect. Plasma protein–drug binding changes little with age.
Drug metabolism and age
Age‐related changes such as reduced blood flow and loss of hepatic volume can cause a reduction in oxidising capacity and therefore altered metabolism of some drugs. It is thought, however, that these changes are clinically, minimally significant. Drugs that undergo microsomal oxidation, such as chlordiazepoxide are likely to be metabolised slower, but the evidence is for this is not established. Conjugation pathways appear unaffected by age.
First‐pass
