Safe Prescribing in Special Populations

Safe Prescribing in Special Populations

Prescribing safely for special populations requires a deep understanding of lifespan physiology to prevent toxicity, fetal harm, or geriatric adverse drug events.

Pediatric Pharmacotherapy: Developing Systems

Pediatric dosing is highly dynamic due to ongoing organ development. Weight-based dosing (mg/kg/day or mg/kg/dose) is the clinical standard.

• Absorption: Neonates and young infants have a higher gastric pH (neutral to slightly acidic) due to immature parietal cells, increasing the absorption of acid-labile drugs (e.g., penicillin) but decreasing the absorption of weak acids (e.g., phenobarbital). Additionally, neonates have irregular gastric emptying and reduced intestinal motility, leading to unpredictable absorption rates.
• Distribution: Children have a higher total body water percentage (up to 80% in neonates compared to 50–60% in adults). Consequently, water-soluble drugs (e.g., aminoglycosides) distribute into a larger relative volume, requiring higher weight-adjusted doses to achieve therapeutic plasma levels. Conversely, they have lower body fat, reducing the distribution of lipid-soluble drugs. Furthermore, neonatal serum albumin levels are low and have a decreased binding affinity, which leads to higher circulating concentrations of free, active drug and increases the risk of toxicity for highly protein-bound agents. For instance, sulfonamides can displace bilirubin from albumin, leading to free bilirubin crossing the blood-brain barrier and causing kernicterus (bilirubin encephalopathy).
• Metabolism and Excretion: Hepatic pathways are immature at birth. The glucuronidation pathway (Phase II) is particularly deficient, which historically led to "gray baby syndrome" in neonates treated with chloramphenicol. Renal clearance is also significantly reduced; GFR in a newborn is only about 30% of adult values, reaching adult capacity around 12 months of age. Renally cleared drugs (like penicillins) require prolonged dosing intervals in infants.

Pregnancy and Lactation: Protecting the Fetus and Infant

Prescribing during pregnancy requires weighing maternal benefit against potential fetal risk, accounting for physiological shifts like a 50% plasma volume expansion and increased GFR.

Teratogenicity and the PLLR

The FDA's Pregnancy and Lactation Labeling Rule (PLLR) replaced the old letter categories (A, B, C, D, X) with descriptive risk summaries, clinical considerations, and data registries under three headings: Pregnancy, Lactation, and Females and Males of Reproductive Potential. This rule provides more detailed, evidence-based safety information.

Key Teratogens in Primary Care

• ACE Inhibitors and ARBs: Contraindicated, especially in the second and third trimesters. They interfere with the fetal renin-angiotensin system, causing fetal renal dysgenesis, oligohydramnios, neonatal skull hypoplasia, and limb contractures.
• Isotretinoin: Highly teratogenic, causing severe craniofacial, cardiac, and central nervous system defects. Prescribers and patients must participate in the strict iPLEDGE safety program.
• Tetracyclines (e.g., doxycycline, minocycline): Avoid after 18 weeks of gestation due to deposition in fetal teeth and bones, leading to permanent yellow-brown tooth discoloration and bone growth suppression.
• Anticonvulsants (e.g., valproic acid): Associated with a high risk of neural tube defects (e.g., spina bifida) and cognitive impairment.
• NSAIDs: Avoid in the third trimester. NSAIDs inhibit prostaglandin synthesis, which can cause premature closure of the fetal ductus arteriosus, neonatal pulmonary hypertension, and maternal/fetal platelet dysfunction.

Lactation Safety

Most drugs enter breast milk via passive diffusion. Properties that prevent significant transfer into breast milk include high molecular weight (>500 Da), low lipid solubility, and high protein binding. The Relative Infant Dose (RID) is used to estimate safety; an RID of less than 10% is generally considered safe. To minimize exposure, lactating mothers should take medications immediately after breastfeeding to allow maternal drug levels to peak and decline before the next feeding.

Summary of Key Physiological Parameters across Lifespan

Geriatrics and the AGS Beers Criteria

Geriatric patients represent a vulnerable population due to age-related physiological changes. Geriatric patients present age-related declines in GFR (the most critical pharmacokinetic change), hepatic blood flow, and albumin, alongside an increased body fat percentage that prolongs lipid-soluble drug half-lives.

The American Geriatrics Society (AGS) Beers Criteria identifies medications that are potentially inappropriate for older adults. Key high-yield Beers Criteria drugs include:

1. First-Generation Antihistamines (e.g., diphenhydramine, hydroxyzine): Possess strong anticholinergic properties, leading to dry mouth, blurred vision, urinary retention, constipation, confusion, delirium, and a high risk of falls.
2. Benzodiazepines (e.g., alprazolam, diazepam): Older adults have increased sensitivity to benzodiazepines. They significantly increase the risk of cognitive impairment, delirium, falls, and fractures.
3. NSAIDs (chronic use): Increase the risk of peptic ulcer disease and gastrointestinal bleeding. They also cause sodium retention and vasoconstriction, which can trigger acute kidney injury and worsen heart failure.
4. Sulfonylureas (specifically glyburide): Glyburide has a prolonged half-life and active metabolites, presenting an exceptionally high risk of severe, protracted hypoglycemia.
5. Skeletal Muscle Relaxants (e.g., cyclobenzaprine, carisoprodol): Highly sedating with significant anticholinergic side effects, increasing fall risk.

Polypharmacy and the Prescribing Cascade

Polypharmacy is defined as the concurrent use of five or more medications. It increases the risk of drug-drug interactions, adverse drug events, and non-adherence. A major consequence is the prescribing cascade, where an adverse drug reaction is misdiagnosed as a new medical condition, prompting the clinician to prescribe a second drug to treat it.

Example: An older patient is prescribed amlodipine for hypertension. The patient develops peripheral edema. Instead of discontinuing amlodipine, the provider prescribes furosemide to treat the edema. Furosemide then causes hypokalemia, leading the provider to prescribe potassium chloride. To break this cascade, NPs must perform systematic deprescribing and complete detailed medication reconciliations at every visit.

Worked Scenario: Anticholinergic Toxidrome in an Older Adult

An 82-year-old female is brought to the clinic by her daughter, presenting with confusion, dry mouth, dilated pupils, and mild tachycardia. The daughter notes the patient has had difficulty sleeping and recently started taking an over-the-counter sleep aid containing diphenhydramine. The NP recognizes the symptoms as an anticholinergic toxidrome (classically described as "blind as a bat, mad as a hatter, red as a beet, hot as a hare, dry as a bone"). Diphenhydramine is a first-generation antihistamine on the Beers Criteria list. The NP directs the immediate discontinuation of the sleep aid, provides supportive care instructions, and educates the patient and daughter on non-pharmacological sleep hygiene strategies.