Prescribing in Special Populations: Safe Dosing Across Patients

The “standard” dose assumes an average adult with normal organs. Real patients aren’t average — pregnancy, the extremes of age, and failing kidneys or liver all change how a drug behaves. Safe prescribing means knowing which five groups need a different approach.

Pregnancy & lactation

• Known teratogens (learn these): warfarin, ACE inhibitors/ARBs, sodium valproate, retinoids (isotretinoin), tetracyclines, methotrexate, lithium, thalidomide.
• Safer choices: methyldopa/labetalol (hypertension), penicillins, paracetamol, low-molecular-weight heparin (anticoagulation).
• Risk is greatest in the first trimester (organogenesis). Prescribe only when benefit outweighs risk.

Paediatrics

• Dose by body weight or surface area, not adult fractions.
• Immature metabolism/excretion: chloramphenicol → grey baby syndrome; avoid aspirin (Reye’s syndrome).
• Tetracyclines stain teeth; fluoroquinolones affect cartilage.

The elderly

• Reduced renal and hepatic clearance and increased sensitivity (especially to sedatives, opioids, anticholinergics).
• Polypharmacy drives interactions and falls — review regularly (Beers / STOPP-START criteria).
• Principle: “start low, go slow.”

Renal impairment

• Dose-reduce or avoid renally-excreted drugs: aminoglycosides, vancomycin, digoxin, lithium, metformin, gabapentin.
• Avoid nephrotoxins: NSAIDs, aminoglycosides, contrast, ACE inhibitors in renovascular disease.
• Adjust by eGFR/creatinine clearance.

Hepatic impairment

• Avoid hepatotoxic drugs (paracetamol overdose, methotrexate, isoniazid).
• Reduced first-pass metabolism raises bioavailability of high-extraction drugs.
• Caution with sedatives and opioids — risk of precipitating encephalopathy.

Exam tip: the teratogen list is almost guaranteed marks — warfarin, ACE inhibitors/ARBs, valproate, retinoids, tetracyclines, methotrexate, lithium. And two one-liners: chloramphenicol → grey baby, aspirin → Reye’s in children.

Five groups, one principle: match the dose to the patient’s physiology. Get pregnancy, paediatrics, the elderly, and renal/hepatic impairment right, and you prescribe safely for anyone who walks through the door.