Prescribing in special groups

Why this matters on day one

The same drug at the same dose is safe in one patient and harmful in another. These are the adjustments most often missed, and the ones the PSA reliably examines.

The sequence

1. 1Renal impairment. Check eGFR and adjust. Avoid or reduce: NSAIDs, metformin (stop if eGFR <30), DOACs, gentamicin, opioids (morphine metabolites accumulate — oxycodone or alfentanil preferred), and nitrofurantoin (ineffective and toxic below eGFR 45). Suspend the ACE inhibitor, ARB and diuretic during acute illness ("sick day rules").
2. 2Hepatic impairment. Reduce or avoid drugs with high first-pass metabolism, sedatives (precipitate encephalopathy), NSAIDs (bleeding and renal), and statins. Watch paracetamol dosing in low body weight or malnutrition.
3. 3Pregnancy. Avoid ACE inhibitors and ARBs, sodium valproate (never, without a pregnancy prevention programme), retinoids, methotrexate, warfarin, tetracyclines, trimethoprim in the first trimester, and NSAIDs after 30 weeks. Use labetalol, nifedipine or methyldopa for hypertension. Folic acid 400 micrograms, or 5 mg where higher risk.
4. 4Breastfeeding. Most drugs are compatible, but check the BNF rather than assuming either way. Avoid codeine (variable maternal metabolism). Time doses after feeds where practical.
5. 5Children. Dose by weight and check against the BNF for Children — not the adult BNF. Never exceed the adult dose. Avoid aspirin under 16 (Reye syndrome).
6. 6Frail older adults. Start low, go slow, and review. Reduce anticholinergic and sedative burden. Use STOPP/START. Deprescribing is an active intervention with measurable benefit, not neglect.

What goes wrong

• Using the adult BNF for a child.
• Continuing metformin, an ACE inhibitor and a diuretic through an acute illness with AKI.
• Prescribing valproate to a woman of childbearing potential.
• Assuming "renally cleared" means "just halve it" — look it up.