Lesson 6.6: Prescribing Skills
Prescribing errors are the most common type of preventable harm in UK hospitals. In PLAB 2, prescribing stations test whether you can write safe, accurate prescriptions and counsel patients appropriately.
Elements of a Safe Prescription
| Element | Rules |
|---|---|
| Drug name | Use generic (non-proprietary) names except for narrow therapeutic index drugs: ciclosporin, lithium, modified-release preparations — where brand name is specified. |
| Dose | Use numerals. Avoid unnecessary decimal points: write '500 mg' not '0.5 g' where possible. Never abbreviate micrograms as 'mcg' — write in full or use 'microgram'. Never write 'units' as 'U' (looks like a zero) — write 'units' in full. |
| Route | Be specific: PO (oral), IV (intravenous), IM (intramuscular), SC (subcutaneous), PR (rectal), INH (inhaled), TOP (topical). |
| Frequency | OD, BD, TDS, QDS, PRN, STAT — use plain English where possible for clarity. |
| Duration | Always specify for antibiotics, steroids, and anticoagulants. |
| Allergies | ALWAYS check and document allergies before prescribing. Write 'NKDA' if no known drug allergies. |
| Signature | Every prescription must be signed with your GMC number (or candidate number in the exam). |
WHO Analgesic Ladder
| Step | Drugs | Key Cautions |
|---|---|---|
| Step 1 — Mild pain | Paracetamol 1g PO QDS (max 4g/24h). NSAIDs: ibuprofen 400 mg TDS with food if not contraindicated. | NSAIDs: avoid in renal impairment, GI ulcer history, anticoagulation, heart failure, elderly. |
| Step 2 — Moderate pain | Codeine 30–60 mg PO QDS or tramadol 50–100 mg PO QDS. Always prescribe a laxative with any opioid. | Codeine ineffective in ~10% (poor metabolisers). Consider dihydrocodeine. |
| Step 3 — Severe pain | Morphine — start low: 2–5 mg PO 4-hourly in opioid-naive patient. Convert to modified release once stable. Always prescribe PRN immediate-release alongside. Regular laxative essential. | Always add: regular laxative, antiemetic PRN. Naloxone available on ward. |
High-Risk Medications — Extra Vigilance Required
These drugs cause the most serious prescribing errors — know them.
- INSULIN: Always check type (rapid vs. long-acting), dose, and timing. Never write 'U' for units — it looks like a zero. Write '10 units' not '10U'. Specify the brand and device.
- ANTICOAGULANTS (warfarin, heparin, DOACs): Check INR before each warfarin dose. Check renal function before DOACs. Heparin infusions: always write units/hour, not mL/hour.
- OPIOIDS: Specify strength and preparation. Equianalgesic conversion when changing drug or route. Always add a laxative.
- METHOTREXATE: In rheumatology, it is WEEKLY, not daily. Fatalities have occurred from daily prescribing. Write 'once weekly' explicitly and clearly.
- CONCENTRATED ELECTROLYTES (KCl concentrate, NaCl 8.4%): Never as IV bolus. Always diluted. Prescribed on the separate electrolyte section of the drug chart.
IV Fluid Prescribing — NICE CG174
| Indication | Fluid and Rate |
|---|---|
| Resuscitation | 0.9% NaCl bolus. For sepsis: 250 mL over 10–15 minutes (NICE NG253). For haemorrhage/anaphylaxis: 500–1000 mL rapidly. Reassess after each bolus. Stop when haemodynamically stable. |
| Routine maintenance (nil by mouth) | 25–30 mL/kg/day water. 1 mmol/kg/day Na, K, Cl. 50–100 g/day glucose. Standard regimen: 1L 0.9% NaCl + 20 mmol KCl over 8h, then 1L 5% glucose + 20 mmol KCl over 8h x2. |
| Replacement of losses | Match fluid to the loss: vomiting (hypochloraemic alkalosis) → 0.9% NaCl + KCl. Diarrhoea → Hartmann's. High output stoma → 0.9% NaCl. |
| Potassium replacement | Max 40 mmol/L peripherally. Max rate 10 mmol/hour peripherally. Cardiac monitoring if >20 mmol/hour. Never as IV bolus. |
Sick Day Rules — Medications to Hold During Acute Illness
- Hold ACE inhibitors and ARBs during acute illness with dehydration, diarrhoea, or vomiting — risk of acute kidney injury.
- Hold NSAIDs during acute illness — nephrotoxic when combined with dehydration.
- Hold metformin if eGFR <30, if patient is having IV contrast (24h before and 48h after), or if acutely unwell — risk of lactic acidosis.
- Hold diuretics if patient is dehydrated or hypotensive.
- Continue statins, most antihypertensives (except as above), and most chronic medications unless a specific contraindication applies.
- Continue insulin but reduce basal dose by 20% if not eating. Switch to sliding scale if nil by mouth.