Why Prescribing Safety Is Non-Negotiable
Prescribing errors are one of the most common sources of patient harm in the NHS — and prescribing safety stations have become a core component of OSCE circuits and the UKMLA. You will be expected to write legible, safe prescriptions; identify and correct errors; recognise contraindications; and use the BNF efficiently under exam conditions.
The good news: prescribing stations are highly systematic. Once you have the framework, they are reliable mark-scorers.
💡 Tip
In every prescribing station, check allergy status before writing a single drug. Write or say: "Before I prescribe anything, I want to confirm — do you have any known allergies or previous adverse reactions to medications?" This is almost always the first mark on the scheme.
The Safe Prescribing Framework
Use the PRODUCT checklist for every prescription:
| Letter | Check |
|---|---|
| Patient | Correct patient (name, DOB, weight if relevant) |
| Reason | Indication documented or clear from context |
| Old medications | Check chart for existing prescriptions |
| Drug | Correct drug — generic name, not brand name |
| Unit | Correct dose, unit, and route |
| Contraindications | Allergies, organ impairment, interactions |
| Timing | Frequency, duration, start date, review date |
💎 Clinical Pearl
Using generic names (paracetamol, not Panadol; metformin, not Glucophage) is a legal requirement in NHS prescribing and will be expected in your OSCE. Brand names in a prescription are a common deduction.
Drug Chart Anatomy
A standard NHS drug chart has sections for:
- 1Regular medications — dose, route, frequency, prescriber signature
- 2PRN (as required) — include maximum frequency and dose
- 3Once-only / stat medications — time, route, indication
- 4Infusions / variable rate — for fluids and insulin sliding scales
- 5Oxygen — target saturations (94–98% for most; 88–92% for hypercapnic COPD)
- 6Allergy box — drug name, nature of reaction, date, prescriber
⚠️ Red Flag
The allergy box must document the reaction, not just the drug name:
- "Rash" (mild — may cross-react with cephalosporins at ~1%)
- "Anaphylaxis" (absolute contraindication — do not give related beta-lactam)
- "Diarrhoea" (side effect, not allergy — different clinical significance)
A vaguely documented allergy is itself a patient safety risk.
The Most Common Prescribing Errors in OSCEs
1. Wrong dose or unit
| Drug | Common error | Correct approach |
|---|---|---|
| Paracetamol | Correct for adults is 1g QDS; errors common in children | Weight-based dosing in paediatrics |
| Morphine | Starting too high in opioid-naive patient | 2.5–5mg IV in opioid-naive |
| Warfarin | Prescribing a fixed dose | Must be individualised; leave blank pending INR |
| LMWH prophylaxis | Forgetting renal dose adjustment | Dalteparin 5000 units SC OD; reduce if eGFR <30 |
| Gentamicin | Prescribing without requesting levels | Once daily dosing; levels at 18–24h |
2. Prescribing a contraindicated drug
Screen every prescription with:
- Allergy — does the patient have an allergy to this drug or its class?
- Renal — is the eGFR too low? (NSAIDs, metformin, gentamicin, DOACs)
- Hepatic — is there liver failure altering metabolism? (statins, opioids, metformin)
- Pregnancy — is the patient pregnant? (NSAIDs >30 weeks, tetracyclines, warfarin)
- Interactions — does this interact with current prescriptions?
🧠 Mnemonic
STOPP criteria — common errors in elderly patients:
- NSAIDs in eGFR <50 or on anticoagulants
- Anticholinergics in dementia or urinary retention
- Benzodiazepines in falls risk
- High-dose PPIs without 8-week review
- Aspirin + warfarin without gastroprotection
3. Missing the review date
Antibiotics, steroids, and opioids must have a documented stop or review date. Writing "review 5/7" (review in 5 days) next to an antibiotic demonstrates safe prescribing practice and will score marks.
4. Not prescribing prophylaxis
A common OSCE task is reviewing a drug chart for omissions:
- VTE prophylaxis: Any hospital inpatient at risk needs LMWH (e.g. enoxaparin 40mg SC OD) unless contraindicated by bleeding or low platelet count
- Gastric protection: Any patient on NSAIDs or high-dose steroids needs a PPI co-prescribed (e.g. omeprazole 20mg OD)
- Laxatives: Any patient started on regular opioids needs a laxative (e.g. senna 15mg ON, or macrogol)
💡 Tip
The opioid prescribing triad — examiners look for all three:
- 1The regular opioid (correct dose and route)
- 2A PRN breakthrough dose (typically 1/6th of the 24-hour total dose)
- 3A laxative prescribed alongside it
Missing any one of these three loses marks.
Renal Dose Adjustments — Key Drugs
| Drug | Threshold | Adjustment |
|---|---|---|
| Metformin | eGFR 30–45: reduce; eGFR <30: stop | Risk of lactic acidosis |
| NSAIDs | Avoid if eGFR <30 | Worsen renal perfusion |
| LMWH (dalteparin) | eGFR <30: reduce dose | Accumulation risk |
| Direct oral anticoagulants | Drug-specific (see BNF) | E.g. apixaban: reduce if 2/3 criteria met |
| Gentamicin | Any renal impairment | Extended intervals; nephrotoxic and ototoxic |
| Gabapentin/pregabalin | eGFR <30 | Significant dose reduction; CNS toxicity |
Using the BNF Efficiently in Exams
Most prescribing OSCEs allow BNF access. Use it quickly:
- 1Go to the drug → check "Indications and dose"
- 2Check "Contra-indications" for absolute bars
- 3Check "Cautions" for renal/hepatic concerns
- 4Check "Interactions" — use Appendix 1 (alphabetical)
- 5Check "Side-effects" only if specifically asked
💡 Tip
BNF time-saver: For drug interactions, go directly to Appendix 1 rather than reading each drug's monograph — it is alphabetically organised and dramatically faster under exam conditions.
Fluid Prescribing — The Basics
Maintenance fluids (adult, ~70kg):
- 25–30 ml/kg/day water
- 1 mmol/kg/day of Na⁺, K⁺, and Cl⁻
- Roughly: 1L 0.9% NaCl + 2L 5% glucose over 24h (with added KCl if normokalaemic)
Fluid resuscitation:
- 500ml IV crystalloid (0.9% NaCl or Hartmann's) over 15 minutes
- Reassess after each bolus
⚠️ Red Flag
Never prescribe 5% glucose for fluid resuscitation — it distributes freely into all body compartments and will not stay in the intravascular space. It is for free water replacement (e.g. hypernatraemia), not volume replacement.
Common Examiner Follow-Up Questions
"You've prescribed amoxicillin — the patient says they're allergic to penicillin. What do you do?"
"I would cross out the prescription immediately and clarify the nature of the reaction. If anaphylaxis, I would not prescribe any beta-lactam and would use an alternative class such as clarithromycin or doxycycline depending on the indication. If it was a mild rash, I would discuss the low cross-reactivity risk with a senior before considering a cephalosporin."
"The patient's eGFR is 25. You've been asked to prescribe metformin — what do you do?"
"An eGFR below 30 is a contraindication to metformin due to the risk of lactic acidosis. I would not prescribe it, document this, and discuss an alternative with the medical team."
"How would you prescribe oxygen on a drug chart?"
"Oxygen is a drug and must be prescribed. I would document target saturations: 94–98% for most patients, or 88–92% for known hypercapnic respiratory failure such as COPD, specifying delivery device and flow rate, titrating to response."