UKMLA Prescribing Safety (PSA-Aligned): Complete Guide

Prescribing is the UKMLA's most technical skill. It integrates pharmacology, pharmacokinetics, patient-specific physiology, and drug-chart convention into a single answer. It is also the skill that new F1s most commonly deliver unsafely in the first six months of practice — the reason the UKMLA and the Prescribing Safety Assessment (PSA) exist in the first place.

This pillar bridges both exams. The AKT tests prescribing through SBAs; the PSA tests it through realistic drug-chart writing. The skills overlap almost completely. If you drill the principles below, you will pass both with the same preparation — and, more importantly, you will prescribe safely on your first on-call.

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1. Why prescribing is UKMLA's most technical skill

A prescribing SBA in the AKT looks simple — pick the correct drug, dose, and frequency. The difficulty is that every choice must satisfy five constraints simultaneously:

1. Indication — right drug for the condition.
2. Contraindications — does the patient's history or physiology exclude this drug?
3. Interactions — does the patient's current chart change the choice?
4. Dose modification — renal, hepatic, age, weight, pregnancy.
5. Monitoring and safety — what do you need to check before and after?

Miss any one and the answer is wrong. That's why prescribing marks feel fragile even when the clinical reasoning is right. The reflex candidates need: before picking a drug, run through I–C–I–D–M (indication, contraindication, interaction, dose modification, monitoring).

The NICE Guidelines + UK Prescribing pillar provides the ladder-level management answers; this pillar is the safe-prescribing layer that makes sure your ladder-step answer is actually prescribable.

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2. PSA ↔ UKMLA overlap and dual preparation

The PSA and UKMLA AKT share the same prescribing substrate, just in different question formats:

Practical preparation takeaway — if you are a UK final-year, rehearsing PSA past papers is arguably the highest-yield prep for the prescribing-heavy UKMLA SBAs. IMGs without PSA can still use the PSA practice papers freely available online as targeted UKMLA revision.

The Ultimate 12-Week UKMLA Study Plan reserves week 9 for a dedicated prescribing block. Use this pillar as the reference for that week, and the UKMLA Question Bank Showdown to compare prescribing-focused banks (Quesmed's prescribing mini-modules are particularly useful).

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3. BNF structure and fast-lookup technique

The BNF is open-book in the PSA and assumed-reference in UKMLA prescribing. Speed of lookup decides marks.

Core sections students under-use:

• Drug monograph interaction appendix — lists interactions at the bottom of each drug entry. Faster than flipping to Appendix 1.
• Renal impairment appendix (Appendix 3) — dose adjustments by eGFR.
• Hepatic impairment appendix (Appendix 2) — avoid/reduce/monitor.
• Treatment summaries — condensed guideline-level decision trees (e.g., "Asthma, acute" → stepwise management).

Fast-lookup routine:

1. Identify the drug class and condition.
2. Go to the treatment summary first — it gives indication + ladder + cautions + doses all in one place.
3. Cross-check the specific drug monograph for dose modifications or interactions.

Using the BNF app (where available) saves time, but the printed BNF is what you get in the PSA — practise with both.

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4. Dose calculations — adult + paediatric by weight

Adult standard dosing — consult BNF; no calculation needed unless weight-based (heparin, gentamicin, chemotherapy).

Paediatric doses are weight-based — always:

Dose (mg) = weight (kg) × dose (mg/kg) × frequency factor

Check maximum dose — does not exceed adult equivalent.

Worked example — amoxicillin for a 15 kg child:

• BNF child dosing: 25 mg/kg TDS (max 500 mg).
• 15 × 25 = 375 mg TDS.
• Prescribe 375 mg PO TDS.

Infusion rates:

Rate (mL/h) = (dose mg/h ÷ concentration mg/mL)

Worked example — IV heparin infusion at 18 units/kg/h, 70 kg patient, 25,000 units in 50 mL saline:

• Hourly dose = 70 × 18 = 1,260 units/h.
• Concentration = 25,000 / 50 = 500 units/mL.
• Rate = 1,260 / 500 = 2.52 mL/h.

Fluid calculations:

• Adult maintenance = 25–30 mL/kg/day water + 1 mmol/kg Na⁺ + 1 mmol/kg K⁺ + 50–100 g glucose/day (NICE CG174).
• Children use Holliday-Segar: 100 mL/kg for first 10 kg + 50 mL/kg for next 10 kg + 20 mL/kg thereafter.

Dose-critical errors to avoid:

• Decimal points — write 0.5 mg, never .5 mg.
• Units — write "units" in full for insulin and heparin (never U).
• Microgram — write in full (never μg, not mcg).

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5. Renal dosing — eGFR-adjusted common drugs

Drugs to stop or reduce in renal impairment — SADMANS:

• SGLT2 inhibitors.
• ACE inhibitors / ARBs.
• Diuretics.
• Metformin.
• Antihypertensives (contextual).
• NSAIDs.
• Sulfonylureas.

Hold these in AKI or intercurrent illness (sick day rules).

Common dose-modified drugs:

Screen every prescription for nephrotoxins:

• Contrast — hold metformin and nephrotoxins 48 h around contrast in eGFR <60.
• NSAIDs + ACEi + diuretic = "triple whammy" for AKI.
• Aminoglycosides + loop diuretic = potentiated nephrotoxicity and ototoxicity.

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6. Hepatic dosing — avoid, dose-reduce, monitor

Drugs to avoid in significant liver disease:

• Paracetamol — maximum 2 g/day (cirrhosis, malnutrition). Still first-line analgesic; just cap the dose.
• Opioids — accumulate; start low, titrate.
• Statins — avoid in decompensated disease.
• Methotrexate, sodium valproate, amiodarone, isoniazid, rifampicin — hepatotoxic; avoid or monitor LFTs.
• NSAIDs — avoid (risk of bleeding, HRS).
• Warfarin — high INR variability; DOACs preferred if appropriate.
• Benzodiazepines — precipitate hepatic encephalopathy; lorazepam safest (non-hepatic metabolism).
• Metformin — avoid if active hepatic disease or alcohol excess.

Drugs that precipitate hepatic encephalopathy:

• Sedatives, opioids, diuretics (via hypokalaemia), GI bleed, constipation, infection, hypoglycaemia.

For ALF King's College criteria and management, see our UKMLA Gastroenterology & Hepatology pillar.

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7. Allergies vs intolerances vs true adverse reactions

Clarifying every "drug allergy" before prescribing is one of the single highest-yield stewardship habits.

True allergy (IgE-mediated):

• Anaphylaxis, urticaria, angioedema, bronchospasm, hypotension.
• Onset minutes to 1 hour.
• Penicillin anaphylaxis → avoid all β-lactams.

Delayed hypersensitivity (T-cell mediated):

• SJS/TEN, DRESS, serum sickness, interstitial nephritis, haemolysis.
• Onset days to weeks.
• Also precludes class.

Intolerance (not allergy):

• GI upset, headache, mild rash without systemic features.
• Does NOT preclude re-use of the drug class.

Adverse effects (predictable):

• Cholestatic jaundice with flucloxacillin/co-amoxiclav.
• C. difficile with any antibiotic.
• Dry cough with ACE inhibitor.

Documentation matters. Always record: drug, reaction (specific), timing, severity. A vague "allergic to penicillin" denies the patient first-line antibiotics for years — most turn out to be intolerances.

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8. Insulin prescribing — regimens, pitfalls, mistakes

Insulin is the most dangerous drug in the BNF — it causes more serious medication errors than any other.

Absolute prescribing rules:

• Write "units" in full. Never "U" (misread as zero, 10-fold overdose).
• Use the full brand name — "NovoMix 30", not "NovoMix".
• IV infusion is 1 unit/mL in 0.9% saline — always.
• Never use an IV syringe for SC injection — 1 mL = 100 units; massive overdose.
• Never inject into a cold, fibrosed, or lipohypertrophied site — erratic absorption.

Common insulin regimens (quick recap):

In-hospital pitfalls to avoid:

• NBM patient — stop rapid-acting, halve long-acting the night before, convert to VRII.
• Steroid-induced hyperglycaemia — morning prednisolone → add lunchtime intermediate insulin.
• DKA — fixed-rate insulin 0.1 units/kg/h; continue long-acting SC.
• Sick day rules — do NOT stop insulin; increase if hyperglycaemic with ketones.
• Hypoglycaemia — give 150–200 mL 10% dextrose IV (not 50% — extravasation injury).

For DKA and HHS detailed management, see our UKMLA Endocrinology Masterclass.

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9. Anticoagulation — warfarin vs DOACs, reversal

Indication-specific choice:

Warfarin targets:

• AF, VTE, most indications — INR 2.0–3.0.
• Mechanical mitral valve, recurrent VTE on warfarin — INR 2.5–3.5.
• Check INR every 1–3 days on initiation, then every 12 weeks once stable.

Warfarin interactions (enzyme inhibitors ↑INR):

• Macrolides, metronidazole, fluconazole, ciprofloxacin, amiodarone, SSRIs, cranberry juice, grapefruit juice.

Enzyme inducers (↓INR): rifampicin, phenytoin, carbamazepine, St John's wort, alcohol (chronic).

Warfarin reversal (major bleeding):

• Stop warfarin.
• Prothrombin complex concentrate (PCC/Beriplex) 25–50 IU/kg IV.
• Vitamin K 5 mg IV.
• FFP only if PCC unavailable.

Minor bleeding or high INR without bleeding:

• INR >8, no bleeding → oral vitamin K 1–5 mg.
• INR >5, no bleeding → hold warfarin, review dose.

DOAC reversal:

• Dabigatran → idarucizumab (Praxbind) 5 g IV.
• Factor Xa inhibitors (apixaban, rivaroxaban, edoxaban) → andexanet alfa (where available) or PCC 50 IU/kg as alternative.

Peri-operative bridging:

• DOACs usually held 24–48 h pre-op; resume 24–72 h post-op based on bleeding risk.
• Warfarin for mechanical valves — bridge with LMWH.
• High-bleeding-risk surgery + high-thrombotic-risk patient needs haematology input.

LMWH prescribing pitfalls:

• Dose-reduce in renal impairment (eGFR <30).
• Check platelets day 4–14 if exposed >4 days (HIT risk).
• Avoid in active major bleeding, severe thrombocytopenia <50.

See the upcoming UKMLA Haematology Essentials pillar for fuller thrombosis and bleeding diathesis coverage.

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10. Analgesia ladder and opioid conversions

WHO pain ladder (still the UKMLA starting-point):

1. Non-opioid — paracetamol ± NSAID.
2. Weak opioid — codeine, dihydrocodeine, tramadol.
3. Strong opioid — morphine, oxycodone, fentanyl.

NICE modifications (NG193 — chronic pain): long-term opioids not recommended for chronic primary pain; focus on non-pharmacological approaches.

Opioid conversion (approximate, oral unless stated):

Breakthrough dose: 1/6 of 24-h total morphine-equivalent dose, PRN.

Worked example: patient on MST (slow-release morphine) 30 mg BD = 60 mg/24h. Breakthrough = 10 mg immediate-release PRN every 4 h.

Opioid side effects:

• Constipation (always prescribe laxative with opioid).
• Nausea (usually resolves in 5–7 days).
• Drowsiness, cognitive impairment.
• Respiratory depression — naloxone 400 μg IV titrated if severe.
• Itch, dry mouth.

Toxicity signs: pinpoint pupils, respiratory depression (<8/min), reduced consciousness → naloxone + supportive care.

Renal/hepatic disease:

• Morphine avoided eGFR <30 (accumulates active metabolite morphine-6-glucuronide).
• Oxycodone or fentanyl safer in renal impairment.
• All opioids need careful titration in liver disease.

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11. Antibiotic prescribing — stewardship + common empirical choices

Start Smart, Then Focus (UKHSA framework):

• Start smart — send cultures before antibiotics where possible; document indication, dose, route, duration; review at 48 h.
• Then focus — stop, switch IV to oral, change antibiotic, continue, or OPAT.

Common empirical choices (UK typical — always check local):

For the full ID pillar including stewardship logic and antibiotic class adverse effects, see our UKMLA Infectious Diseases pillar.

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12. Prescribing in pregnancy / breastfeeding

Pregnancy categories simplified:

Breastfeeding — broadly safer options:

• Paracetamol, ibuprofen safe.
• Penicillins, cephalosporins, macrolides safe.
• Sertraline is the preferred SSRI.
• Warfarin safe (doesn't cross breast milk).
• Avoid aspirin (Reye's risk), codeine (ultra-rapid metaboliser infants), lithium.

Key rule: if a drug is safe in pregnancy, it's usually safe in breastfeeding; the reverse isn't always true.

For pregnancy emergencies — pre-eclampsia magnesium sulphate, anti-D for sensitising events, PPH uterotonic ladder — see our UKMLA Obstetrics & Gynaecology pillar.

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13. Drug interactions — the high-yield 10

The ten UKMLA-examinable interactions:

1. Warfarin + macrolide/metronidazole/ciprofloxacin → ↑INR.
2. Warfarin + rifampicin/phenytoin/carbamazepine → ↓INR.
3. ACE inhibitor + potassium-sparing diuretic + NSAID → hyperkalaemia, AKI.
4. ACE inhibitor + NSAID + diuretic → "triple whammy" AKI.
5. SSRI + NSAID → GI bleeding risk.
6. Macrolide + statin → rhabdomyolysis (CYP3A4 inhibition).
7. Amiodarone + warfarin/digoxin/statin → increased levels; monitor.
8. Lithium + thiazide/NSAID/ACEi → lithium toxicity.
9. SSRI + triptan + tramadol → serotonin syndrome.
10. Clarithromycin + simvastatin → rhabdomyolysis (avoid combination; switch to atorvastatin or hold statin during antibiotic course).

Drug-level monitoring (narrow therapeutic index):

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14. PRN vs regular vs once-only vs STAT

PRN (pro re nata — "as needed"):

• Documents indication, maximum frequency, maximum in 24 h.
• Examples: PRN paracetamol 1 g QDS max, PRN morphine 5 mg every 4 h max.

Regular:

• Fixed-time doses on the chart.
• Examples: amoxicillin 500 mg TDS at 06:00/14:00/22:00.

Once-only:

• Single dose (e.g., induction of anaesthesia, premedication, single vaccine).

STAT:

• Immediate single dose for acute need.
• Examples: STAT IV adrenaline 500 μg IM in anaphylaxis; STAT IV hydrocortisone 100 mg in Addisonian crisis.

Chart-writing rules (PSA-critical):

• Sign and date every prescription.
• Print name below signature.
• Specify drug, dose, frequency, route, start date, duration if appropriate.
• For PRN — specify indication ("for pain", "for nausea").
• Never prescribe "as required" for antibiotics, antihypertensives, or DMARDs.

Common chart errors PSA penalises:

• Missing signature or date.
• Ambiguous drug name (brand vs generic without specification where it matters — e.g., lithium, anticonvulsants).
• Wrong units ("mg" instead of "μg" for levothyroxine 25 μg).
• Wrong frequency ("QDS" when it should be "TDS").
• Unsafe rounding (e.g., amoxicillin 500 mg TDS instead of weight-based paediatric dose).

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15. Controlled drugs — prescription essentials

Schedule 2 and 3 CDs (the UK-examinable tier) require specific prescription details:

1. Signed and dated in the prescriber's own handwriting (can be printed — but signature must be ink).
2. Name and address of patient.
3. Form and strength of preparation.
4. Dose clearly stated (not "as directed").
5. Quantity in both words AND figures (e.g., "60 (sixty) tablets").
6. Prescriber's registered address.
7. Maximum 30-day supply for outpatient.

Common Schedule 2 drugs: morphine, oxycodone, fentanyl, diamorphine, methadone, pethidine, methylphenidate, cocaine.

Common Schedule 3 drugs: buprenorphine, temazepam, tramadol (moved to S3 in 2014), midazolam.

Destruction and witnessing: CDs stored in locked cabinet, register entries witnessed and signed. Denatured destruction of unused CDs documented.

Prescribing errors to avoid:

• Quantity only in figures — invalid.
• Missing dose.
• Unsigned.

The PSA routinely includes a CD prescription. Practise writing one for morphine 10 mg PO every 4 h PRN for 7 days until it becomes automatic:

"Morphine sulphate 10 mg tablets. One tablet (10 mg) orally every 4 hours as required for pain. Supply 42 (forty-two) tablets. [Signature, date, printed name, registered address]."

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Exam technique — prescribing SBA archetypes

Five stem patterns you will see in the AKT:

1. "65-year-old on warfarin INR 8 no bleeding" → hold warfarin + oral vitamin K 1–5 mg; recheck INR 24 h.
2. "Type 2 diabetic on metformin, HbA1c 62, recent MI" → add SGLT2 inhibitor (empagliflozin/dapagliflozin) — CVD benefit irrespective of HbA1c.
3. "Elderly patient on digoxin + furosemide + ACEi, confused, nauseated, bradycardic" → digoxin toxicity → check U&Es (hypokalaemia potentiates), level; DigiFab if severe.
4. "Post-op patient on morphine PCA, RR 6, pinpoint pupils" → opioid toxicity → naloxone 100–200 μg IV titrated + O₂ + reduce opioid.
5. "Pregnant woman + hypertension + protein:creatinine ratio raised" → pre-eclampsia → labetalol first-line + magnesium sulphate if seizures.

Common prescribing traps:

• Don't prescribe ACEi in bilateral renal artery stenosis.
• Don't prescribe NSAIDs + ACEi + diuretic together (triple whammy).
• Don't prescribe clarithromycin with simvastatin (rhabdomyolysis).
• Don't prescribe trimethoprim in 1st trimester or nitrofurantoin at term.
• Don't forget laxative + antiemetic with opioid initiation.

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Summary — five reflexes that win prescribing SBAs

1. Run I–C–I–D–M on every drug choice: indication, contraindication, interaction, dose modification (renal/hepatic/weight), monitoring.
2. Hold the SADMANS drugs in AKI or sick-day context.
3. Warfarin reversal ladder — INR-driven. Major bleeding = PCC + vitamin K.
4. Insulin = units, in full, never "U". 10% dextrose for unconscious hypo, not 50%.
5. Check every allergy before prescribing any antibiotic. Intolerance ≠ allergy.

Pair this pillar with the Cardiology Masterclass for cardiac drug prescribing, the Respiratory Masterclass for asthma/COPD step-ladders, and the Infectious Diseases pillar for empirical antibiotic choices. Prescribing marks are cumulative across the AKT — every specialty tests them. The reflexes above cover 80% of the question stems at the borderline.