Contents

1. Introduction
2. Why Prescribing Errors Matter
3. The Five Rs of Prescribing
   • Right Patient
   • Right Drug
   • Right Dose
   • Right Route
   • Right Time
4. Prescribing in Special Populations
   • Paediatrics
   • Older Adults
   • Pregnancy and Breastfeeding
   • Renal Impairment
   • Hepatic Impairment
5. Writing a Prescription in the UK
6. Controlled Drug Prescriptions
7. The PReSCRIBER Checklist
8. Common Errors to Avoid
9. Summary
10. Quiz

Abstract

• Around 1 in 20 prescriptions written in UK hospitals contains an error, and prescribing errors contribute to a substantial fraction of preventable hospital admissions.
• The traditional safety check is the Five Rs: right patient, right drug, right dose, right route, right time. Each step has predictable failure modes worth recognising.
• Special populations: children, older adults, pregnant women, and patients with renal or hepatic impairment: have different pharmacokinetics and pharmacodynamics and therefore different prescribing rules.
• UK prescriptions follow specific legal and practical requirements set out in the British National Formulary (BNF). The PReSCRIBER mnemonic is the standard memory aid taught in UK medical schools for completing a hospital drug chart safely.

Core

Introduction

Prescribing is the most common therapeutic intervention any doctor performs, and it is also one of the most error-prone. A junior doctor in a UK hospital writes hundreds of prescriptions a week, and even a small error rate produces a steady stream of adverse events. Safe prescribing is therefore not a separate clinical skill but an attitude: the recognition that the act of writing a drug, a dose, a route and a frequency is a high-risk activity that deserves full attention every time.

This article covers the principles taught at UK pre-clinical level. The two foundational sciences are covered separately in Pharmacokinetics and Pharmacodynamics: both are essential to understanding why the rules below exist.

Why Prescribing Errors Matter

The 2009 GMC EQUIP study found that around 8.9% of inpatient prescriptions written by junior doctors contained an error of some kind, and similar numbers have been replicated across UK trusts. The figure usually quoted in teaching is "around 1 in 20", reflecting the proportion of errors that are clinically significant.

Prescribing errors are estimated to contribute to between 5% and 8% of UK hospital admissions. Most are preventable. The drugs most commonly involved in serious adverse events are:

• Anticoagulants: warfarin and the direct oral anticoagulants (DOACs).
• Insulins.
• Opioids.
• Antibiotics, particularly through allergy oversights.
• Methotrexate (weekly, not daily, dosing).
• Chemotherapy.

These same drugs reappear at the top of every list of dispensing errors, hospital incident reports, and litigation cases. They are worth recognising as a group because they share three properties: a narrow therapeutic index, a high frequency of prescription, and serious consequences if dosed wrongly.

The Five Rs of Prescribing

The classical safety checklist taught throughout UK undergraduate teaching is the Five Rs: right Patient, right Drug, right Dose, right Route, right Time. Each is a place where a prescription can go wrong.

Right Patient

Confirm the patient's identity by name, date of birth and a unique identifier (hospital or NHS number) at every prescribing event. Look at the wristband, not the chart cover. Wrong-patient errors are particularly common on ward rounds and in handover situations.

Right Drug

Two failure modes dominate this step:

• Look-alike, sound-alike (LASA) errors. Generic names that differ by a letter or two (clobazam vs clonazepam, hydralazine vs hydroxyzine, vinblastine vs vincristine) are a documented source of harm.
• Allergy oversights. Always check the patient's allergy status before prescribing: on the chart, by asking the patient, and by clarifying what kind of reaction occurred. A "rash with penicillin in childhood" is not the same as anaphylaxis, but until clarified it must be treated as such.

Always prescribe by the generic name in lower case, except for biological products (e.g. insulins, biosimilars, modified-release preparations) where the brand name matters because formulations are not interchangeable.

Right Dose

Dose errors are the largest single category of prescribing error and the most likely to cause serious harm. Calculation errors are particularly common in:

• Paediatric prescribing (weight-based dosing, see below).
• Renally cleared drugs at low GFR (digoxin, gentamicin, low-molecular-weight heparins). The principles are explained in Glomerular Filtration Rate and the Measurement of Kidney Function.
• Drugs given as a fraction of a vial: insulin, opioids and chemotherapy.
• Drugs with multiple strengths or formulations: modified-release versus immediate-release preparations are not interchangeable.

Three units to be careful with on a UK drug chart:

• Always write "micrograms" in full: the abbreviation "µg" can be misread as "mg", a thousand-fold error.
• Always write "units" in full for insulin and heparin: "U" can be misread as a zero, producing a tenfold overdose.
• Never use a trailing zero (write "1 mg", not "1.0 mg") or a naked decimal (write "0.5 mg", not ".5 mg"): both are routine sources of tenfold dose errors.

Right Route

Confirm that the route written matches the formulation prescribed and the clinical indication. Several drugs are dangerous if given by the wrong route; the most notorious is vincristine, which is fatal if given intrathecally instead of intravenously, an error that has caused multiple deaths in the UK and led to the introduction of dedicated vincristine-only minibags.

If a patient is nil by mouth or vomiting, an oral prescription is the wrong route; alternatives such as buccal, sublingual, rectal, intramuscular, subcutaneous or intravenous administration may be required.

Right Time

Frequency and timing matter for two reasons. First, a drug given at the wrong frequency cannot maintain a steady-state plasma concentration in its therapeutic window. Second, certain drugs have time-dependent rules; methotrexate is given weekly for rheumatological indications, not daily, and daily prescription has caused fatalities. Bisphosphonates are given on an empty stomach with a clear hour before food. Levothyroxine is given at least 30 minutes before breakfast for the same reason.

Prescribing in Special Populations

Drug handling differs substantially in five groups, each with its own pre-clinical principles.

Paediatrics

Children are not small adults. Doses are almost always calculated by weight (mg/kg) or, for some drugs, body surface area (mg/m²). Two reference resources are used in the UK:

• The BNF for Children (BNFc) for licensed paediatric doses.
• Local trust paediatric prescribing guidelines for drugs used off-licence.

Important principles:

• Neonates have immature hepatic enzymes and renal function, so drug clearance is slow.
• Infants and small children have a higher proportion of body water, increasing the volume of distribution of water-soluble drugs.
• Several drugs are contraindicated or restricted in specific paediatric age groups: aspirin below age 16 (Reye's syndrome), tetracyclines below age 12 (tooth and bone effects), codeine below age 12 (variable CYP2D6 metabolism, MHRA warning), systemic fluoroquinolones (ciprofloxacin etc.) restricted to use only when other antibiotics are inappropriate, because of MHRA cautions about disabling and persistent musculoskeletal and neurological side effects.

Older Adults

Older patients have reduced renal clearance (eGFR falls with age), reduced hepatic blood flow, lower lean body mass with relatively more adipose tissue, and frequently have multiple comorbidities. The combination produces marked sensitivity to many drugs.

Particular caution is required with:

• Benzodiazepines: increased fall and confusion risk.
• Anticholinergics: cognitive impairment and constipation.
• NSAIDs: renal impairment, GI bleeding and heart failure.
• Opioids: reduced clearance, increased sensitivity.

Polypharmacy (the regular use of five or more medicines) is the single biggest risk factor for harm in older adults. The STOPP-START tool, developed in the UK and Ireland, helps identify medicines that should be stopped (potentially inappropriate) and others that should be started in the elderly. STOPP-START is covered in detail in the article on Poisoning and STOPP-START.

Pregnancy and Breastfeeding

Most medicines crossing the placenta carry some risk to the fetus. The risks vary by trimester; the first trimester (organogenesis) is the period of greatest risk for structural teratogenesis, while later trimesters carry risks of growth restriction, neurodevelopmental effects and fetal toxicity.

Notable teratogens that pre-clinical students are expected to know include:

• ACE inhibitors and angiotensin receptor blockers (renal failure, oligohydramnios)
• Warfarin (warfarin embryopathy)
• Sodium valproate (neural tube defects, neurodevelopmental effects: now strictly controlled by the MHRA Pregnancy Prevention Programme)
• Lithium (Ebstein's anomaly)
• Methotrexate (multiple defects)
• Retinoids (multiple defects, requires Pregnancy Prevention Programme)
• Tetracyclines (tooth and bone effects after the first trimester)
• Aminoglycosides (ototoxicity)

The principle in clinical practice is: do not prescribe in pregnancy unless the benefit is clear and the alternatives have been considered. The BNF Appendix on prescribing in pregnancy is the standard reference.

Renal Impairment

Renal impairment requires dose adjustment of renally excreted drugs. The general approach is to estimate the patient's renal function (using eGFR for most drugs, but the Cockcroft-Gault formula for narrow-therapeutic-index drugs such as digoxin, gentamicin and vancomycin), and then consult the BNF for the specific reduction required.

Drugs that require particular care in renal impairment include the aminoglycosides, vancomycin, low-molecular-weight heparins, metformin, digoxin, lithium, and most NSAIDs. The principles are covered in Acute Kidney Injury and Chronic Kidney Disease.

Hepatic Impairment

The liver is the major site of drug metabolism, and significant hepatic impairment can prolong the half-life of many drugs. There is no equivalent of eGFR for the liver; the BNF advises caution and dose reduction based on clinical assessment, with the Child-Pugh classification used in specialist settings.

Drugs that require particular caution include opioids (reduced clearance, sedation), benzodiazepines, statins, paracetamol (in chronic alcohol use), and any drug with significant first-pass metabolism. See Hepatic, Biliary and Pancreatic Pathology for the underlying disease processes.

Writing a Prescription in the UK

A legally and practically valid UK prescription contains the following elements, set out in the BNF:

• Patient identification: full name, date of birth, address (for FP10 prescriptions in primary care) or hospital number (for inpatient charts).
• Date.
• Drug name: in lower case, generic, written legibly or printed.
• Dose, with the units written in full ("milligrams" or "mg"; "micrograms"; "units"). Avoid trailing zeros and naked decimals.
• Route.
• Frequency: written either as a Latin abbreviation (e.g. od, bd, tds, qds, prn) or in plain English. Most UK trusts now prefer plain English.
• Duration or quantity.
• Prescriber's signature, printed name, GMC number and contact details (a bleep number on a hospital chart).

For community (FP10) prescriptions, additional rules apply: the prescriber's address must be on the form and the prescription must be signed in indelible ink.

Controlled Drug Prescriptions

Drugs covered by the Misuse of Drugs Regulations 2001 (controlled drugs, "CDs") have additional legal prescribing requirements. The most clinically relevant for pre-clinical students are Schedule 2 (e.g. morphine, oxycodone, methadone, diamorphine) and Schedule 3 (e.g. tramadol, midazolam, gabapentin, pregabalin) drugs.

For Schedule 2 and 3 controlled drugs prescribed in the community, the following must be written by hand or by computer (post-2015 amendments allow computer-generated prescriptions):

• The name and address of the patient.
• The form and strength of the preparation.
• The total quantity in both words and figures.
• The dose.
• The prescriber's signature in indelible ink.

The "total quantity in both words and figures" requirement is the most easily missed and the most likely to be queried by a pharmacist.

The PReSCRIBER Checklist

The PReSCRIBER mnemonic is the standard memory aid taught in UK medical schools to ensure nothing is missed when completing a hospital drug chart on admission. It is widely used in the Prescribing Safety Assessment (PSA) and on the wards.

P: Patient details (full identification and weight)
Re: Reaction (allergies and the reaction that occurred)
S: Sign the front of the chart
C: Contraindications check (renal/hepatic function, pregnancy, interactions)
R: Route for each drug
I: IV fluids prescribed if needed
B: Blood-clot (VTE) prophylaxis prescribed
E: antieEmetic prescribed if needed
R: pain Relief prescribed if needed

The mnemonic doubles as a structured ward-round task list: every patient on the take should have all nine boxes ticked before the team moves on.

Common Errors to Avoid

The recurring patterns identified in UK incident reports are worth knowing:

• Failing to take a complete drug history on admission. Around half of all medication errors begin here. Always ask about over-the-counter medicines, herbal remedies, recent antibiotics, and the actual versus prescribed dose for any drug the patient self-administers.
• Failing to check allergies, or accepting "allergic to penicillin" without clarifying the reaction. A patient who experienced anaphylaxis to amoxicillin should not receive any beta-lactam without specialist advice; a patient who had nausea is much more likely to tolerate a different agent in the same class.
• Continuing a drug indefinitely without review. Antibiotics, proton pump inhibitors, hypnotics and steroids are all routinely continued past their original indication.
• Writing as µg, ng, U, IU or with trailing zeros. Spell units out in full.
• Failing to consider non-pharmacological alternatives. Sleep hygiene before zopiclone, exercise before metformin, dietary advice before a statin.
• Not double-checking high-risk prescriptions. The "second pair of eyes" is a near-zero-cost intervention that catches a high proportion of serious errors. It is mandatory for chemotherapy and intrathecal drugs in the UK.

Summary

Safe prescribing is a habit, not a one-off skill. The principles taught at pre-clinical level translate directly into daily clinical practice and the UK Prescribing Safety Assessment.

• The Five Rs (right patient, drug, dose, route, time) are the basic safety check.
• Drug handling differs in children, older adults, pregnancy, renal impairment and hepatic impairment.
• The drugs most often involved in serious harm are anticoagulants, insulins, opioids, antibiotics, methotrexate and chemotherapy.
• UK prescriptions follow rules set out in the BNF; controlled drugs have additional legal requirements.
• The PReSCRIBER mnemonic is the standard safety check for completing a hospital drug chart.
• Prescribing is a high-risk activity; the "second pair of eyes" catches a high proportion of preventable errors.

Reviewed by: Dr. Marcus Judge

Quiz

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