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PH10.{3-4,6} | PH10.{3-4,6} | Rational Prescription Writing and Appraisal — SDL Guide — SDL Guide (Part 2)
Prescribing in Practice: Worked P-Drug Examples
Applying the WHO six-step method to real cases demonstrates how the framework converts clinical reasoning into a complete, rational prescription. Two worked examples follow — one infection, one chronic disease — to illustrate both acute and long-term P-drug selection.
Worked Example 1 — Uncomplicated Lower UTI in a non-pregnant adult:
- Step 1 (Problem): Dysuria, frequency, suprapubic pain; no fever; dipstick positive; no renal calculi, no pregnancy
- Step 2 (Objective): Eradicate uropathogen, resolve symptoms within 3–5 days
- Step 3 (P-drug evaluation): Candidates — nitrofurantoin, trimethoprim, co-trimoxazole, ciprofloxacin. Local resistance data in India shows >30% E. coli resistance to co-trimoxazole in many settings; ciprofloxacin reserved for complicated UTI (antimicrobial stewardship); nitrofurantoin — effective for lower UTI, minimal systemic absorption, low resistance in India, cheap, available. P-drug: Nitrofurantoin 100 mg (modified-release) BD for 5 days (or nitrofurantoin 50 mg QID × 7 days if MR formulation unavailable)
- Step 4 (Prescription): Write complete prescription as above with all mandatory elements
- Step 5 (Inform): Take with food to reduce nausea; complete the full course; return if fever develops (suggests upper UTI requiring review)
- Step 6 (Monitor): Review if symptoms persist beyond 5 days; urine culture/sensitivity if recurrent
Worked Example 2 — New diagnosis: Mild Stage 1 Hypertension, no comorbidities:
- Step 1 (Problem): BP 150/95 mmHg on two occasions; no diabetes, no CKD, no heart failure, non-smoker, no target organ damage
- Step 2 (Objective): Reduce BP to <140/90 mmHg, reduce long-term cardiovascular risk
- Step 3 (P-drug evaluation): JNC/BHS/AHA guidelines and Indian hypertension guidelines support amlodipine (CCB), amlodipine + ACE inhibitor, or thiazide as first-line. For monotherapy in mild hypertension without comorbidities: amlodipine 5 mg OD — evidence-based (ASCOT-BPLA), good tolerability, once-daily dosing (adherence), generic available (inexpensive). Alternatives: Ramipril 5 mg OD if patient has associated proteinuria/diabetes; avoid beta-blockers as first-line in uncomplicated hypertension per current guidelines.
- Step 4 (Prescription): Write complete prescription
- Step 5 (Inform): This medication is lifelong — do not stop if BP normalises; ankle oedema may occur (dose-related, mostly mild); continue lifestyle modification (DASH diet, exercise, salt restriction)
- Step 6 (Monitor): Recheck BP at 4–6 weeks; electrolytes and renal function at 3 months
These examples demonstrate that the P-drug method produces not just a drug name but a complete clinical rationale — which is what makes it teachable, auditable, and improvable.
SELF-CHECK
You write a prescription for nitrofurantoin 100 mg BD for 5 days for the UTI patient above. A colleague reviews it and points out it is incomplete. Which of the following omissions would make the prescription LEGALLY invalid in India?
A. A) Omitting the patient's address
B. B) Omitting the prescriber's medical registration number
C. C) Not writing 'complete the full course' in the patient instructions
D. D) Not writing the generic name alongside the brand name
Reveal Answer
Answer: B. B) Omitting the prescriber's medical registration number
B is correct. The prescriber's medical registration number is a mandatory legal element for Schedule H drug prescriptions in India — its absence makes the prescription invalid for dispensing purposes. Patient address is recommended but not legally mandatory for Schedule H (it is mandatory for Schedule X). Patient instructions, while best practice, are not a legal validity requirement. Generic name writing is mandatory in practice per NMC rules, but the question asks what makes the prescription legally invalid at the pharmacist level.
Prescription Appraisal: Evaluating and Improving a Given Prescription
Prescription appraisal is the systematic evaluation of an existing prescription to identify errors, omissions, irrational choices, and missed opportunities for improvement. It is a core clinical skill: in referral medicine, you will frequently need to review another doctor's prescription; in prescribing audits, it is a quality-improvement tool; in clinical pharmacology assessments (NMC CBME), it is a formal competency task (PH10.6).
A structured prescription appraisal covers five domains:
Domain 1 — Completeness (legal elements). Are all mandatory elements present? Patient details (name, age, sex), date, drug name, strength, formulation, route, dose, frequency, duration, prescriber name, registration number, and signature.
Domain 2 — Legibility and clarity. Is the handwriting unambiguous? Are abbreviations standard and unambiguous? Are the dose and unit clearly stated (e.g. '0.5 mg' not '0.5 mgs' or '.5mg')? Are leading zeros present (0.5, not .5) and trailing zeros absent (5 mg, not 5.0 mg)?
Domain 3 — Rational drug choice. Is the drug the most appropriate for the diagnosis? Is it consistent with current guidelines? Are there better alternatives (first-line vs second-line)? Is combination therapy rational (fixed-dose combinations — are the components needed at the specified ratio)?
Domain 4 — Dosing appropriateness. Is the dose correct for the indication and patient? Is it adjusted for renal/hepatic impairment, age, or weight? Is the duration correct (e.g. 3 days vs 7 days for uncomplicated UTI — both may be defensible depending on the drug, but 1 day or 30 days would be wrong)?
Domain 5 — Drug interactions and contraindications. Does the prescription contain potentially harmful interactions (e.g. ciprofloxacin + antacid reducing absorption; metformin + iodinated contrast medium risk)? Are any prescribed drugs contraindicated in this patient given documented allergies, comorbidities, or concurrent medications?
| Appraisal Domain | Key Questions | Common Errors |
|---|---|---|
| Completeness | All legal elements present? | Missing registration number, missing dose strength |
| Legibility | Unambiguous? Standard abbreviations? | Illegible handwriting, trailing zeros, U for units |
| Rational drug choice | First-line? Evidence-based? | Second-line used without indication; irrational FDC |
| Dosing | Correct dose, frequency, duration? | Subtherapeutic dose; no renal adjustment stated |
| Interactions/Contraindications | Checked for patient-specific factors? | Allergy not checked; interaction missed |
CLINICAL PEARL
Never write '5.0 mg' — always write '5 mg.' Trailing zeros are a well-documented cause of 10-fold dosing errors: '5.0 mg' is misread as '50 mg' when the decimal point is blurred. The reverse applies to sub-milligram doses: always write '0.5 mg', never '.5 mg' (a missing leading zero has been misread as '5 mg'). This WHO-recommended convention — no trailing zeros, always leading zeros for decimal doses — should become automatic in every prescription you write.
Self-Assessment: Prescription Writing and Appraisal
Consolidate your learning by completing the following two structured exercises before the next clinical session.
Exercise 1 — Write a complete prescription:
Patient: Rani Devi, 28 years, female. Diagnosis: uncomplicated lower UTI. Your P-drug choice: co-trimoxazole 960 mg (trimethoprim 160 mg + sulfamethoxazole 800 mg) BD for 5 days (assume local resistance is not a problem for this exercise). Write out the complete prescription with all mandatory elements.
Exercise 2 — Appraise and correct a prescription:
A prescription reads: 'Metronidazole 400 mg BD. Amoxicillin-clavulanate (Augmentin) TDS. — Dr R. Sharma.'
Apply the five-domain appraisal checklist and list all errors or omissions.
Expected findings for Exercise 2:
- Domain 1 (Completeness): Missing — patient name, age, sex, date, duration for both drugs, formulation, prescriber registration number, address
- Domain 2 (Legibility): No issues visible in text, but handwritten version would require checking
- Domain 3 (Rational choice): Unclear — what is the diagnosis? The combination of metronidazole + amoxicillin-clavulanate may be rational for dental infections, peritonitis, or mixed aerobe/anaerobe infections, but without a stated diagnosis or indication, the rationality cannot be assessed
- Domain 4 (Dosing): Dose of amoxicillin-clavulanate not specified — 625 mg (250/125 mg) TDS vs 1 g (500/125 mg) BD are different; duration absent for both drugs
- Domain 5 (Interactions/contraindications): Penicillin allergy not checked; no mention of whether diagnosis is confirmed; both drugs alter gut flora and should not be co-prescribed without clear indication
Key learning: A prescription without a clearly stated diagnosis (or at least a clinical context that makes the prescription intelligible) is nearly impossible to appraise rationally. The indication is as important as the drug.