Chapter 1 · Module 6 of 6 · General Principles

Rational Prescribing and Drug Information

The five-element prescribing framework, drug safety in pregnancy and lactation, interpreting clinical evidence, and preventing medication errors

Rational prescribing integrates five elements — right drug, right dose, right route, right duration, right patient — with therapeutic drug monitoring indicated when a narrow therapeutic index drug shows significant pharmacokinetic variability and a well-established concentration-effect relationship
Drug safety in pregnancy depends on timing: the embryonic period (weeks 3–8) carries the highest teratogenic risk; high-yield teratogens include isotretinoin, valproic acid, warfarin, ACE inhibitors/ARBs, methotrexate, and tetracyclines — each requiring a specific clinical action
Relative risk reduction is a misleading summary statistic when baseline event rates are low; absolute risk reduction and number needed to treat are the clinically actionable numbers — a 50% relative reduction from 4% to 2% means 49 of every 50 patients treated receive no benefit
Evidence quality runs from systematic reviews and meta-analyses of randomized controlled trials at the top to expert opinion and case series at the bottom; prescribing decisions should be anchored to the highest available level
Medication errors cluster at four stages — prescribing, dispensing, administration, and monitoring — and are prevented by system-level safeguards (electronic prescribing, barcode administration, pharmacist review) combined with individual prescriber habits applied consistently

Section 1 — The Rational Prescribing Framework

Section 1

The Five Elements of Rational Prescribing

Right Drug

Evidence-based selection for this specific indication — not habit or familiarity

Right Dose

Individualized by weight, renal and hepatic function, age, and pharmacogenomics

Right Route

Matched to clinical urgency and the drug's bioavailability profile

Right Duration

Defined therapeutic endpoint; not open-ended; taper where required

Right Patient

Contraindications, drug interactions, adherence, and monitoring all assessed

Therapeutic drug monitoring (TDM) is indicated when a drug has a narrow therapeutic index AND significant interpatient pharmacokinetic variability AND a well-established concentration-effect relationship. Classic TDM drugs: aminoglycosides, vancomycin, digoxin, lithium, phenytoin, cyclosporine, tacrolimus.

Section 2 — Prescribing in Pregnancy and Lactation

Section 2

High-Yield Teratogens and Lactation Principles

Drug	Fetal Risk	Clinical Action
Isotretinoin	Severe craniofacial, cardiac, and CNS malformations; spontaneous abortion	Absolutely contraindicated. Mandatory iPLEDGE pregnancy prevention program required in the US.
Warfarin	Warfarin embryopathy (nasal hypoplasia, bone stippling) in 1st trimester; fetal bleeding later	Switch to heparin during pregnancy — heparin does not cross the placenta.
Valproic acid	Neural tube defects, cognitive impairment, autism spectrum disorder	Highest-risk antiepileptic. Avoid when alternatives exist. Folic acid supplementation if required.
ACE inhibitors / ARBs	Fetal renal dysgenesis, oligohydramnios, skull hypoplasia (2nd and 3rd trimester)	Contraindicated in pregnancy. Switch to methyldopa, labetalol, or nifedipine.
Methotrexate	Fetal death, growth restriction, CNS and limb malformations	Contraindicated. Washout period required before conception.
Tetracyclines	Discoloration and dysplasia of fetal teeth and bones	Avoid after week 14 of gestation. Safe alternatives exist for most infections.

Prescribing in lactation: Infant drug exposure depends on maternal plasma concentration, the drug's protein binding and lipophilicity, milk-to-plasma ratio, and infant dose per body weight. The LactMed database (NIH/NLM) is the authoritative resource. Most drugs are compatible with breastfeeding at therapeutic doses; the drugs of greatest concern include cytotoxic agents, radioactive compounds, and lithium.

Section 3 — Drug Information and Evidence Appraisal

Section 3

Absolute vs. Relative Risk Reduction and Hierarchy of Evidence

Same trial — two ways of presenting the same result

Drug reduces myocardial infarction: placebo group 4% event rate → treated group 2% event rate

50%

Relative Risk Reduction

Absolute Risk Reduction

Number Needed to Treat

Always ask for the ARR and NNT. The RRR alone cannot tell you whether the absolute benefit is clinically meaningful. A 50% relative reduction sounds impressive, but 49 of every 50 patients treated receive no benefit from this drug — all 50 bear its risks and cost.

Hierarchy of Evidence — Highest to Lowest Quality

Systematic reviews and meta-analyses of randomized controlled trials

Individual well-conducted randomized controlled trials

Cohort studies · Case-control studies

Case series · Expert opinion (lowest quality)

Section 4 — Medication Errors and Patient Safety

Section 4

Error Types and Prevention Strategies

Prescribing

Most common category

Wrong drug, dose, route, or omission of an indicated drug. Decimal place errors, weight-based calculation errors, look-alike/sound-alike name errors (hydroxyzine vs. hydralazine; vinblastine vs. vincristine). Electronic prescribing with clinical decision support is the primary system-level safeguard.

Dispensing

Pharmacy stage

Wrong drug selected, wrong strength, misread handwriting. Substantially reduced by electronic prescribing and pharmacist verification. Tall-man lettering (hydrOXYzine vs. hydrALAzine) at point of dispensing.

Administration

Point of patient care

Wrong patient, wrong time, wrong rate. Barcode administration systems and independent double-checks reduce risk for high-alert drugs (insulin, anticoagulants, concentrated electrolytes).

Monitoring

Failure to track response or detect toxicity

Missing drug levels for narrow therapeutic index drugs, not checking renal function after starting nephrotoxic agents, missing early toxicity signals. Systematic TDM protocols and structured medication review reduce this risk.

Individual prescriber safety habits: Never abbreviate drug names · Always specify complete route and frequency · Double-check unusually high or low doses · Verify weight-based calculations independently · Confirm renal function before prescribing renally cleared drugs · Review the complete medication list for interactions before adding any new drug

Chapter 1 Complete · General Principles of Pharmacology

This chapter established the conceptual foundation for all of clinical pharmacology. The pharmacokinetics/pharmacodynamics framework gives every drug problem a systematic structure: absorption, distribution, metabolism, and elimination determine how much drug reaches its target; receptor mechanisms, agonism and antagonism, potency and efficacy, and the therapeutic index determine what happens when it gets there.

Drug safety — adverse reactions, interactions, dangerous combinations, and contraindications — emerges predictably from the same pharmacological principles. Variability in drug response across patients is accounted for by pharmacogenomics, age-related physiological changes, and disease-induced alterations in clearance and protein binding. Rational prescribing integrates all of these frameworks into every prescribing decision.

The subsequent chapters in this site apply these general principles to individual drug classes and therapeutic areas. The concepts introduced here — especially PK/PD, the therapeutic index, receptor regulation, and dose individualization — recur throughout every chapter that follows.

Author / Source	Title	Publication
Katzung BG, ed.	Basic and Clinical Pharmacology, 15th edition	McGraw-Hill, 2021
Brunton LL, Knollmann BC, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 14th edition	McGraw-Hill, 2023
World Health Organization	Guide to Good Prescribing: A Practical Manual	WHO, 1994 (WHO/DAP/94.11)
Brunton LL, Hilal-Dandan R, Knollmann BC, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 13th edition	McGraw-Hill, 2018
Briggs GG, Freeman RK, Towers CV, Forinash AB	Drugs in Pregnancy and Lactation, 11th edition	Lippincott Williams & Wilkins, 2017
National Institutes of Health, National Library of Medicine	LactMed: Drugs and Lactation Database	nlm.nih.gov/medlineplus/druginfo/mommy
Myles PS, Gin T	Statistical methods for anaesthesia and intensive care	Butterworth-Heinemann, 2001
Waller DG, Sampson AP	Medical Pharmacology and Therapeutics, 5th edition	Elsevier, 2018