Prescribing Principles

Topic Overview

Prescribing principles form the foundation of safe and effective pharmacotherapy in advanced nursing practice. As a Family Nurse Practitioner (FNP), you are responsible for selecting appropriate medications, determining correct dosages, monitoring for adverse effects, and educating patients. This section covers the core concepts and legal/ethical frameworks that guide prescribing decisions. On the exam, expect questions on pharmacokinetics, pharmacodynamics, controlled substance regulations, and prescription writing components.

Key Concepts and Definitions

• Pharmacokinetics – What the body does to the drug: absorption, distribution, metabolism, excretion (ADME).
• Pharmacodynamics – What the drug does to the body: receptor binding, dose-response relationships, therapeutic and toxic effects.
• Bioavailability – Fraction of an administered dose that reaches systemic circulation unchanged (IV = 100%, oral varies).
• Half-life (t½) – Time required for plasma concentration to decrease by 50%; determines dosing interval.
• Steady state – Plateau drug concentration achieved after 4–5 half-lives of regular dosing.
• Therapeutic index (TI) – Ratio of toxic dose to effective dose; narrow TI drugs (e.g., warfarin, digoxin) require close monitoring.
• Agonist – Binds to receptor and produces a response. Antagonist – Blocks receptor and prevents response.
• Schedule of controlled substances – DEA classification: Schedule I (high abuse potential, no accepted medical use) through Schedule V (lowest abuse potential).

Core Principles of Prescribing

Legal and Regulatory Requirements

• FNP must hold a current state license, DEA registration (if prescribing controlled substances), and national certification.
• Prescriptions must include: patient name and address, date, drug name, strength, dosage form, quantity, directions for use, prescriber name, address, DEA number (for controls), and signature.
• For Schedule II drugs, written prescription required (no refills); electronic prescribing is now mandated in many states.
• Schedule III–V can be phoned in or faxed, with up to 5 refills in 6 months.
• State practice authority varies: some states require collaborative agreement with a physician; others grant full practice authority.

Patient Assessment & Drug Selection

1. Complete medication history – Include OTC, herbal supplements, and adherence patterns.
2. Assess allergies & adverse drug reactions – Document specific reaction type and severity.
3. Evaluate organ function – Renal/hepatic impairment affects drug clearance and dosing.
4. Consider age, weight, pregnancy/lactation status – Pediatric and geriatric dosing often differs.
5. Review concurrent medications – Assess for drug-drug interactions (e.g., CYP450 enzyme inhibition/induction).
6. Select drug based on efficacy, safety, cost, and patient preference – Use evidence-based guidelines (e.g., JNC 8 for hypertension, ADA for diabetes).

Dosing and Titration

• Start low, go slow – Especially in elderly, hepatic/renal impairment, or narrow TI drugs.
• Loading dose – Used for drugs with long half-lives to reach steady state quickly (e.g., amiodarone, digoxin).
• Maintenance dose – Keeps plasma concentration within therapeutic window.
• Titration – Adjust dose based on therapeutic response and side effects; monitor at appropriate intervals.

Assessment and Monitoring

• Baseline labs – Renal function (SCr, eGFR), liver enzymes (ALT, AST), CBC, electrolytes before starting certain medications.
• Therapeutic drug monitoring (TDM) – Required for drugs with narrow TI (e.g., lithium, valproic acid, digoxin, aminoglycosides).
• Follow-up visits – Assess efficacy, side effects, adherence; adjust therapy accordingly.
• Documentation – Clearly record indication, plan, patient education, and monitoring schedule in the medical record.

Patient Education and Counseling

• Explain purpose – What the medication treats and expected onset of action.
• Dosing instructions – Timing, with/without food, missed dose protocol.
• Common side effects – What to expect and when to call the clinician.
• Serious adverse effects – “Red flags” requiring immediate medical attention.
• Drug interactions – Avoid alcohol, grapefruit juice (affects CYP3A4), OTC NSAIDs with anticoagulants.
• Storage and disposal – Keep out of reach of children; flush or return unused controlled substances per FDA guidelines.

Safety Precautions and Complications

• Polypharmacy – Common in older adults; use Beers Criteria to identify potentially inappropriate medications.
• High-risk medications – Anticoagulants, insulin, opioids, chemotherapy agents require extra vigilance.
• Prescription drug abuse – Screen for substance use disorder; use prescription drug monitoring program (PDMP) for controlled substances.
• Medication errors – Use “do not use” abbreviations (e.g., “U” for units → write “units”); double-check calculations for pediatric weight-based dosing.
• Boxed warnings (black box warnings) – Label alerts for serious risks (e.g., antidepressants and suicidality, NSAIDs and cardiovascular risk).

Exam Tips and High-Yield Points

• Know the four phases of pharmacokinetics (ADME) and how disease states alter each phase.
• Memorize CYP450 enzyme inducers and inhibitors: inducers (e.g., rifampin, carbamazepine, phenytoin) decrease drug levels; inhibitors (e.g., ketoconazole, erythromycin, amiodarone) increase drug levels.
• Understand drug half-life calculations: time to steady state = 4–5 × t½.
• For controlled substances: Schedule II = no refills; Schedule III–V = up to 5 refills in 6 months.
• Differentiate agonist vs. antagonist vs. partial agonist (e.g., buprenorphine is partial mu agonist).
• Common exam scenarios: Adjusting dose in renal failure (e.g., gabapentin, metformin, lithium), avoiding contraindicated drugs in pregnancy (Category D/X).
• Memory aid for prescription components: “Patient, Date, Drug, Dose, Route, Quantity, Directions, Refills, Signature” – use mnemonic “PDDDRQDRS”.

Quick Comparison Table: Drug Schedule Highlights