6.2 Pharmacology Principles & Major Drug Classes

Pharmacokinetics, Pharmacodynamics, and Safe Administration

Pharmacological and Parenteral Therapies is the single highest-yield subcategory on both exams — roughly 14-20% of REx-PN scored items — so pharmacology deserves the largest share of your study time.

Pharmacokinetics is what the body does to a drug: Absorption, Distribution, Metabolism (mainly the liver), and Excretion (mainly the kidneys). Impaired liver or kidney function raises drug levels and toxicity risk, which is why the very young, older adults, and clients with hepatic or renal disease need dose adjustments and closer monitoring. Pharmacodynamics is what the drug does to the body — its therapeutic action, side effects, and adverse effects.

Routes include PO (oral), SL (sublingual), IM (intramuscular), SC/SubQ (subcutaneous), IV (intravenous, fastest onset), topical/transdermal, and inhaled. The PN matches route to the drug order and the client's condition (for example, hold PO meds if the client is NPO or has an unsafe swallow).

The rights of medication administration

Before every dose, verify the rights: right client (two identifiers such as name and date of birth against the record), right drug, right dose, right route, right time, right documentation, right reason, and right response. Always check allergies, and for high-alert medications (insulin, heparin, opioids, IV potassium, chemotherapy) perform an independent double-check. Controlled substances must be counted and locked. Unregulated care providers cannot administer medications, and assessment cannot be delegated — a frequently tested trap.

Major Drug Classes and Nursing Considerations

Analgesics and antibiotics

Acetaminophen relieves mild-moderate pain and fever; the ceiling is 4000 mg/day for healthy adults because overdose causes hepatotoxicity. Opioids are covered in depth in Section 6.4. Antibiotics must be taken as the full prescribed course even after symptoms resolve — stopping early promotes antibiotic resistance and recurrence. Always screen for allergy (a penicillin allergy history is high-yield).

Anticoagulants — do not confuse the labs

A classic distractor pairs the wrong drug with the wrong test. Warfarin → INR; heparin → aPTT. For all anticoagulants, monitor for bleeding (bruising, blood in urine/stool, gum bleeding) and teach bleeding precautions.

Antihypertensives and diuretics

ACE inhibitors (–pril) can cause a persistent dry cough (report it), hyperkalemia, and rare angioedema. Loop diuretics like furosemide lower potassium — monitor for hypokalemia (muscle weakness, dysrhythmias) and encourage potassium-rich foods, while potassium-sparing diuretics do the opposite and risk hyperkalemia.

Digoxin and insulin

Digoxin slows and strengthens the heartbeat: count the apical pulse for one full minute and withhold the dose if it is below 60 bpm in an adult, then notify the prescriber. Toxicity presents with nausea, visual halos/yellow-green vision, and dysrhythmias, and hypokalemia increases toxicity risk. Insulin is high-alert and timed to meals; rapid-acting insulin (aspart, lispro) is given about 15 minutes before eating, and the ever-present hazard is hypoglycemia — treat a conscious client with 15 g of fast-acting carbohydrate (the 'rule of 15').

Iron and other teaching points

Oral iron absorbs best with a vitamin C source (orange juice) and can cause dark stools and constipation. Always teach clients the drug's purpose, common side effects, and which symptoms to report.

Dosage-calculation basics

The REx-PN includes dosage-calculation items, so master the core formula Dose to give = Desired dose / dose on Hand (D/H), converting units first. Example: an order for 500 mg with 250 mg tablets on hand gives 500/250 = 2 tablets. For liquids, multiply by the volume: 1 g ordered (= 1000 mg) from a vial of 500 mg/mL gives (1000/500) x 1 mL = 2 mL. Always double-check whether the answer is reasonable — if a calculation says to give many tablets or an unusually large volume, re-verify before administering.

Adverse reactions, interactions, and the older adult

Distinguish a side effect (a predictable, usually minor effect such as drowsiness) from an adverse drug reaction (harmful, e.g., an allergic or toxic response) and from a drug interaction (one drug altering another's effect). Before giving a new drug, the PN checks compatibility and existing therapy. Older adults are especially vulnerable because of polypharmacy and reduced hepatic and renal clearance, which raise the risk of accumulation, falls, and confusion; 'start low and go slow' and reconcile medications at every transition of care.

Common-mistake callout

Two recurring test traps deserve emphasis. First, never crush an enteric-coated or extended-release tablet — doing so destroys the protective coating or dumps the full dose at once, risking gastric irritation or overdose. Second, when an order is unclear, illegible, or seems unsafe (a dose outside the usual range), the correct action is to clarify with the prescriber before administering, never to guess. Safe medication practice always favours verification over assumption, and the PN documents the drug, dose, route, time, and the client's response after every administration.