Master routes of administration, dosage forms, pharmacokinetics (ADME), and drug interactions — the core pharmacology topics tested on the PTCE.
Practice with Flashcards →The Four Pharmacology Pillars
Every pharmacology question on the PTCE traces back to one of these four foundational areas
The route determines onset speed, bioavailability, and whether the drug undergoes first-pass metabolism. IV delivers 100% bioavailability; oral routes are reduced by absorption and first-pass hepatic metabolism.
Immediate-release (IR) vs. extended-release (ER/XR/XL/SR/LA), enteric-coated tablets, suspensions, patches, inhalers, and suppositories each have critical rules — especially which forms must NEVER be crushed or chewed.
Absorption, Distribution, Metabolism, Excretion — four sequential phases that determine drug concentration in the body. Key concepts include bioavailability, protein binding, CYP450 metabolism, half-life, and steady-state timing.
CYP450 inhibitors increase drug levels; inducers decrease them. Interactions are additive, synergistic, or antagonistic. Drugs with a narrow therapeutic index (NTI) require the closest monitoring when interactions occur.
How It Works
Core concepts, mechanisms, and key rules for every pharmacology pillar
Bioavailability & First-Pass Effect
When a drug is taken orally, it is absorbed in the GI tract and travels via the portal vein directly to the liver before reaching systemic circulation. The liver metabolizes a portion of the drug, reducing the amount that reaches the bloodstream. This is called the first-pass effect.
Sublingual (SL), buccal, transdermal, rectal (partially), IV, IM, SubQ, and inhalation routes all deliver drug into systemic circulation without passing through the hepatic portal system first. Sublingual drugs dissolve under the tongue and absorb directly into capillaries — onset is very rapid (1–5 minutes).
Intravenous administration delivers drug directly into the bloodstream — no absorption step, no first-pass, no barrier to cross. This gives IV the fastest onset and highest bioavailability of any route. It also means IV dosing errors are immediately irreversible, making accuracy critical.
Fastest to slowest: IV (seconds) → Inhalation (minutes) → SL (1–5 min) → IM (10–30 min) → SubQ (15–30 min) → Oral PO (30–60 min) → Transdermal (hours). Choose route based on desired onset, patient condition, and available dosage forms.
IR vs. ER, Special Forms & Handling Rules
How the Body Processes Drugs
Many drugs bind to plasma proteins (primarily albumin). Only the free (unbound) fraction of drug is pharmacologically active. A drug that is 98% protein-bound leaves only 2% free to exert effects. If another drug displaces it (drug interaction), the free fraction suddenly increases — potentially causing toxicity.
The cytochrome P450 enzyme system in the liver is responsible for metabolizing most drugs. Key isoforms include CYP3A4 (metabolizes ~50% of drugs), CYP2D6, CYP2C9, CYP2C19, and CYP1A2. These enzymes can be inhibited or induced by other drugs, foods (e.g., grapefruit juice), or patient genetics.
Some drugs are administered in an inactive form (prodrug) and must be metabolized in the liver to become active. Example: codeine is a prodrug that is converted to morphine by CYP2D6. Poor metabolizers (genetic variation) may get little or no pain relief; ultra-rapid metabolizers may get dangerously high morphine levels.
Most drugs and their metabolites are eliminated by the kidneys in urine. Patients with renal impairment (low creatinine clearance / GFR) will accumulate drugs — requiring dose reductions or extended dosing intervals. The pharmacist must verify dosing is appropriate for kidney function.
CYP450, Interaction Types & NTI Drugs
The combined effect equals the sum of the individual effects. Example: taking two CNS depressants (e.g., benzodiazepine + opioid) produces additive sedation. The effect is predictable but still clinically important — especially with CNS, cardiovascular, or anticoagulant drugs.
The combined effect is GREATER than the sum of individual effects. This can be therapeutic (desired) or dangerous (undesired). Example: alcohol + sedatives produce a synergistic CNS depression far beyond what either drug alone would cause — a dangerous and potentially fatal combination.
One drug reduces or blocks the effect of another. Can be intentional (e.g., naloxone reverses opioid overdose; flumazenil reverses benzodiazepines) or unintentional (e.g., an antibiotic reducing the effectiveness of an oral contraceptive via gut flora disruption).
NTI drugs have a very small margin between therapeutic and toxic doses. Even minor interactions, dose changes, or renal/hepatic impairment can cause toxicity or treatment failure. Key NTI drugs: warfarin, digoxin, lithium, phenytoin, theophylline, methotrexate, cyclosporine. These require TDM (therapeutic drug monitoring).
Compare
Filter by pillar to compare routes, dosage forms, and drug interaction types side by side
| Item | Category | Onset / Timing | Key Feature | PTCB Exam Tip |
|---|---|---|---|---|
| Intravenous (IV) | Route | Seconds | 100% bioavailability; no absorption step | Fastest onset; immediately irreversible — errors are critical |
| Sublingual (SL) | Route | 1–5 min | Absorbed under tongue; bypasses first-pass | Nitroglycerin SL is the classic example; do not swallow |
| Oral (PO) | Route | 30–60 min | Most common; subject to first-pass effect | Bioavailability varies by drug; food can increase or decrease absorption |
| Intramuscular (IM) | Route | 10–30 min | Bypasses GI tract; good absorption | Used for vaccines, antipsychotics, some antibiotics |
| Subcutaneous (SubQ) | Route | 15–30 min | Slower than IM; good for insulin and biologics | Inject into fatty tissue (abdomen, thigh); rotate sites |
| Transdermal (TD) | Route | Hours | Bypasses first-pass; continuous slow delivery | Patches for fentanyl, nicotine, estrogen, nitroglycerin |
| Rectal (PR) | Route | 15–30 min | Partial first-pass bypass; used when PO not possible | Lower rectum → bypasses portal circulation; upper → does not |
| Inhalation (INH) | Route | Minutes | Large surface area; rapid systemic or local effect | MDI inhalers need coordination; use spacer for children/elderly |
| Immediate Release (IR) | Form | Fast: ~30–60 min | Full dose released immediately | Can be crushed (unless EC coated); multiple daily doses required |
| Extended Release (ER/XR/XL/SR/LA) | Form | Slow: hours | Drug released gradually over 12–24 hours | NEVER crush or chew — causes bolus overdose |
| Enteric-Coated (EC) | Form | Delayed (intestine) | Coating resists stomach acid; dissolves in intestine | NEVER crush — protects GI lining or acid-labile drugs |
| Suspension | Form | Variable | Drug particles in liquid — settles on standing | ALWAYS shake well before each use; measure with syringe |
| Orally Disintegrating (ODT) | Form | Fast: ~1–5 min | Dissolves on tongue; no water needed | Useful for patients with dysphagia; do NOT swallow whole |
| Transdermal Patch | Form | Hours | Controlled drug release through skin | Dispose of properly; old patch must be removed before new one applied |
| Elixir | Form | Fast (liquid) | Alcohol-based solution; drug fully dissolved | Contraindicated in alcoholics, children; do NOT confuse with suspension |
| Absorption (A) | ADME | Phase 1 | Drug moves from site of administration into blood | Affected by route, food, GI pH, dissolution rate, and first-pass effect |
| Distribution (D) | ADME | Phase 2 | Drug spreads to tissues; protein binding limits free drug | Only FREE (unbound) drug is pharmacologically active |
| Metabolism (M) | ADME | Phase 3 | Liver CYP450 converts drug to metabolites | Prodrugs become active here; inhibitors/inducers alter levels |
| Excretion (E) | ADME | Phase 4 | Kidneys eliminate drug/metabolites via urine | Renal impairment requires dose adjustment; monitor BUN/creatinine |
| Half-Life / Steady State | ADME | ~4–5 half-lives | 50% of drug eliminated per half-life; SS = 4–5 half-lives | After 5 half-lives ≈ 97% of drug is eliminated from body |
| CYP450 Inhibition | Interaction | Usually rapid onset | Blocks CYP enzyme → drug metabolism slows → levels rise | Grapefruit juice inhibits CYP3A4 — can cause dangerous drug level spikes |
| CYP450 Induction | Interaction | Onset: days to weeks | Up-regulates CYP enzyme → drug metabolized faster → levels fall | Rifampin, St. John's Wort — reduce levels of warfarin, OCP, many drugs |
| Additive Effect | Interaction | — | Combined effect = sum of individual effects (1+1=2) | Two antihypertensives: double the blood pressure lowering effect |
| Synergistic Effect | Interaction | — | Combined effect > sum of individual effects (1+1>2) | Alcohol + sedative = synergistic CNS depression — potentially fatal |
| Antagonistic Effect | Interaction | — | One drug reduces the effect of another (1+1<2) | Naloxone antagonizes opioid receptors — reverses overdose |
| Narrow Therapeutic Index (NTI) | Interaction | — | Small margin between therapeutic and toxic dose | Warfarin, digoxin, lithium, phenytoin — require TDM and dose precision |
Real Examples
Clinical scenarios showing how pharmacology concepts apply in pharmacy practice
Practice Quiz
10 PTCE-style questions covering all four pharmacology pillars — get instant explanations after each answer
Drug Advisor
Answer a few questions and get targeted pharmacology guidance — routes, forms, ADME, and interactions
Memory Hooks
Click any card to flip it and reveal the answer — 8 high-yield pharmacology mnemonics for the PTCE
