Study Guide | Actual verified study complete solutions |
2026/27 Updates | 100% correct
FOUNDATIONS IN PHARMACOLOGY
1. Pharmacokinetics = “What the body does to the drug”
ADME (know this cold)
• Absorption → how drug enters blood
o Affected by: route, blood flow, pH
• Distribution → where drug goes
o Protein binding matters (low albumin = more free drug )
• Metabolism → liver breakdown
o Mainly via CYP450 enzymes- group of enzymes in the liver that metabolize
xenobiotics and endogenous substances (steroids/fatty acids)
• Excretion → kidneys remove drug
Half-life (HIGH-YIELD)
• Time for drug level to ↓ by 50%
• 4–5 half-lives = steady state
o Ex: A drug has a half-life of 12 hours. Approximately how long will it take to
reach steady state? 48 - 60 hours
• Long half-life → risk of accumulation/toxicity
Hepatic Drug Metabolism
, • Done by
Cytochrome P450 enzyme system
Two key ideas:
• Inducers → ↓ drug levels (faster metabolism)
o Inducers = decrease in drug levels
• Inhibitors → ↑ drug levels (toxicity risk)
o Inhibitors = inhibit metabolism = increase of drug levels
Agonists vs Antagonists
• Agonist → activates receptor
• Antagonist → blocks receptor
Easy:
Agonist = ON
Antagonist = OFF
Drug Interactions (common test topic)
• Additive (↑ effect)
• Antagonistic (↓ effect)
• CYP450 interactions (biggest source)
Classic Examples
Inducers (↓ drug levels) by increasing the enzyme synthesis
• Rifampin “Rifampin = Reduces other drug levels”
• Phenytoin
• Carbamazepine
• St. John’s Wort
Inhibitors (↑ drug levels)
• Amiodarone
, • Grapefruit juice
• Azole antifungals (e.g., ketoconazole)
• Macrolides (e.g., erythromycin)
Clinical Tip
• Toxicity after starting a new drug → likely CYP inhibitor
• Therapy fails after starting a new drug → likely CYP inducer
Clinical Scenarios
Scenario Likely Mechanism
Drug becomes toxic after starting new med CYP inhibitor
Drug becomes ineffective after starting new med CYP inducer
Unexpected bleeding on warfarin CYP inhibitor
Oral contraceptive fails on new med CYP inducer
Exam Example
A patient stabilized on warfarin starts erythromycin and develops a high INR. Why?
• Erythromycin = CYP inhibitor → warfarin metabolism ↓ → INR ↑ → bleeding risk
Adverse Drug Reactions (ADR) – how to prevent
• Start low, go slow
• Check kidney/liver function
• Review all meds (polypharmacy!)
• Adjust for age
Boxed Warnings