Chapter 1 · Module 4 of 6 · General Principles

Drug Safety and Adverse Effects

Classifying adverse drug reactions, recognizing pharmacokinetic and pharmacodynamic interactions, and identifying dangerous drug combinations and contraindications

Adverse drug reactions are classified by mechanism: Type A reactions are dose-dependent extensions of pharmacological action (common, manageable by dose reduction); Type B reactions are idiosyncratic and unpredictable (rare, often requiring permanent discontinuation); Type C reactions are cumulative with chronic use
Drug interactions are either pharmacokinetic — one drug alters the absorption, metabolism, or elimination of another — or pharmacodynamic, where combined effects are additive, synergistic, or antagonistic; CYP enzyme inhibition raises drug levels (toxicity risk) and CYP induction lowers them (efficacy failure)
Four combinations demand particular vigilance: MAOIs with serotonergic drugs (serotonin syndrome), MAOIs with tyramine-rich foods (hypertensive crisis), multiple QT-prolonging drugs (torsades de pointes), and opioids with benzodiazepines (fatal respiratory depression)
Drug-disease contraindications arise when a drug's pharmacological mechanism directly worsens the patient's underlying condition — the same property that makes a drug therapeutically useful in one context makes it harmful in another
Systematic contraindication checking — not recall of a list — is the clinical skill: ask how each drug's mechanism interacts with each active problem on the patient's problem list

Section 1 — Types of Adverse Drug Reactions

Section 1

ADR Classification — Types A, B, C, and Allergic Reactions

Type A

Dose-Dependent — Predictable

Extension of the drug's pharmacological action. Most common type. Managed by dose reduction. Examples: warfarin bleeding, insulin hypoglycemia, beta-blocker bradycardia.

Type B

Idiosyncratic — Unpredictable

Unrelated to the drug's known mechanism. Rare. Often requires permanent discontinuation. Examples: isoniazid hepatotoxicity, chloramphenicol aplastic anemia, SJS.

Immediate Allergy

Minutes — IgE-Mediated

Urticaria, angioedema, anaphylaxis. Treat immediately with epinephrine. Permanent avoidance required. Classic example: penicillin anaphylaxis.

Delayed Allergy

Hours to Days — Cell-Mediated

Rash, contact dermatitis, and at the severe end, SJS and TEN. Implicated drugs: allopurinol, sulfonamides, antiepileptics. Genetic risk markers exist for some.

Section 2 — Drug Interactions

Section 2

Pharmacokinetic and Pharmacodynamic Interactions

CYP Inhibition

Raises drug levels → toxicity risk

Azole antifungals, macrolides, amiodarone, ritonavir, and grapefruit juice inhibit hepatic CYP enzymes. The object drug accumulates. Consequences: warfarin bleeding, statin myopathy, QT prolongation.

CYP Induction

Lowers drug levels → efficacy failure

Rifampin, phenytoin, carbamazepine, phenobarbital, and St. John's wort accelerate metabolism. Consequences: oral contraceptive failure, antiretroviral failure, transplant rejection from reduced immunosuppressant levels.

Additive

Combined effect equals sum — exploitable therapeutically or hazardous

Intentional: combination antihypertensives. Hazardous: two QT-prolonging drugs producing additive interval prolongation and torsades de pointes risk.

Synergistic

Combined effect exceeds sum — exploited in antimicrobial therapy

Trimethoprim + sulfamethoxazole: each blocks a different step in bacterial folate synthesis. Together far more effective than either alone.

Antagonistic

Combined effect less than sum — the basis of reversal agents

Naloxone reverses opioids, flumazenil reverses benzodiazepines, protamine reverses heparin. Pharmacodynamic antagonism exploited deliberately in toxicology.

Section 3 — Dangerous Drug Combinations

Section 3

Four High-Risk Combination Patterns

Serotonin Syndrome

MAOIs + SSRIs / tramadol / meperidine / linezolid

Excess synaptic serotonin → neuromuscular excitability (clonus, hyperreflexia), autonomic instability (hyperthermia, tachycardia), altered mental status. Potentially fatal. Two-week washout required between MAOI and any serotonergic drug.

Hypertensive Crisis

MAOIs + tyramine-rich foods (aged cheese, cured meats, fermented foods)

Dietary tyramine normally degraded in gut by monoamine oxidase. When enzyme is blocked, tyramine is absorbed intact → massive norepinephrine release → severe hypertension → risk of intracerebral hemorrhage. Dietary counseling is mandatory for all patients on MAOIs.

Torsades de Pointes

Two or more QT-prolonging drugs (antiarrhythmics + macrolides / fluoroquinolones / antipsychotics)

Additive QT interval prolongation → risk of fatal ventricular arrhythmia. Electrolyte abnormalities (low potassium, low magnesium) compound risk. Baseline electrocardiogram required before adding a second QT-prolonging drug.

Respiratory Depression

Opioids + benzodiazepines (also: alcohol, non-benzodiazepine sedative-hypnotics)

Additive CNS and respiratory depression. Major contributor to opioid overdose deaths. Avoid combination when possible. If co-prescribing is necessary: lowest effective doses, explicit patient counseling, co-prescribe naloxone.

Section 4 — Drug-Disease Contraindications

Section 4

High-Yield Drug-Disease Contraindications

Drug	Avoid In	Mechanism of Harm
Beta-blockers	Decompensated heart failure, high-degree AV block, severe asthma	Reduce contractility; block AV conduction; beta-2 blockade causes bronchoconstriction
NSAIDs	Heart failure, chronic kidney disease, volume depletion, active peptic ulcer disease	Block prostaglandins maintaining renal perfusion; cause sodium retention worsening heart failure; damage gastric mucosa
ACE inhibitors	Pregnancy, bilateral renal artery stenosis, history of angioedema	Fetal renal toxicity; precipitate renal failure in stenotic kidneys; bradykinin accumulation causes angioedema
Metformin	Severe renal impairment, tissue hypoxia states	Accumulates when renal clearance is reduced; inhibits lactate metabolism → lactic acidosis
Anticholinergics	Narrow-angle glaucoma, urinary retention, benign prostatic hyperplasia	Block aqueous humor drainage (acute angle-closure glaucoma); relax detrusor, contract bladder neck (urinary retention)
Statins	Pregnancy, active liver disease	Cholesterol essential for fetal development (teratogenic); may worsen active hepatic disease

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
Ritter JM, Flower R, Henderson G, Loke YK, MacEwan D, Rang HP	Rang & Dale's Pharmacology, 9th edition	Elsevier, 2019
Brunton LL, Hilal-Dandan R, Knollmann BC, eds.	Goodman & Gilman's The Pharmacological Basis of Therapeutics, 13th edition	McGraw-Hill, 2018
Edwards IR, Aronson JK	Adverse drug reactions: definitions, diagnosis, and management	Lancet, 2000; 356(9237):1255–1259
Cascorbi I	Drug interactions: principles, examples, and clinical consequences	Deutsches Ärzteblatt International, 2012; 109(33–34):546–556
Waller DG, Sampson AP	Medical Pharmacology and Therapeutics, 5th edition	Elsevier, 2018
Katzung BG, Trevor AJ, eds.	Basic and Clinical Pharmacology, 15th edition	McGraw-Hill, 2021