12.2 Interactions & Adverse Effects
Key Takeaways
- CYP450 inhibitors raise substrate drug levels toward toxicity; inducers lower them toward therapeutic failure — warfarin, phenytoin, and theophylline are the classic victims
- Hypokalemia from loop and thiazide diuretics potentiates digoxin toxicity; maintain potassium ≥ 4.0 mEq/L in patients on digoxin
- NSAIDs, many antibiotics (TMP-SMX, macrolides, fluoroquinolones), and acute alcohol use all elevate the INR in patients on warfarin
- Peak and trough timing matters: draw aminoglycoside troughs just before the next dose; vancomycin trough goal is typically 15–20 mcg/mL for serious infections
- Lithium's narrow therapeutic range (0.6–1.2 mEq/L) means dehydration, NSAIDs, thiazides, and ACE inhibitors can push levels into toxicity
How Drug–Drug Interactions Happen
Interactions are either pharmacokinetic (what the body does to the drug — absorption, distribution, metabolism, excretion) or pharmacodynamic (what the drug does to the body — effects at receptors).
CYP450 Metabolism
The cytochrome P450 enzyme family in the liver metabolizes most drugs. Two mechanisms dominate exam questions:
- Enzyme inhibitors (e.g., azole antifungals, macrolides, grapefruit juice, cimetidine) slow metabolism of a substrate drug → higher levels → toxicity
- Enzyme inducers (e.g., rifampin, phenytoin, carbamazepine, St. John's wort, chronic alcohol) speed metabolism → lower levels → therapeutic failure
The drugs most often harmed are those with narrow therapeutic indexes: warfarin, phenytoin, digoxin, theophylline, lithium.
Pharmacodynamic Interactions
- Additive — 1 + 1 = 2 (two sedatives)
- Synergistic — 1 + 1 = 3 (aspirin plus warfarin on bleeding risk)
- Antagonistic — one drug blunts another (naloxone reversing opioids; beta-agonists vs beta-blockers)
Classic Tested Interaction Pairs
| Interaction | Mechanism | Nursing action |
|---|---|---|
| Warfarin + antibiotics (TMP-SMX, macrolides, fluoroquinolones, metronidazole) | CYP inhibition and gut flora reduction (less vitamin K) | Expect INR to rise; monitor closely, anticipate dose reduction |
| Warfarin + NSAIDs/aspirin | Antiplatelet effect + GI mucosal injury | Increased bleeding; avoid combination when possible |
| Digoxin + loop/thiazide diuretics | Diuretic-induced hypokalemia potentiates digoxin toxicity | Keep K⁺ ≥ 4.0 mEq/L; watch for anorexia, nausea, halos, arrhythmias |
| ACE inhibitor + potassium-sparing diuretic (spironolactone) or K supplements | Both raise potassium | Risk of hyperkalemia; monitor K⁺ |
| MAOI + tyramine-rich foods (aged cheese, cured meats, tap/draft beer, soy sauce) | Tyramine not broken down | Hypertensive crisis — teach dietary avoidance |
| Statins + grapefruit juice | CYP3A4 inhibition raises statin levels | Risk of myopathy/rhabdomyolysis; avoid grapefruit with simvastatin and atorvastatin |
Also tested: MAOIs + SSRIs or meperidine → serotonin syndrome (agitation, hyperthermia, clonus — treat by stopping drugs and supportive care); aminoglycosides + loop diuretics → additive ototoxicity; tetracyclines/fluoroquinolones + calcium, iron, antacids → chelation and poor absorption (separate by 2 hours).
Food–Drug Interactions
- Vitamin K and warfarin: patients do not eliminate green leafy vegetables — they keep intake consistent week to week so the INR stays stable. Sudden increases lower the INR; sudden decreases raise it
- Grapefruit juice inhibits intestinal CYP3A4 for 24–72 hours: avoid with many statins, calcium channel blockers (felodipine, nifedipine), amiodarone, and some benzodiazepines
- Dairy/calcium chelates tetracyclines and fluoroquinolones
- High-protein meals can reduce levodopa absorption
- Enteral feedings bind phenytoin — hold tube feeds 1–2 hours before and after the dose
Adverse Drug Reaction Taxonomy
- Side effect — predictable, often unavoidable effect at therapeutic doses (dry mouth from anticholinergics)
- Adverse drug reaction (ADR) — any noxious, unintended response at normal doses; includes side effects but implies harm
- Toxicity — dose-related organ damage (aminoglycoside nephrotoxicity); often predictable from levels
- Idiosyncratic reaction — unpredictable, genetically based, not dose-related (e.g., hemolysis in G6PD deficiency)
- Allergic (hypersensitivity) reaction — immune-mediated, not dose-related
Allergic Severity and Anaphylaxis
Reactions range from mild urticaria to anaphylaxis, a life-threatening type I hypersensitivity with airway edema, bronchospasm, and hypotension. Priorities: stop the drug, maintain the airway, give epinephrine 0.3–0.5 mg IM (1:1,000) into the anterolateral thigh — epinephrine is always first, before antihistamines or steroids. Then position flat with legs elevated, high-flow oxygen, IV fluids, and monitor for a biphasic reaction. Document the allergy and notify pharmacy; a true allergy is never "tested" by re-challenge outside a controlled setting.
Black Box Warnings
A black box warning is the FDA's strongest labeling alert for serious or fatal risks. Examples to know: fluoroquinolones (tendon rupture), clozapine (agranulocytosis), valproate (hepatotoxicity, teratogenicity), opioids (respiratory depression), and metoclopramide (tardive dyskinesia).
Cross-Reactivity and Severe Cutaneous Reactions
Penicillin–cephalosporin cross-reactivity is lower than the historically quoted 10% (closer to 1–2%), but a patient with a documented anaphylactic penicillin reaction should not receive a cephalosporin without specialist evaluation. Stevens-Johnson syndrome (SJS) and toxic epidermal necrolysis (TEN) are life-threatening mucocutaneous reactions triggered by drugs such as sulfonamides, allopurinol, lamotrigine, and carbamazepine — a new rash with mucosal lesions or blistering after starting one of these drugs means stop it and escalate immediately. Distinguish serotonin syndrome (hyperreflexia, clonus, rapid onset, serotonergic drugs) from neuroleptic malignant syndrome (lead-pipe rigidity, slow onset over days, dopamine antagonists, elevated CK); both cause hyperthermia and both require stopping the offending drug.
Monitoring Labs Nurses Must Know
| Drug | Key lab | Target/toxicity |
|---|---|---|
| Aminoglycosides (gentamicin) | Peak 30–60 min after infusion; trough just before next dose | Gentamicin trough >2 mcg/mL → nephro-/ototoxicity risk |
| Vancomycin | Trough before 4th dose (AUC-guided dosing increasingly used) | Trough 15–20 mcg/mL for serious infections; watch for red-man syndrome (slow the infusion — a rate reaction, not allergy) |
| Lithium | Level 12 hours after last dose | Therapeutic 0.6–1.2 mEq/L; toxicity >1.5; severe >2.5 (tremor, confusion, seizures). Dehydration, low sodium, NSAIDs, thiazides, ACE inhibitors raise levels |
| Digoxin | Level 6–8 hours post dose | Therapeutic 0.5–2.0 ng/mL (often 0.5–0.9 for heart failure); hold and notify for apical pulse <60 bpm or signs of toxicity |
| Warfarin | INR | 2.0–3.0 for most indications; 2.5–3.5 for mechanical mitral valves |
| Heparin | aPTT or anti-Xa | Follow protocol nomograms |
| Phenytoin | Level | Therapeutic 10–20 mcg/mL |
| Theophylline | Level | Therapeutic 10–20 mcg/mL |
A patient on warfarin for atrial fibrillation is started on trimethoprim-sulfamethoxazole for a urinary tract infection. The nurse should anticipate which change?
A patient taking digoxin and furosemide reports nausea, anorexia, and seeing yellow-green halos around lights. Which laboratory value most likely explains these symptoms?
A patient develops urticaria, wheezing, tongue swelling, and a blood pressure of 78/40 mmHg minutes after an IV antibiotic is started. After stopping the infusion, which action takes priority?