Everything You Need to Know About Drug-drug interactions for Step 1

Drug–drug interactions (DDIs) are one of those “easy points if you’re prepared, brutal if you’re not” Step topics—because the exam loves pairing a classic medication with a second agent that quietly flips its metabolism, binding, or physiologic effect. The good news: DDIs are highly pattern-based. If you learn a small set of mechanisms and a short list of culprit drugs, you can predict most testable interactions on Step 1 (and a surprising number on Step 2).

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What exactly is a drug–drug interaction?

A drug–drug interaction occurs when one drug alters the effect of another drug, causing:

• Increased toxicity
• Decreased efficacy
• Unexpected clinical effects (e.g., serotonin syndrome, QT prolongation, bleeding)

Big classification: PK vs PD

First Aid cross-reference: General Principles of Pharmacology (drug metabolism, enzyme induction/inhibition, therapeutic index), plus drug-specific sections (CYP450 table, anticoagulants, autonomics, psych, antimicrobials).

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Pathophysiology (the mechanisms Step loves)

1) CYP450 enzyme inhibition (fast → toxicity risk)

Inhibition decreases metabolism of substrate drugs → increased levels → toxicity.

• Time course: often rapid (hours to days)
• Classic “giveaway” vignette: “Patient stable on warfarin, starts antibiotic, now bleeding + INR high.”

High-yield CYP inhibitors to memorize

A classic mnemonic is SICKFACES.COM, but on exams the most common culprits include:

• Macrolides (especially erythromycin, clarithromycin)
• Azole antifungals (ketoconazole, itraconazole, fluconazole; variable strength)
• Protease inhibitors (ritonavir is a big one)
• Amiodarone
• Cimetidine
• Grapefruit juice (intestinal CYP3A4 inhibition)
• Isoniazid (can inhibit CYP enzymes)

What Step asks: “Which medication caused toxicity by inhibiting CYP?” or “What happens to the level of drug X?”

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2) CYP450 enzyme induction (slow → therapeutic failure risk)

Induction increases metabolism of substrate drugs → decreased levels → treatment failure.

• Time course: slow onset (days to weeks) because it requires increased enzyme transcription/translation
• Classic vignette: “Breakthrough seizures / pregnancy / clot / transplant rejection after starting rifampin.”

High-yield CYP inducers (Step favorites)

• Rifampin
• Carbamazepine
• Phenytoin
• Phenobarbital
• St. John’s wort
• Griseofulvin (shows up with OCP failure)

Clinical consequences to recognize:

• Oral contraceptive failure (spotting, unexpected pregnancy)
• Warfarin failure (↓ INR → thrombosis)
• Calcineurin inhibitor failure (↓ tacrolimus/cyclosporine → transplant rejection)
• Antiretroviral failure (depending on regimen)

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3) P-glycoprotein (P-gp) transport interactions

P-gp is an ATP-dependent efflux pump (e.g., in gut, BBB, kidney) that reduces intracellular drug levels.

• Inhibiting P-gp can increase drug absorption/levels
• Inducing P-gp can decrease levels

Board-relevant example:

• Digoxin levels can rise with P-gp inhibitors (clinically important; Step sometimes frames this as “inhibits renal secretion/transport”).

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4) Protein binding displacement (less common than students think)

Highly protein-bound drugs (e.g., warfarin) can be displaced, transiently increasing free drug.
But: the body often re-equilibrates via metabolism/clearance, so Step more often tests CYP effects than pure displacement.

Still, know the concept:

• Warfarin is highly albumin-bound → displacement can increase bleeding risk (often paired with metabolism inhibition in real life).

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5) Altered absorption (chelators, pH changes, gut flora)

These are often “mechanism straightforward → free points.”

Chelation (big Step 1 favorite)

Divalent/trivalent cations bind drugs → reduced absorption:

• Tetracyclines + Ca²⁺/Mg²⁺/Fe²⁺ (milk, antacids, iron) → ↓ absorption
• Fluoroquinolones + cations → ↓ absorption
• Levothyroxine + iron/calcium → ↓ absorption (common clinically)

Gastric pH changes

• PPIs/H2 blockers can reduce absorption of drugs needing acid (classically discussed with antifungals like ketoconazole; real-world varies)

Gut flora effects (classic warfarin tie-in)

• Broad-spectrum antibiotics ↓ vitamin K–producing gut flora → ↑ warfarin effect → bleeding, ↑ INR

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6) Altered excretion (renal tubular secretion; urine pH)

Competition for secretion

• Probenecid decreases tubular secretion of weak acids (e.g., penicillin) → increases levels
• NSAIDs can reduce renal perfusion → raise levels of renally cleared drugs (e.g., lithium)

Urine alkalinization/acidification (weak acids/bases)

• Weak acids (e.g., aspirin) are “trapped” in basic urine → increased excretion
• Weak bases (e.g., amphetamines) are “trapped” in acidic urine → increased excretion

This is more “general principles” than common DDI vignettes, but it’s fair game on Step 1.

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Clinical presentation: how DDIs show up in vignettes

Think in patterns—toxicity vs failure vs syndrome.

A) Toxicity from increased levels (PK)

Common “target” drugs with narrow therapeutic index (NTI):

• Warfarin → bleeding, ↑ INR
• Digoxin → nausea, yellow vision, arrhythmias
• Lithium → tremor, ataxia, nephrogenic diabetes insipidus
• Theophylline → seizures, arrhythmias
• Phenytoin → nystagmus, ataxia, gingival hyperplasia (toxicity adds CNS findings)

B) Therapeutic failure from decreased levels (PK)

• Breakthrough seizures (antiepileptics affected by inducers/inhibitors)
• Pregnancy on OCPs (enzyme inducers)
• Transplant rejection (tacrolimus/cyclosporine levels drop)
• Clotting while “on warfarin” (inducers lower INR)

C) Additive/synergistic physiologic effects (PD)

• Bleeding: anticoagulants + antiplatelets + NSAIDs
• Hypotension/bradycardia: nitrates + PDE-5 inhibitors; beta-blockers + non-DHP CCBs
• CNS/respiratory depression: opioids + benzodiazepines + alcohol
• Hyperkalemia: ACEi/ARB + K⁺-sparing diuretics + K⁺ supplements
• Serotonin syndrome: serotonergic combinations (see below)
• QT prolongation: multiple QT-prolongers together

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Diagnosis: a Step-friendly approach

1) Identify a “trigger” change

DDIs are often tested as: “A stable patient started medication X and now has symptom Y.”

Ask:

• What drug was added (or stopped)?
• Is it a CYP inhibitor/inducer?
• Is there a PD synergy (same physiologic direction)?

2) Check for an NTI drug

If the patient is on warfarin, digoxin, lithium, theophylline, phenytoin—your DDI radar should immediately go up.

3) Use key labs/monitoring clues

• Warfarin: ↑ INR (bleeding risk) or ↓ INR (clot risk)
• Digoxin: arrhythmia + GI symptoms; level may be provided
• Lithium: neurologic symptoms; kidney issues; level may be provided
• Serotonin syndrome: hyperreflexia/clonus is a major clue (see below)

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Treatment (and prevention): what to do about DDIs

General management principles

• Stop or replace the offending interacting drug when possible
• Adjust dose of the affected drug (especially if must continue both)
• Monitor levels or physiologic endpoints:
  • INR for warfarin
  • Drug levels for digoxin, lithium, phenytoin (context-dependent)
  • EKG for QT risk or arrhythmias
• Provide antidotes/supportive care for acute toxicity (high yield below)

High-yield specific “what to do” scenarios

• Warfarin toxicity/bleeding: hold warfarin; give vitamin K ± PCC/FFP depending on severity (Step 2 leans into this)
• Opioid overdose (PD synergy common): naloxone
• Benzodiazepine overdose (careful): flumazenil (rarely used clinically due to seizure risk; Step may ask mechanism)
• Serotonin syndrome: stop serotonergic agents, supportive care, cyproheptadine (5-HT antagonist)
• Torsades from QT prolongation: IV magnesium

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HY “association list” you should know cold

1) Warfarin: the Step 1 interaction magnet

Warfarin effect increases (↑ INR, bleeding) with:

• CYP inhibitors (macrolides, azoles, amiodarone, cimetidine, etc.)
• Broad-spectrum antibiotics (↓ gut flora → ↓ vitamin K)
• Aspirin/NSAIDs (PD: platelet inhibition + GI bleeding risk)

Warfarin effect decreases (↓ INR, clot risk) with:

• CYP inducers (rifampin, carbamazepine, phenytoin, phenobarbital, St. John’s wort)
• Increased vitamin K intake (not a drug-drug interaction, but commonly paired as a distractor)

First Aid cross-reference: Anticoagulants (warfarin), CYP450 inducers/inhibitors tables.

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2) Nitrates + PDE-5 inhibitors = dangerous hypotension

• Nitroglycerin/isosorbide + sildenafil/tadalafil/vardenafil
• Mechanism: excessive cGMP-mediated vasodilation → severe hypotension, syncope

First Aid cross-reference: Autonomics/vasodilators; erectile dysfunction drugs.

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3) Serotonin syndrome combos (PD)

Common triggers:

• SSRI/SNRI + MAOI
• SSRI + linezolid (MAOI activity)
• SSRI + tramadol
• SSRI + triptans (classically taught; real-world risk variable, but Step-likely)
• Multiple serotonergic agents together

Clinical triad (think: “hot + altered + hyperreflexic”):

• Mental status changes (agitation, confusion)
• Autonomic instability (hyperthermia, diaphoresis, diarrhea)
• Neuromuscular hyperactivity (clonus, hyperreflexia)

Treatment: stop drugs, supportive care, cyproheptadine

First Aid cross-reference: Antidepressants (SSRIs, SNRIs, MAOIs), antimicrobials (linezolid), analgesics (tramadol).

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4) QT prolongation stacking (PD)

QT risk increases when combining multiple QT-prolongers. Big categories:

• Class Ia and Class III antiarrhythmics (quinidine, procainamide, sotalol, amiodarone*)
• Macrolides
• Fluoroquinolones
• Antipsychotics (especially typicals, some atypicals)
• Methadone
• Ondansetron (commonly taught)

*Amiodarone prolongs QT but less torsades than expected; Step may still treat it as QT-prolonging.

Presentation: syncope, palpitations; polymorphic VT (torsades)
Treatment: IV magnesium

First Aid cross-reference: Antiarrhythmics; antimicrobial sections; psych pharm.

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5) Lithium toxicity: dehydration + NSAIDs/ACEi/diuretics

Lithium levels rise with:

• Thiazide diuretics (↑ lithium reabsorption in proximal tubule)
• NSAIDs (↓ renal perfusion → ↓ clearance)
• ACE inhibitors/ARBs (can reduce GFR/alter handling)

Presentation: coarse tremor, ataxia, confusion, GI upset, nephrogenic DI
Management: stop offending agent; hydration; severe cases may need dialysis

First Aid cross-reference: Mood stabilizers (lithium); renal physiology tie-ins.

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6) Digoxin toxicity: beware transport/clearance changes + hypokalemia

Digoxin toxicity risk increases with:

• Hypokalemia (often from loop/thiazide diuretics): less K⁺ competition at Na⁺/K⁺-ATPase → more digoxin effect
• Drugs that raise digoxin levels (classically amiodarone, some macrolides; transport interactions may be referenced)

Presentation: nausea/vomiting, confusion, yellow vision, arrhythmias
Treatment: digoxin immune Fab (Digibind) for serious toxicity

First Aid cross-reference: Cardiac glycosides (digoxin); diuretics; antiarrhythmics.

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7) Chelation: “don’t take with milk/antacids/iron”

• Tetracyclines: doxycycline + dairy/antacids/iron → ↓ absorption
• Fluoroquinolones: ciprofloxacin + cations → ↓ absorption

First Aid cross-reference: Antibiotics—tetracyclines, fluoroquinolones.

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8) Beta-blocker + non-DHP calcium channel blocker = bradycardia/heart block

• Metoprolol/atenolol/etc. + verapamil/diltiazem
• PD synergy: ↓ AV node conduction + negative inotropy

Presentation: bradycardia, hypotension, dizziness, possible AV block
First Aid cross-reference: Cardiovascular pharm (beta-blockers, CCBs).

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A quick “DDI algorithm” for Step questions

1. Is the adverse effect from too much drug or too little drug?

   • Toxicity → think inhibitor, reduced clearance, transport effects
   • Failure → think inducer, reduced absorption

2. Is there an NTI drug involved?
   Warfarin, digoxin, lithium, theophylline, phenytoin are classic.

3. Does this look like PD synergy?

   • Bleeding (anticoagulant + antiplatelet/NSAID)
   • Hypotension (nitrates + PDE-5)
   • CNS depression (opioid + benzo)
   • Serotonin syndrome (stack serotonergic drugs)

4. Pick the most “board-famous” interacting agent in the list.
   Rifampin, macrolides, azoles, amiodarone, cimetidine, grapefruit juice, St. John’s wort show up constantly.

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High-yield tables (memorize-friendly)

CYP450 inducers vs inhibitors (Step essentials)

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Classic “pairings” the exam likes

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First Aid cross-references (where to anchor this)

Use these as “home bases” while you annotate UWorld/NBME patterns:

• General Principles: CYP450 induction/inhibition, first-pass metabolism, therapeutic index
• Autonomics/Cardio: nitrates, PDE-5 inhibitors, beta-blockers, CCBs, antiarrhythmics (QT)
• Antimicrobials: macrolides, azoles, rifampin, tetracyclines, fluoroquinolones, linezolid
• Psych: SSRIs/SNRIs/MAOIs, lithium
• Heme: warfarin and monitoring (INR)

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Final rapid-fire high-yield facts

• Inhibitors act fast → toxicity soon after starting the new drug.
• Inducers act slow → failure after days to weeks.
• Warfarin interactions are disproportionately tested; always ask “INR up or down?”
• QT prolongation is additive; torsades treatment = IV magnesium.
• Serotonin syndrome clue is clonus/hyperreflexia (not just “fever + confusion”).
• Chelation (tetracyclines/fluoroquinolones) is a common “missed freebie.”