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Pharmacy — Interactions, Adverse Effects & Counseling flashcards

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Classic drug interactions (warfarin, CYP450 and P-gp, serotonergic, QT, renal and absorption), hallmark adverse effects, must-say counseling points for patches, sprays, insulin, iron and the pill, narrow-therapeutic-index drugs and Beers Criteria concepts, mostly as applied cases.

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Learn deck · part of PTCB PTCE / NAPLEX · written against: Top-drug lists and federal pharmacy law as of 2026; PTCE content outline effective 6 January 2026. Updated 2026-10-10.

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  1. A patient stable on warfarin is prescribed sulfamethoxazole/trimethoprim for a UTI. What happens to the INR, and why?
    The INR rises — sulfamethoxazole inhibits CYP2C9, the main pathway for the more potent S-enantiomer of warfarin, raising bleeding risk.

    Watch out: check the INR within days or choose a non-interacting antibiotic.
    Warfarin Interactions
  2. A patient on simvastatin is prescribed clarithromycin for pneumonia. What is the concern, and what is a common fix?
    Clarithromycin is a strong CYP3A4 inhibitor, so simvastatin levels rise sharply and rhabdomyolysis can follow — the combination is contraindicated.

    Watch out: hold the statin for the course or use azithromycin, which barely inhibits CYP3A4.
    CYP4Interactions
  3. A patient taking phenelzine asks for an OTC pseudoephedrine decongestant. What should the pharmacist say?
    Do not take it — sympathomimetic decongestants with an MAOI can trigger a hypertensive crisis; refer to the prescriber for a safer option such as saline spray.
    Serotonergic & MAOI Interactions
  4. Which commonly dispensed drug groups are classic QT-prolonging risks?
    Macrolides and fluoroquinolones, antipsychotics (haloperidol, ziprasidone), methadone, and ondansetron or citalopram — plus class III antiarrhythmics.

    Watch out: risk climbs when two or more QT drugs are combined, especially in older women and patients with low potassium or magnesium.
    QT Prolongation
  5. A heart failure patient on lisinopril is started on spironolactone. Which lab is the priority?
    Serum potassium (with creatinine) — both drugs reduce potassium excretion, so hyperkalemia is the key risk.

    Watch out: potassium supplements and potassium-based salt substitutes are avoided.
    Pharmacodynamic & Renal Interactions
  6. A patient on ciprofloxacin also takes a calcium supplement and an antacid. How should they be scheduled?
    Separate them by several hours, taking the ciprofloxacin first — calcium, magnesium, aluminum, iron and zinc chelate fluoroquinolones and can cause treatment failure.
    Absorption & Chelation
  7. A patient starting atorvastatin reports unexplained muscle pain and weakness. What adverse effect is suspected, and which lab is checked?
    Statin-associated myopathy, checked with a creatine kinase (CK) level; dark urine plus a very high CK suggests rhabdomyolysis and the statin is stopped.
    Hallmark Adverse Effects
  8. How should a patient take an oral bisphosphonate such as alendronate?
    First thing in the morning on an empty stomach with a full glass of plain water, then stay upright and wait before eating or taking other medicines; this prevents esophageal injury and preserves the very poor absorption.
    Counseling Points
  9. Which four drugs are the textbook examples of narrow-therapeutic-index (NTI) medications?
    Warfarin, digoxin, lithium and phenytoin — others include levothyroxine, carbamazepine, tacrolimus and cyclosporine.
    Narrow Therapeutic Index
  10. What are the Beers Criteria, and who publishes them?
    The American Geriatrics Society's list of potentially inappropriate medications for adults 65 and older, updated periodically (the 2023 edition was still the current one as of mid-2026).
    Older Adults & Beers Criteria

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About these cards

Review status: Second-reader reviewed, with corrections applied.

Checked against:

  • Checked against FDA prescribing information on DailyMed (US National Library of Medicine; public domain) and the FDA Drug Development and Drug Interactions table of substrates, inhibitors and inducers; reference only, no text copied.
  • Older-adult items checked against the American Geriatrics Society 2023 Updated Beers Criteria (reference only); disposal guidance against FDA's drug-disposal pages (take-back first, then the flush list; public domain).
  • Missed-pill and backup rules checked against the CDC US Selected Practice Recommendations for Contraceptive Use (2024); triptan–SSRI evidence against the American Headache Society position statement (reference only; no text copied).
  • Torsades de pointes treatment checked against the 2025 American Heart Association Guidelines for CPR and ECC, Part 9: Adult Advanced Life Support; reference only, no text copied.
  • NAPLEX® is a registered trademark of the National Association of Boards of Pharmacy (NABP), and PTCB is a trademark of the Pharmacy Technician Certification Board; neither is affiliated with or endorses this product.

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