Drug Interactions & Contraindications

1. A patient stable on warfarin asks the technician to recommend an OTC pain reliever for a headache. Which product should be flagged for pharmacist consultation before recommending?

NSAIDs like ibuprofen increase bleeding risk in patients on warfarin through additive antiplatelet effects and GI mucosal irritation. Acetaminophen (in moderate doses) is generally the preferred OTC analgesic for patients on warfarin; loratadine (an antihistamine) and docusate (a stool softener) do not carry this bleeding interaction.

2. A patient on long-term warfarin therapy should be counseled to keep their intake of which nutrient consistent, since large swings can destabilize their INR?

Vitamin K is required for synthesis of clotting factors and directly opposes warfarin's mechanism of action; sudden increases or decreases in vitamin K intake (leafy greens) push the INR out of therapeutic range. Vitamin C, calcium, and vitamin D do not meaningfully affect warfarin's anticoagulant effect.

3. A patient taking the MAOI phenelzine should be counseled to avoid which type of food due to the risk of a hypertensive crisis?

Aged and fermented foods are high in tyramine. MAOIs block the enzyme that normally breaks down tyramine, allowing it to build up and trigger a surge of norepinephrine that can cause a hypertensive crisis. Fresh produce and plain starches like rice do not carry significant tyramine content.

4. Combining an MAOI with an SSRI carries a significant risk of which potentially life-threatening condition?

Both drug classes increase serotonergic activity; combining them can flood the system with serotonin, causing agitation, hyperthermia, tremor, and autonomic instability. Hypoglycemia is associated with diabetes medications, angioedema with ACE inhibitors, and Stevens-Johnson syndrome with certain antibiotics and anticonvulsants, not this combination.

5. A patient newly started on simvastatin asks about dietary restrictions. What should the technician relay as a significant interaction concern?

Grapefruit juice inhibits intestinal CYP3A4, the enzyme that metabolizes simvastatin (and other statins like atorvastatin and lovastatin), raising statin blood levels and increasing the risk of myopathy and rhabdomyolysis. Orange juice, milk, and coffee do not inhibit CYP3A4 in this way.

6. A patient taking lisinopril presents a new prescription for potassium chloride supplements. Why should the technician flag this combination for pharmacist review?

ACE inhibitors like lisinopril already reduce aldosterone-mediated potassium excretion; adding a potassium supplement on top can push serum potassium to dangerous levels and cause cardiac arrhythmia. The combination does not cause hypokalemia, reduce blood pressure control, or affect blood glucose.

7. A patient on digoxin who also takes furosemide is at increased risk of digoxin toxicity, primarily because furosemide can cause which electrolyte disturbance?

Loop diuretics like furosemide waste potassium. Low serum potassium increases the myocardium's sensitivity to digoxin, raising the risk of toxicity (arrhythmias) even when the digoxin level itself is within the therapeutic range. Furosemide does not typically cause hyperkalemia, hypernatremia, or hypercalcemia.

8. A technician counsels a patient to separate their levothyroxine dose from calcium carbonate supplements by several hours. What is the reason for this instruction?

Calcium (and iron) forms insoluble complexes with levothyroxine in the gut, reducing how much is absorbed and potentially undermining thyroid control. This is an absorption interaction in the GI tract, not a bottle-storage issue, and it decreases rather than increases levothyroxine's effect.

9. A patient picking up a prescription for tetracycline should be counseled that taking it with milk or calcium-fortified products may result in what?

Calcium and other divalent cations chelate with tetracycline, forming an insoluble complex that is poorly absorbed, reducing the antibiotic's effectiveness. Photosensitivity is a separate tetracycline side effect unrelated to dairy intake, and the interaction reduces rather than enhances absorption.

10. A patient should be counseled to separate ciprofloxacin dosing from aluminum- or magnesium-containing antacids by roughly how much time to avoid reduced antibiotic absorption?

Aluminum, magnesium, and calcium in antacids chelate fluoroquinolones like ciprofloxacin, forming an insoluble complex that reduces absorption. Spacing the doses by roughly 2 hours before or 6 hours after minimizes this interaction; a 30-minute gap is insufficient and no separation would significantly blunt the antibiotic's effect.

1. A patient on stable warfarin therapy is prescribed metronidazole for a dental infection. What should the technician flag to the pharmacist?

Metronidazole inhibits CYP2C9, the enzyme responsible for metabolizing warfarin, which raises the INR and increases bleeding risk. It does not reduce warfarin's effect, and a dose increase would be the opposite of the appropriate response — a dose reduction or closer INR monitoring is typically needed instead.

2. A patient taking low-dose methotrexate for rheumatoid arthritis is prescribed trimethoprim/sulfamethoxazole for a UTI. Why is this combination dangerous?

Methotrexate and trimethoprim/sulfamethoxazole both interfere with folate metabolism through related pathways, and combining them can cause severe, additive bone marrow suppression (pancytopenia). This is an additive toxicity issue, not an absorption or effectiveness problem.

3. A patient on lithium is being started on a diuretic for hypertension. Which class of diuretic should be flagged as most likely to raise lithium to toxic levels?

Thiazide diuretics reduce renal sodium excretion, which causes the kidneys to reabsorb more lithium along with it, raising lithium levels and the risk of toxicity (tremor, confusion, arrhythmia). Loop diuretics carry a smaller risk, and osmotic or carbonic anhydrase inhibitor diuretics are not the classic culprits in this interaction.

4. A pharmacy receives a new prescription for sildenafil for a patient whose profile shows an active nitroglycerin prescription. What should the technician do?

Sildenafil and nitrates both cause vasodilation through the nitric oxide/cGMP pathway; combined, they can cause severe, life-threatening hypotension. This is a contraindicated combination requiring pharmacist intervention, not a routine fill or a technician-level substitution decision.

5. Concurrent use of omeprazole may reduce the effectiveness of which antiplatelet medication by inhibiting its conversion to its active form?

Clopidogrel is a prodrug that must be converted to its active metabolite by CYP2C19. Omeprazole is a strong CYP2C19 inhibitor and can reduce this conversion, blunting clopidogrel's antiplatelet effect. Aspirin, warfarin, and apixaban do not require this same enzymatic activation.

6. A prescriber orders metoprolol for a patient already taking verapamil. What is the primary safety concern with this combination?

Both a beta blocker and a non-dihydropyridine calcium channel blocker like verapamil slow conduction through the AV node; combining them significantly increases the risk of severe bradycardia and heart block. This interaction is not primarily about potassium, glucose, or QT interval effects.

7. A patient asks if it's safe to take St. John's Wort, an herbal supplement, along with her oral contraceptive. What concern should the technician relay to the pharmacist?

St. John's Wort induces CYP3A4, which increases the metabolism of estrogen and progestin, lowering hormone levels and reducing contraceptive efficacy — this can lead to breakthrough bleeding or unintended pregnancy. It does not increase hormone levels, and its interaction potential extends well beyond antibiotics.

8. A patient taking metronidazole should be advised to avoid alcohol during treatment and for how long afterward, to avoid a disulfiram-like reaction?

Metronidazole can interfere with alcohol metabolism, causing a buildup of acetaldehyde that produces flushing, nausea, vomiting, and headache. Because the drug and its effects persist after the last dose, patients are typically counseled to avoid alcohol for about 3 days after finishing therapy, not just on dosing days.

9. A pharmacy technician notices a profile for a patient with chronic kidney disease shows a new prescription for ibuprofen 800 mg three times daily. Why should this be flagged for pharmacist review?

NSAIDs inhibit prostaglandins that help maintain blood flow to the kidneys; in a patient with chronic kidney disease, this can further reduce renal perfusion and worsen kidney function. The concern is a drug-disease interaction specific to renal impairment, not liver disease or a pediatric-only issue.

10. A non-selective beta blocker such as propranolol should be used cautiously or avoided in patients with which condition?

Non-selective beta blockers block beta-2 receptors in the lungs in addition to beta-1 receptors in the heart, which can cause bronchoconstriction and worsen asthma or COPD symptoms. Propranolol is actually used to treat hypertension, migraine prevention, and essential tremor, so those are not contraindications.

1. A patient with poorly controlled hypertension asks the technician to recommend an OTC decongestant. Which ingredient should be avoided due to its potential to raise blood pressure?

Pseudoephedrine is a sympathomimetic that causes vasoconstriction and can raise both blood pressure and heart rate, making it a concern in patients with uncontrolled hypertension. Diphenhydramine (antihistamine), guaifenesin (expectorant), and dextromethorphan (cough suppressant) do not carry this same pressor effect.

2. A patient scheduled for a CT scan with iodinated contrast dye is taking metformin. What concern should the technician flag to the pharmacist?

Iodinated contrast can cause contrast-induced nephropathy, impairing renal clearance of metformin and raising the risk of lactic acidosis. This is why metformin is often held before and after contrast procedures until renal function is confirmed stable — it is a lab/renal function concern, not an absorption or imaging-interference issue.