Drug Interactions Quick Reference

Drug Interactions Quick Reference

Pharmacy technicians should be familiar with clinically significant drug interactions. While pharmacists are responsible for clinical decision-making, technicians play a key role in flagging potential interactions during prescription processing. This cheat sheet covers the most commonly tested interactions.

Major Drug-Drug Interactions

Drug A	Drug B	Interaction	Clinical Concern
Warfarin	NSAIDs (ibuprofen, naproxen)	Increased bleeding risk	NSAIDs inhibit platelet function and may increase warfarin levels
Warfarin	Metronidazole	Increased warfarin effect	Metronidazole inhibits warfarin metabolism, raising INR
Warfarin	Vitamin K (high dietary intake)	Decreased warfarin effect	Vitamin K opposes warfarin's mechanism
ACE inhibitors	Potassium supplements / potassium-sparing diuretics	Hyperkalemia	Both increase potassium levels
Metformin	Iodinated contrast dye	Risk of lactic acidosis	Metformin should typically be held before and after contrast procedures
SSRIs	MAOIs	Serotonin syndrome	Potentially fatal - requires washout period between drugs
SSRIs	Tramadol	Serotonin syndrome risk	Both increase serotonin activity
Fluoroquinolones (ciprofloxacin)	Antacids, calcium, iron, zinc	Decreased antibiotic absorption	Divalent/trivalent cations chelate fluoroquinolones
Tetracyclines (doxycycline)	Antacids, calcium, iron, dairy	Decreased antibiotic absorption	Same chelation mechanism as fluoroquinolones
Methotrexate	NSAIDs	Increased methotrexate toxicity	NSAIDs decrease renal clearance of methotrexate
Statins (simvastatin, atorvastatin)	Grapefruit juice	Increased statin levels	Grapefruit inhibits CYP3A4, raising statin blood levels
Statins	Gemfibrozil	Increased risk of myopathy/rhabdomyolysis	Both can cause muscle damage; risk multiplied together
Digoxin	Amiodarone	Increased digoxin levels	Amiodarone reduces digoxin clearance
Theophylline	Ciprofloxacin	Increased theophylline levels	Ciprofloxacin inhibits theophylline metabolism
Sildenafil	Nitrates (nitroglycerin)	Severe hypotension	Combination can cause dangerous drop in blood pressure
Lithium	NSAIDs or ACE inhibitors	Increased lithium levels	Both reduce renal clearance of lithium
Carbamazepine	Oral contraceptives	Decreased contraceptive efficacy	Carbamazepine induces metabolism of estrogen/progestin

Drug-Food Interactions

Drug	Food	Effect	Recommendation
Warfarin	Vitamin K-rich foods (leafy greens)	Decreased anticoagulant effect	Maintain consistent vitamin K intake
Simvastatin, atorvastatin	Grapefruit juice	Increased drug levels (CYP3A4 inhibition)	Avoid grapefruit or limit intake
MAOIs	Tyramine-rich foods (aged cheese, cured meats, fermented foods)	Hypertensive crisis	Strict dietary avoidance of tyramine
Tetracyclines	Dairy products	Decreased absorption	Take on empty stomach; avoid dairy within 2 hours
Levothyroxine	Calcium, iron, coffee, high-fiber foods	Decreased absorption	Take on empty stomach 30-60 min before food
Metronidazole	Alcohol	Disulfiram-like reaction (nausea, vomiting, flushing)	Avoid alcohol during and for several days after treatment
Potassium-sparing diuretics	Potassium-rich foods (bananas, oranges)	Hyperkalemia	Monitor potassium levels; moderate intake

Drug-Disease Interactions

Drug	Condition	Concern
NSAIDs	Kidney disease	Can worsen renal function
NSAIDs	GI ulcers	Increased risk of GI bleeding
Beta blockers	Asthma	May trigger bronchospasm (especially non-selective agents)
Metformin	Severe kidney disease	Increased risk of lactic acidosis
Anticholinergics	Glaucoma (closed-angle)	Can increase intraocular pressure
Anticholinergics	Benign prostatic hyperplasia	Can worsen urinary retention
Decongestants (pseudoephedrine)	Hypertension	Can raise blood pressure

Cytochrome P450 (CYP450) Basics

Many drug interactions occur through the CYP450 enzyme system in the liver. Key concepts:

• Inhibitors slow down metabolism of other drugs, increasing their blood levels (risk of toxicity).
• Inducers speed up metabolism of other drugs, decreasing their blood levels (risk of therapeutic failure).

CYP Enzyme	Common Inhibitors	Common Inducers
CYP3A4	Ketoconazole, erythromycin, grapefruit juice, diltiazem	Rifampin, carbamazepine, phenytoin, St. John's wort
CYP2D6	Fluoxetine, paroxetine, bupropion, quinidine	(Few clinically significant inducers)
CYP1A2	Ciprofloxacin, fluvoxamine	Smoking (tobacco), rifampin
CYP2C9	Fluconazole, amiodarone	Rifampin
CYP2C19	Omeprazole, fluoxetine	Rifampin

Exam Tips

• Warfarin interactions are among the most commonly tested - know the major ones.
• Remember the chelation interaction: fluoroquinolones and tetracyclines with divalent cations (Ca, Fe, Mg, Zn, Al).
• MAOI + tyramine-rich food = hypertensive crisis is a classic exam question.
• SSRI + MAOI = serotonin syndrome is another frequently tested interaction.
• Grapefruit juice and CYP3A4 inhibition comes up often.