Every entry is a story with the same skeleton: mechanism → clinical consequence → what a pharmacist does about it. The severity words are honest. Most interactions are managed, not forbidden.
A curated educational library, not a comprehensive clinical interaction checker.
ACE inhibitor / ARB + diuretic + NSAID (the 'triple whammy')
pharmacodynamicavoid
Mechanism
Diuretic depletes volume, RAAS blockade relaxes the efferent arteriole, NSAID constricts the afferent: glomerular perfusion loses every defense at once.
Consequence
Acute kidney injury, classically in a dehydrated older adult who bought ibuprofen for a sore knee.
Management
Avoid the triple combination; if an NSAID is truly needed, shortest course, hydration, and early creatinine check. Counsel every ACEi+diuretic patient about OTC NSAIDs.
The most common triple whammy is two prescriptions plus one supermarket purchase.
Apixaban / rivaroxaban + Rifampin, carbamazepine, phenytoin, St John's wort
pharmacokineticavoid
Mechanism
Strong CYP3A4 + P-gp induction accelerates DOAC clearance.
Consequence
Sub-therapeutic anticoagulation: stroke or VTE on a 'therapeutic' dose.
Management
Choose warfarin (titratable by INR) when a strong inducer is unavoidable.
Induction interactions kill silently. Nothing to measure, nothing to see, until the clot.
Ciprofloxacin + Theophylline
pharmacokineticavoid
Mechanism
Potent CYP1A2 inhibition throttles theophylline clearance; theophylline's window is narrow.
Consequence
Theophylline toxicity: nausea, tachyarrhythmia, seizures.
Management
Choose another antibiotic, or halve theophylline and level-check.
Old drug, narrow window, strong inhibitor: the three-ingredient recipe for classic toxicity.
Simvastatin / atorvastatin + Clarithromycin, itraconazole, ritonavir (strong CYP3A4 inhibitors)
pharmacokineticavoid
Mechanism
CYP3A4 inhibition multiplies statin exposure several-fold (simvastatin worst).
Consequence
Myopathy up to rhabdomyolysis: muscle pain, dark urine, AKI.
Management
Pause the statin for a short inhibitor course, or switch to pravastatin/rosuvastatin for chronic co-therapy.
A one-week clarithromycin course is the textbook reason to pause, not power through, a 3A4 statin.
SSRIs / SNRIs + MAO inhibitors (including linezolid)
pharmacodynamicavoid
Mechanism
Blocking reuptake and degradation simultaneously floods the serotonin synapse.
Consequence
Serotonin syndrome: agitation, clonus, hyperthermia: potentially fatal.
Management
Contraindicated. Fourteen-day washout (five weeks after fluoxetine). Remember linezolid is an MAOI in antibiotic clothing.
The linezolid prescription in an SSRI patient is the modern version of this classic collision.
Statins + Gemfibrozil
pharmacokineticavoid
Mechanism
Gemfibrozil inhibits statin glucuronidation and OATP uptake, raising exposure. A PK interaction dressed as a PD one.
Consequence
Sharply increased myopathy risk.
Management
If a fibrate is truly needed with a statin, fenofibrate is the compatible choice.
Fibrate + statin questions are really gemfibrozil questions.
Warfarin + NSAIDs (e.g. ibuprofen, naproxen)
pharmacodynamicavoid
Mechanism
Pharmacodynamic stacking: platelet inhibition and gastric mucosal injury on top of anticoagulation. The INR often does not move: the bleeding risk does.
Consequence
Major GI bleeding risk multiplies while the INR looks reassuring.
Management
Avoid routine NSAID use; prefer acetaminophen for analgesia. If an NSAID is unavoidable, shortest course, gastroprotection, and explicit bleeding counselling.
The most dangerous warfarin interactions are the ones the INR cannot see.
Warfarin + Trimethoprim-sulfamethoxazole
pharmacokineticavoid
Mechanism
Sulfamethoxazole strongly inhibits CYP2C9, the enzyme clearing the potent S-enantiomer of warfarin.
Consequence
One of the highest-risk warfarin combinations: INRs of 6+ and hospitalization-grade bleeding are well documented.
Management
Choose a different antibiotic where possible. If truly necessary: preemptive dose reduction and INR within 3 days.
TMP-SMX is the classic 'warfarin patient in the ED' story. Recognize it before dispensing, not after.
ACE inhibitors / ARBs + Potassium-sparing diuretics, K supplements, salt substitutes
pharmacodynamiccaution
Mechanism
RAAS blockade lowers aldosterone (potassium retention) while the partner adds potassium or blocks its excretion.
Consequence
Additive hyperkalemia, worst with renal impairment layered on top.
Management
Deliberate combinations (HFrEF spironolactone) are legitimate WITH scheduled potassium checks; accidental ones (OTC salt substitute) are found by asking.
Salt substitutes are potassium chloride wearing a health halo. A counter question, not a chart review, finds them.
ACE inhibitors / ARBs / thiazides + Lithium
pharmacokineticcaution
Mechanism
Reduced renal perfusion and increased proximal reabsorption raise lithium levels; lithium's therapeutic index is unforgiving.
Consequence
Lithium toxicity: tremor, confusion, ataxia, renal injury.
Management
If co-prescribed, lithium level within a week of any change and at steady state; counsel on dehydration days.
Lithium is cleared like sodium: anything that makes the kidney hoard sodium hoards lithium too.
Beta blockers + Verapamil / diltiazem
pharmacodynamiccaution
Mechanism
Additive sinus-node suppression and AV-node blockade from two different directions.
Consequence
Bradycardia, AV block, hypotension, occasionally pacing-level.
Management
Usually avoid; when deliberately combined (specialist rate control), monitor HR and PR interval closely.
One node, two brakes: check the ECG maths before the chart maths.
Ciprofloxacin + Warfarin
pharmacokineticcaution
Mechanism
CYP1A2/3A4 inhibition plus gut-flora suppression.
Consequence
INR rise during the course.
Management
INR check within 3-5 days of starting.
File under 'every antibiotic moves warfarin until proven otherwise'.
Ciprofloxacin (and all fluoroquinolones, tetracyclines) + Calcium, magnesium, aluminum, iron, zinc. Antacids, dairy, supplements
pharmacokineticcaution
Mechanism
Divalent/trivalent cations chelate the antibiotic in the gut lumen; absorption can fall by more than half.
Consequence
Sub-therapeutic antibiotic levels: treatment failure that looks like resistance.
Management
Separate: antibiotic 2 hours before or 6 hours after the cation source. Ask specifically about antacids, multivitamins, and dairy.
The commonest quinolone failure is a breakfast interaction, not a resistant organism.
Furosemide (high-dose IV) + Aminoglycosides
pharmacodynamiccaution
Mechanism
Additive cochlear toxicity; both injure the same hair cells.
Consequence
Ototoxicity, potentially permanent.
Management
Slow the furosemide infusion, level-guide the aminoglycoside, and separate where clinically possible.
Rate matters: bolus furosemide at high dose is an ear question.
Hydrochlorothiazide + Lithium
pharmacokineticcaution
Mechanism
Thiazide-induced sodium depletion drives compensatory proximal reabsorption of sodium, and lithium with it.
Consequence
Lithium levels rise 25-40% within days to weeks.
Management
Prefer alternatives; if combined, recheck lithium within 5-7 days and after any dose change.
Among diuretics, thiazides raise lithium the most; loops less; amiloride least.
Metformin + Iodinated contrast media
pharmacokineticcaution
Mechanism
Contrast-induced kidney injury impairs metformin clearance; accumulated metformin risks lactic acidosis.
Consequence
Lactic acidosis if AKI follows contrast with metformin still on board.
Management
Hold per local protocol (commonly at/after imaging in at-risk patients), restart after renal function is confirmed stable.
The interaction is conditional: metformin plus contrast is fine unless the kidney stumbles. Protocols exist for exactly that 'unless'.
Non-selective beta blockers + Salbutamol
pharmacodynamiccaution
Mechanism
Direct pharmacological antagonism at beta-2: the reliever's receptor is occupied by its blocker.
Consequence
Blunted bronchodilation exactly when the patient reaches for rescue.
Management
Avoid non-selective beta blockers in reactive airway disease; cardioselective agents at low dose when compelling indications demand.
Propranolol for migraine plus a blue inhaler in the same profile is a real-world flag, not an exam trick.
NSAIDs + Lithium
pharmacokineticcaution
Mechanism
Prostaglandin-dependent renal perfusion falls; lithium clearance falls with it.
Consequence
Lithium rise within days of regular NSAID use.
Management
Prefer acetaminophen; if NSAID unavoidable, lithium level within a week.
The OTC ibuprofen bottle is the commonest unprescribed lithium interaction.
NSAIDs + Methotrexate (especially high-dose)
pharmacokineticcaution
Mechanism
Competition for renal tubular secretion plus reduced renal perfusion slows methotrexate clearance.
Consequence
Methotrexate toxicity: mucositis, cytopenias, renal injury (chiefly with high-dose oncology regimens).
Management
Avoid around high-dose MTX; low-dose weekly rheumatology MTX usually coexists with NSAIDs under monitoring.
Dose context decides this one: the same pair is routine in rheumatology and dangerous in oncology.
SSRIs / SNRIs + Tramadol
pharmacodynamiccaution
Mechanism
Tramadol inhibits serotonin reuptake in its own right and lowers seizure threshold; effects stack.
Consequence
Serotonin syndrome (usually milder spectrum) and seizures, especially at higher doses or in the elderly.
Management
Prefer non-serotonergic analgesia; if combined, counsel on early symptoms (restlessness, tremor, sweating).
Tramadol is the serotonergic hiding inside the analgesic aisle.
Vancomycin + Piperacillin-tazobactam, aminoglycosides, NSAIDs, contrast
pharmacodynamiccaution
Mechanism
Additive nephrotoxicity by different mechanisms converging on the same kidney.
Consequence
AKI rates rise notably with vancomycin + piperacillin-tazobactam co-therapy.
Management
Daily creatinine, level-guided dosing, and the stewardship question: does the spectrum still need both?
The best management of this interaction is usually de-escalation, not just monitoring.
Warfarin + Clarithromycin / erythromycin
pharmacokineticcaution
Mechanism
CYP inhibition (3A4, some 2C9 effect) plus gut-flora suppression reducing vitamin K synthesis: warfarin exposure and effect both rise.
Consequence
INR climbs within days of starting the antibiotic.
Management
Recheck INR within 3-5 days of starting; anticipate a temporary warfarin dose reduction. Azithromycin interacts less.
Any new antibiotic in a warfarin patient buys an early INR check.
Warfarin + Amiodarone
pharmacokineticcaution
Mechanism
Amiodarone inhibits CYP2C9 and 3A4. Slowly, with a half-life measured in weeks, so the interaction creeps in over a month.
Consequence
Gradual INR rise over 2-6 weeks after amiodarone starts.
Management
Anticipate 30-50% warfarin dose reduction; INR weekly during the ramp-in.
Slow-onset inhibitors cause slow-onset interactions; monitoring must outlast the first normal INR.
Acetaminophen (sustained ≥2-3 g/day) + Warfarin
pharmacodynamicmonitor
Mechanism
A metabolite interferes with the vitamin K cycle at sustained high intake.
Consequence
INR creeps up over 1-2 weeks of round-the-clock use.
Management
Still the analgesic of choice in warfarin patients, with an INR check if use becomes regular and high-dose.
'Safest' is relative: even acetaminophen earns an INR check at full-time doses.
Amlodipine / felodipine + Strong CYP3A4 inhibitors; grapefruit (felodipine)
pharmacokineticmonitor
Mechanism
Intestinal and hepatic CYP3A4 inhibition raises dihydropyridine exposure; grapefruit's effect on felodipine is the canonical example.
Consequence
More vasodilation: flushing, edema, hypotension.
Management
Counsel on grapefruit with susceptible agents; watch BP when macrolides or azoles join.
Grapefruit acts in the gut wall: even a single glass can matter for high-first-pass drugs.
Beta blockers + Insulin / sulfonylureas
pharmacodynamicmonitor
Mechanism
Adrenergic warning symptoms of hypoglycemia are blunted; recovery (glycogenolysis) may slow with non-selective agents.
Consequence
Hypoglycemia without its alarm bells; sweating persists as the surviving symptom.
Management
Counsel explicitly; prefer cardioselective agents; encourage glucose monitoring rather than symptom reliance.
This interaction is managed by counselling, not by avoiding either drug.
Broad-spectrum antibiotics (any) + Warfarin
pharmacodynamicmonitor
Mechanism
Gut flora synthesize part of the vitamin K supply; antibiotics thin that supply while illness itself reduces intake.
Consequence
Modest INR rise, additive with any CYP effect of the specific agent.
Management
INR within 3-5 days of starting any antibiotic course.
Two mechanisms often stack in the same prescription. Flora plus CYP.
Loop diuretics + Lithium
pharmacokineticmonitor
Mechanism
Volume contraction increases proximal lithium reabsorption (less than thiazides, more than nothing).
Consequence
Lithium level drift upward.
Management
Level check after initiation or dose change.
All diuretics are lithium questions; only the magnitude differs.
Omeprazole / esomeprazole + Clopidogrel
pharmacokineticmonitor
Mechanism
Clopidogrel is a prodrug activated by CYP2C19; omeprazole inhibits 2C19, reducing active metabolite formation.
Consequence
Reduced antiplatelet effect (biomarker-solid, outcome-debated).
Management
Where gastroprotection is needed, pantoprazole sidesteps the axis entirely.
A within-class PPI switch dissolves the whole controversy. Classic pharmacist intervention.
SSRIs + NSAIDs / anticoagulants
pharmacodynamicmonitor
Mechanism
Platelets depend on absorbed serotonin for aggregation; SSRIs deplete it, adding to NSAID mucosal injury or anticoagulation.
Consequence
Upper GI bleeding risk roughly doubles versus either alone.
Management
Ask about OTC NSAID habits at every SSRI dispense; gastroprotection for the high-risk.
The bleeding-risk conversation belongs at the SSRI counter, not just the warfarin clinic.
Warfarin + Vitamin K-rich foods (leafy greens)
pharmacodynamicmonitor
Mechanism
Dietary vitamin K feeds VKORC1's product back into the factor-carboxylation cycle, directly opposing warfarin.
Consequence
Fluctuating intake makes the INR seesaw; a sudden salad enthusiasm drops it.
Management
Consistency, not avoidance: keep weekly vitamin K intake steady and dose warfarin around it.
The counselling is 'be consistent', never 'stop eating greens'.