This afternoon I met a perscription that consist of nine different agents, which have twenty drug interactions. Many of them are cardiovascular drugs. They are  Beta-Blockers, Electrolytes, Statins, ARBs, Inotropic Agents, Thiazide, Calcium Channel Blockers, and Benzodiazepines. I list them and their dosage below.

  1. Alprazolam 0.4 mg po Qd
  2. Metoprolol 12.5 mg po Bid
  3. Potassium Chloride 1 g po Bid
  4. Simvastatin 20 mg po Qn
  5. Irbesartan 0.15 g po Qd
  6. Irbesartan/Hydrochlorothiazide 1 tablet po Qd
  7. Digoxin 0.125 mg po Qd
  8. Amlodipine 5 mg po Qd
  9. Warfarin 2.5 mg po Qd

I check these nine drug in Multi-Dug Interaction Chechker and find there are twenty drug interactions between these nine drugs. They are:

Serious – Use alternative

Amlodipine + Simvastatin. Amlodipine increases levels of simvastatin by Other (See comment). Possible serious or life-threatening interaction. Monitor closely. Use alternatives if available. Comment: Benefits of combination therapy should be carefully weighed against the  potential risks of combination. Potential for increased risk of myopathy/rhabdomyolysis. Limit simvastatin dose to no more than 20 mg/day when used concurrently.

Significant – Monitor Closely

Hydrochlorothiazide + Digoxin. Hydrochlorothiazide increases effects of digoxin by pharmacodynamic synergism. Significant interaction possible, monitor closely. Hypokalemia increases digoxin effects.

Potassium Chloride + Hydrochlorothiazide. Potassium chloride increases and hydrochlorothiazide decreases serum potassium. Effect of interaction is not clear, use caution. Potential for dangerous interaction. Use with caution and monitor closely.

Simvastatin + Warfarin. Simvastatin, warfarin. Either increases effects of the other by affecting hepatic/intestinal enzyme CYP3A4 metabolism. Significant – Monitor Closely. Competition by each drug for CYP3A4-mediated metabolism may result in increased INR and increased risk of rhabdomyolysis.

Simvastatin + Digoxin. Simvastatin will increase the level or effect of digoxin by P-glycoprotein (MDR1) efflux transporter. Significant – Monitor Closely.

Digoxin + Hydrochlorothiazide. Digoxin will increase the level or effect of hydrochlorothiazide by basic (cationic) drug competition for renal tubular clearance. Significant – Monitor Closely.

Metoprolol + Irbesartan. Metoprolol, irbesartan. Mechanism: pharmacodynamic synergism. Significant – Monitor Closely. Risk of fetal compromise if given during pregnancy.

Alprazolam + Digoxin. Alprazolam increases levels of digoxin by unknown mechanism. Significant – Monitor Closely.

Metoprolol + Digoxin. Metoprolol increases effects of digoxin by pharmacodynamic synergism. Significant – Monitor Closely. Enhanced bradycardia.

Metoprolol + Amlodipine. Metoprolol and amlodipine both increase anti-hypertensive channel blocking. Significant – Monitor Closely.

Irbesartan + Potassium Chloride. Irbesartan and potassium chloride both increase serum potassium. Significant – Monitor Closely.

Metoprolol + Potassium Chloride. Metoprolol and potassium chloride both increase serum potassium. Significant – Monitor Closely.

Potassium Chloride + Digoxin. Potassium chloride and digoxin both increase serum potassium. Significant – Monitor Closely.

Irbesartan + Metoprolol. Irbesartan and metoprolol both increase serum potassium. Significant – Monitor Closely.

Irbesartan + Digoxin. Irbesartan and digoxin both increase serum potassium. Significant – Monitor Closely.

Irbesartan + Hydrochlorothiazide. Irbesartan increases and hydrochlorothiazide decreases serum potassium. Effect of interaction is not clear, use caution. Significant – Monitor Closely.

Metoprolol + Digoxin. Metoprolol and digoxin both increase serum potassium. Significant – Monitor Closely.

Metoprolol + Hydrochlorothiazide. Metoprolol increases and hydrochlorothiazide decreases serum potassium. Effect of interaction is not clear, use caution. Significant – Monitor Closely.

Digoxin + Hydrochlorothiazide. Digoxin increases and hydrochlorothiazide decreases serum potassium. Effect of interaction is not clear, use caution. Significant – Monitor Closely.

Irbesartan + Irbesartan/Hydrochlorothiazide (Not listed in the multi-drug interaction checker. I add this interaction by myself). Both Irbesartan and Irbesartan/Hydrochlorothiazide increase the level or effects of each other. The combination use of these two drugs is repeated administration and may enhance adverse effects of Irbesartan. I think this combination use should be avoid absolutely.

I alysis these twenty drug interactions. These twenty drug interactions include ten refered to  potassium disorder, three refered to pharmacodynamic synergism, one refered to myopathy/rhabdomyolysis, one hepatic/instestinal enzyme, one MDR1 (P-gp), one renal tubular excretion, one unknown, one anti-hypertension channel, and one repeated administration.

I think the next thing to do is to modify this pharmacotherapy regimen as there are many drug interactions in this regimen.