Clinically relevant drug-drug interactions in primary care

Drug interactions are common in the primary care setting and are usually predictable. Identifying the most important and clinically relevant drug interactions in primary care is essential to patient safety. Strategies for reducing the risk of drug-drug interactions include minimizing the number of drugs prescribed, re-evaluating therapy on a regular basis, considering nonpharmacologic options, monitoring for signs and symptoms of toxicity or effectiveness, adjusting dosages of medications when indicated, and adjusting administration times. Inhibition or induction of cytochrome P450 drug metabolizing isoenzymes is the most common mechanism by which clinically important drug interactions occur. The antimicrobials most likely to affect the international normalized ratio significantly in patients receiving warfarin are trimethoprim/sulfamethoxazole, metronidazole, and fluconazole. An empiric warfarin dosage reduction of 30% to 50% upon initiation of amiodarone therapy is recommended. In patients receiving amiodarone, limit dosages of simvastatin to 20 mg per day and lovastatin to 40 mg per day. Beta blockers should be tapered and discontinued several days before clonidine withdrawal to reduce the risk of rebound hypertension. Spironolactone dosages should be limited to 25 mg daily when coadministered with potassium supplements. Avoid prescribing opioid cough medicines for patients receiving benzodiazepines or other central nervous system depressants, including alcohol. Physicians should consider consultation with a clinical pharmacist when clinical circumstances require the use of drugs with interaction potential.