Methods. We identified a national cohort of insured patients aged 18+ years receiving care from 10 integrated delivery systems (N=959,546). Prescription drug and medical claims data were used to compile information on dispensings and other factors during baseline (07/01/1999 – 12/31/1999) and follow-up (01/01/2000 – 06/30/2001). FDA warning labeling, Hansten and Horn Ratings and solicited input from pharmacists/physicians were used to identify risky medication pairs that have potentially significant clinical sequelea and for which there are available risk-reducing actions. Two definitions of concomitant use were used: same day dispensings and dispensings with overlap according to the dispensed “days supply.” Among patients with a dispensing during follow-up for one of four object drugs (warfarin, digoxin, cyclosporine, and lovastatin/simvastatin), we report the number and percent with a concomitant risky dispensing.

Results. Among the cohort, 76,617 patients were dispensed at least one of the 4 object drugs. The most frequent risky concomitant dispensing with warfarin was NSAIDs (>44.0%). Among risky digoxin, cyclosporine and lovastatin/simvastatin dispensings, 77.0%, 77.0% and 66.1%, respectively, were co-dispensed with verapamil, ditiazem, or bepridil. The proportion of object drug users who received a risky concomitant dispensing ranged from 19.0% (N=10,346 lovastatin/simvastatin users) to 29.4% (N=210 cyclosporine users) when concomitant use was defined using “days supply” overlap and from 8.7% (N=1,668 warfarin users) to 18.5% (N=132 cyclosporine users) when only same day dispensings were considered. When extrapolated to the insured, US 2000 population, between 1.6 (same day definition) and 3.2 (days supply definition) million people nationwide are dispensed one of the four object drugs in combination with another medication that can be considered ‘risky' co-dispensing. Over half those were dispensed lovastatin or simvastatin in combination with a potentially contraindicated medication.

Conclusions. To date, most research in the area of risky concomitant medication use has focused on co-prescribing in the hospital setting. However, we found evidence of risky concomitant medication dispensing among ambulatory care patients. Because using prescription claims data to monitor concomitant prescribing in the outpatient setting is relatively simple and inexpensive, an opportunity exists to augment current benchmarking and other quality improvement efforts to better understand how risky concomitant medication dispensing translates into adverse events and ultimately to improve medication safety in the outpatient setting.