We developed a program to automatically screen patients' medication pr
ofiles for pairs of interacting drugs. Since some drug-drug interactio
ns are indicated by changes in physiological parameters (e.g., ciprofl
oxacin and theophylline leading to an elevation of theophylline levels
), the program considered the patients' relevant laboratory parameters
prior to generating the alerts. We developed an editor to facilitate
maintenance of the knowledge base. We evaluated the program for 3 week
s in two satellite pharmacies. The program reported 160 alerts of whic
h 5 resulted in a change in the patients' therapies (one per 500 patie
nt-days of care). These five interactions were potentially very seriou
s. An additional 3 alerts led to changes in medication administration
times. Subjectively, the program is well received and continues to be
in routine clinical use.