Drugs with
narrow therapeutic index (NTI-drugs) are drugs with small differences
between therapeutic and toxic doses. The pattern of drug-related
problems (DRPs) associated with these drugs has not been explored.
Objective: To investigate how, and to what extent drugs, with
a narrow therapeutic index (NTI-drugs), as compared with other
drugs, relate to different types of drug-related problems (DRPs)
in hospitalised patients.
Methods: Patients from internal medicine and rheumatology departments
in five Norwegian hospitals were prospectively included in 2002.
Clinical pharmacists recorded demographic data, drugs used, medical
history and laboratory data. Patients who used NTI-drugs (aminoglycosides,
ciclosporin, carbamazepine, digoxin, digitoxin, flecainide, lithium,
phenytoin, phenobarbital, rifampicin, theophylline, warfarin)
were compared with patients not using NTI-drugs. Occurrences of
eight different types of DRPs were registered after reviews of
medical records and assessment by multidisciplinary hospital teams.
The drug risk ratio, defined as number of DRPs divided by number
of times the drug was used, was calculated for the various drugs.
Results: Of the 827 patients included, 292 patients (35%) used
NTI-drugs. The NTI-drugs were significantly more often associated
with DRPs than the non-NTI-drugs, 40% versus 19% of the times
they were used. The drug risk ratio was 0.50 for NTI-drugs and
0.20 for non-NTI-drugs. Three categories of DRPs were significantly
more frequently found for NTI-drugs: non-optimal dose, drug interaction,
and need for monitoring.
Conclusion: DRPs were more frequently associated with NTI-drugs
than with non-NTI-drugs, but the excess occurrence was solely
related to three of the eight DRP categories recorded. The drug
risk ratio is a well-suited tool for characterising the risk attributed
to various drugs.
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