title: A limited number of prescribed drugs account for the great majority of drug-drug interactions.
abstract: OBJECTIVE
  The purpose of this study was to investigate the prevalence of prescribed combinations of interacting drugs in the Swedish population.
  
  
  METHODS
  This study design was retrospective and cross-sectional, based on a national register of dispensed prescription drugs during the period from January 1 to April 30, 2010. Prescription data was linked to the drug-drug interaction database SFINX to yield the prevalence of interacting combinations dispensed in the population. The study focused in particular on C- (clinically relevant interactions that can be handled, e.g. by dose adjustments), and D-interactions (clinically relevant interactions that should be avoided).
  
  
  RESULTS
  Thirty-eight and 3.8 % of the population were dispensed combinations of drugs classified as C- or D- interactions, respectively, i.e. clinically relevant, involving all therapeutic areas. Half of the D-interactions were associated with increased risk of adverse drug reactions whereas the other half were considered interactions with a potential to cause therapeutic failure. We identified a top 15 list of D-interactions that included 80 % of the total number of interacting drug combinations. Regarding individual drugs, a group of only ten drugs was involved in as much as 94 % of all D-interactions.
  
  
  CONCLUSIONS
  This study reveals that the majority of prescribed interacting drug combinations in Sweden involve a limited number of drugs. The findings may increase the awareness among prescribers of these most common drug interactions in clinical practice and highlight an area for pharmacological education. It may also serve as an inventory of potential interactions within different therapeutic areas for further research.
authors.affiliation: Division of Clinical Pharmacology, Department of Laboratory Medicine, Karolinska Institutet, Karolinska University Hospital Huddinge, Stockholm, Sweden.
authors.forename: Johan
authors.initials: J
authors.lastname: Holm
authors.forename: Birgit
authors.initials: B
authors.lastname: Eiermann
authors.forename: Erik
authors.initials: E
authors.lastname: Eliasson
authors.forename: Buster
authors.initials: B
authors.lastname: Mannheimer
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D002648:Child; D002675:Child, Preschool; D003430:Cross-Sectional Studies; D016208:Databases, Factual / Q000706:statistics & numerical data; D004347:Drug Interactions; D011307:Drug Prescriptions / Q000706:statistics & numerical data*; D005260:Female; D006801:Humans; D007223:Infant; D007231:Infant, Newborn; D008297:Male; D008875:Middle Aged; D015995:Prevalence; D012189:Retrospective Studies; D013548:Sweden / Q000453:epidemiology; D055815:Young Adult