Adherence to guidelines for avoiding drug interactions associated with warfarin--a Nationwide Swedish Register Study.
OBJECTIVE
To investigate the extent to which clinicians avoid well-established drug-drug interactions associated with warfarin. We hypothesised that clinicians would avoid combining non-steroidal anti-inflammatory drugs (NSAIDs), tramadol and sulfamethoxazole with warfarin.


METHODS
A cross-sectional analysis of nationwide dispensing data was performed in Swedish individuals 18 years or older (n =  7,563,649). Odds ratios of interacting NSAIDs, tramadol and sulfamethoxazole versus respective prevalence of comparator drugs codeine, and ciprofloxacin in patients co-dispensed interacting warfarin versus patients unexposed was calculated.


RESULTS
The odds of receiving an interacting NSAID versus the comparator codeine was markedly lower in patients with warfarin than in the remaining population (adjusted OR 0.21; 95% CI 0.20 - 0.22). Also, the interacting drugs tramadol and sulfamethoxazole were less common among patients dispensed warfarin as compared to the remaining population, although the decrease was much more modest (adjusted OR 0.83; CI 0.80-0.87 and 0.81; CI 0.73 - 0.90).


CONCLUSIONS
In conclusion, Swedish doctors in the vast majority of cases refrain from prescribing NSAIDs to patients already on warfarin. Tramadol and sulfamethoxazole are however rarely avoided.
Dept of Laboratory Medicine, Division of Clinical Pharmacology, Karolinska University Hospital, Huddinge, Karolinska Institutet, Stockholm, Sweden.
Jonatan D
JD
Lindh
Dept of Laboratory Medicine, Division of Clinical Pharmacology, Karolinska University Hospital, Huddinge, Karolinska Institutet, Stockholm, Sweden.
Marine L
ML
Andersson
Karolinska Institutet, Department of Clinical Science and Education at Södersjukhuset, Stockholm, Sweden.
Buster
B
Mannheimer
D000328:Adult; D000368:Aged; D000369:Aged, 80 and over; D000894:Anti-Inflammatory Agents, Non-Steroidal / Q000008:administration & dosage; D003430:Cross-Sectional Studies; D004347:Drug Interactions; D019983:Guideline Adherence; D006801:Humans; D008875:Middle Aged; D016017:Odds Ratio; D012042:Registries; D012189:Retrospective Studies; D013420:Sulfamethoxazole / Q000008:administration & dosage; D013548:Sweden; D014147:Tramadol / Q000008:administration & dosage; D014859:Warfarin / Q000008:administration & dosage / Q000009:adverse effects*
D000894:Anti-Inflammatory Agents, Non-Steroidal; D014147:Tramadol; D014859:Warfarin; D013420:Sulfamethoxazole