Retrospective evaluation of a potential interaction between azithromycine and warfarin in patients stabilized on warfarin.
OBJECTIVE
To investigate a potential interaction between azithromycin and warfarin.


METHODS
Retrospective case-control study.


METHODS
Veterans Affairs medical center.


METHODS
Fifty-two patients stable on anticoagulation therapy.


METHODS
Patients who received a prescription for azithromycin and warfarin at any time since the hospital was opened, June 1, 1995, to July 22, 1999, were identified through a computerized report generated from the pharmacy prescription package.


RESULTS
Patients having a stable international normalized ratio (INR; defined as a therapeutic INR +/- 0.2) for at least two consecutive visits before receiving an azithromycin prescription were reviewed. Changes in INR from before and after addition of azithromycin were compared with changes in a control group. Controls were identified from a computer-generated report of patients who received a prescription for terazosin and warfarin at any time since the hospital was opened to July 22, 1999 (terazosin was chosen as it has no known interaction with warfarin). These patients also had a stable INR for at least two consecutive visits before receiving the terazosin prescription. In patients with INRs on record within 14 days after starting azithromycin or terazosin (9 patients/group), the average change in INR was 0.18 +/- 0.48 in the azithromycin group and 0.07 +/- 0.49 in the terazosin group (p=0.60). For patients with an INR on record within 30 days after starting azithromycin or terazosin (26 patients/group), the average change in INR was 0.25 +/- 0.67 in the azithromycin group and 0.05 +/- 0.55 in the terazosin group (p=0.18).


CONCLUSIONS
An interaction between azithromycin and warfarin was not observed in this retrospective review of patients with a stable INR receiving the combination.
Beckey
N P
NP
Department of Veterans Affairs Medical Center, West Palm Beach, Florida 33410, USA.
Parra
D
D
Colon
A
A
D000368:Aged; D000369:Aged, 80 and over; D000900:Anti-Bacterial Agents / Q000493:pharmacokinetics*; D000925:Anticoagulants / Q000493:pharmacokinetics*; D017963:Azithromycin / Q000493:pharmacokinetics*; D016022:Case-Control Studies; D004347:Drug Interactions; D006801:Humans; D019934:International Normalized Ratio / Q000706:statistics & numerical data*; D008297:Male; D008875:Middle Aged; D010975:Platelet Aggregation Inhibitors / Q000493:pharmacokinetics; D011224:Prazosin / Q000031:analogs & derivatives / Q000493:pharmacokinetics; D012189:Retrospective Studies; D014859:Warfarin / Q000493:pharmacokinetics*
D000900:Anti-Bacterial Agents; D000925:Anticoagulants; D010975:Platelet Aggregation Inhibitors; D014859:Warfarin; D017963:Azithromycin; C041226:Terazosin; D011224:Prazosin