Effect of concomitantly administered drugs on the control of long term anticoagulant therapy.

The stability of control of long term anticoagulant therapy was studied in a group of 277 patients with respect to drug interactions. Increased instability (changes of dose/month of therapy) was related to the number of concomitant drugs taken irrespective of, and in addition to, known specific interactions. Fluctuating cardiac status in patients requiring digoxin and diuretic therapy was a major factor in stabilizing anticoagulant control.