Interaction between doxycycline and some antiepileptic drugs

The mean half life of doxycycline given to seven patients on long-term diphenylhydantoin treatment was 7.2 +/- 0.4 hours. In five patients on long-term carbamazepine treatment the half life was 8.4 +/- 1.4 hours. In four patients on combined diphenylhydantoin and carbamazepine treatment the half life was 7.4 +/- 0.7 hours. All these were significantly shorter than a mean half life of 15.1 +/- 1.0 hours when doxycycline was given to nine control patients. Therefore doxycycline in normal doses given to patients taking diphenylhydantoin or carbamazepine may fail to maintain the minimum inhibitory concentration necessary for proper bacteriostasis. When doxycycline is given in association with agents known to induce drug metabolism the serum concentration of the antibiotic should be watched to see that bacteriostatic levels are maintained.