Drug Interactions between fosphenytoin and moxifloxacin / triamcinolone

MONITOR CLOSELY: Concomitant administration of corticosteroids may potentiate the risk of tendinitis and tendon rupture associated with fluoroquinolone treatment. The mechanism is unknown. Tendinitis and tendon rupture have most frequently involved the Achilles tendon, although cases involving the rotator cuff (the shoulder), the hand, the biceps, and the thumb have also been reported. Some have required surgical repair or resulted in prolonged disability. Tendon rupture can occur during or up to several months after completion of fluoroquinolone therapy.

MANAGEMENT: Caution is recommended if fluoroquinolones are prescribed in combination with corticosteroids, particularly in patients with other concomitant risk factors (e.g., age over 60 years; recipient of kidney, heart, and/or lung transplant). Patients should be advised to stop taking the fluoroquinolone, avoid exercise and use of the affected area, and promptly contact their physician if they experience pain, swelling, or inflammation of a tendon. In general, fluoroquinolones should only be used to treat conditions that are proven or strongly suspected to be caused by bacteria and only if the benefits outweigh the risks.

MONITOR: Phenytoin and other hydantoins may induce the CYP450 3A4 hepatic metabolism of corticosteroids and increase their clearance and decrease their half-lives, possibly reducing their therapeutic efficacy. In addition, results of the dexamethasone adrenal suppression test may be unreliable in patients concurrently taking phenytoin. Hydrocortisone appears to be affected to a lesser extent than other corticosteroids. Some corticosteroids have also been reported to cause increases or decreases in serum phenytoin levels; however, data have been inconsistent.

MANAGEMENT: Patients should be closely monitored for clinical and laboratory evidence of reduced corticosteroid effects and changes in phenytoin concentrations during concomitant therapy. Some patients may require increased corticosteroid dosages.