Phenytoin

Certain medications present specific challenges when administered through feeding tubes. The medication studied most thoroughly is phenytoin. Most studies have shown significant decreases in phenytoin absorption when the medication was administered enterally to patients receiving EN. Several mechanisms have been proposed for this apparent interaction. Many institutions have adopted a policy of holding tube feedings for 1 or 2 hours before and after administration of phenytoin to decrease the interaction, although some EN patients subjected to this routine still will require relatively high dosages of phenytoin to achieve therapeutic serum concentrations. Holding the feeding around medication administration can make meeting nutritional requirements difficult with continuous feedings, especially if the phenytoin is administered several times daily. Diligent monitoring of phenytoin serum concentrations is necessary for the patient on EN receiving this medication. In some cases, use of IV phenytoin or another anticonvulsant medication may be prudent.

Warfarin

EN formulas contain vitamin K, which can antagonize the pharmacologic activity of warfarin. Vitamin K content of EN formulas generally has been adjusted down over the past several decades, resulting in products today that contain amounts of vitamin K unlikely to affect anticoagulation by warfarin significantly. However, inadequate warfarin anticoagulation in EN patients receiving formulas containing minimal vitamin K has been reported. There is some thought that a component of certain tube feedings, perhaps protein, may bind warfarin and result in suboptimal activity of the drug. A recent small study indicated a better response in terms of the International Normalized Ratio (INR) when feedings were held for one hour before and after warfarin ad ministration compared to administration of the drug without holding the feedings. When tube feedings are started, changed, or discontinued, the INR should be monitored closely.

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