Fosphenytoin dosage

I Advantages. Phenytoin has been used for over 60 years, and its risk-to-benefit ratio is well developed. It is available in oral solid, oral liquid, extended-release oral solid, and parenteral (phenytoin and fosphenytoin) dosage forms, allowing flexibility in dosing and use in emergent conditions. 

Fosphenytoin. Dosage is based on the phenytoin equivalent 750 mg of fosphenytoin is equivalent to 500 mg of phenytoin. (For example, a loading dose of 1 g phenytoin would require a dose of 1.5 g fosphenytoin.) Dilute twofold to tenfold in 5% dextrose or normal saline and administer no faster than 225 mg/min. 

Absorption of phenytoin is highly dependent on the formulation of the dosage form. Particle size and pharmaceutical additives affect both the rate and the extent of absorption. Absorption of phenytoin sodium from the gastrointestinal tract is nearly complete in most patients, although the time to peak may range from 3 to 12 hours. Absorption after intramuscular injection is unpredictable, and some drug precipitation in the muscle occurs this route of administration is not recommended for phenytoin. In contrast, fosphenytoin, a more soluble phosphate prodrug of phenytoin, is well absorbed after intramuscular administration. 

Fosphenytoin is a prodrug, a compound that undergoes chemical conversion in the body to become the therapeutically active compound phenytoin. Cerebyx dosage will continue to be expressed in PEs. This terminology was adopted in an effort to simplify therapeutic conversions between phenytoin sodium and fosphenytoin sodium (i.e., 500 mg of phenytoin sodium injection is equal to 500 mg PE of fosphenytoin sodium injection). The manufacturer pointed out that by using PEs, prescribers will not have to make dosing... 

If oral administration is not feasible, intravenous administration of phenytoin is preferred over intramuscular administration. Fos-phenytoin is a prodrug for phenytoin and is available as a parenteral dosage form. It is very water-soluble and is converted rapidly to phenytoin systemically. Fosphenytoin can be given rapidly intravenously and intramuscularly with reliable absorption and minimal pain. It is significantly better tolerated than phenytoin. 

The rise in serum phenytoin levels with intravenous chloramphenicol in adults is well documented and clinically important. A two to fourfold rise can occur within a few days. Concurrent use should be avoided unless the effects can be closely monitored and appropriate phenytoin dosage reductions made as necessary. The use of a single prophylactic dose of phenytoin or fosphenytoin may be an exception to this. It seems very doubtful if enough chloramphenicol is absorbed from eye drops or ointments for an interaction to occur. 

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