A parent drug may be modified by the addition of side chains to alter activity or solubility. The equivalent dose of the modified drug depends on how the bioavailability and activity have been altered by the change. However, many drugs will show an equivalent dose based on the molarity of active component.


Sodium phenytoin (Dilantin, MW 274.3) is relatively insoluble. Fosphenytoin (Cerebyx, MW 406.24) has the addition of a phosphonooxylmethyl side chain that enhances water solubility.


Additions in fosphenytoin over phenytoin: (1 sodium) + (1 PO4) + (1 methyl) = 132 grams.


ratio of 1 mole fosphenytoin to 1 mole dilantin =

= 406.24 / 274.3 = 1.5


equivalent dose of fosphenytoin in mg =

= (mg of phenytoin sodium) * 1.5


Fosphenytoin is formulated as a liquid solution listed in phenytoin equivalents (PE) in order to reduce the risk of misdosing. For example, the 2 mL vial (100 mg PE) contains 150 mg fosphenytoin equivalent to 100 mg phenytoin sodium.


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