IV preparations of phenytoin contain a preservative that may cause hypotension and arrhythmias if administered too rapidly.
Thus, the total loading dose of phenytoin should usually be administered over about half an hour and be given through an infusion pump.
Fosphenytoin is a water-soluble prodrug of phenytoin. The dosage of fosphenytoin is expressed in terms of phenytoin equivalents (PE); 150 mg of fosphenytoin sodium is equivalent to 100 mg of phenytoin sodium. While it can be given IM and IV and may have a lower side effect profile it is much more expensive than phenytoin and has nor yet clearly established a place in therapy.
Phenytoin is ineffective in theophylline induced seizures and in animal studies has actually been shown to decrease the seizure threshold and lower the lethal dose.
It is also ineffective in animal models of tricyclic antidepressant poisoning.
Paloucek FP, Rodvold KA. Evaluation of theophylline overdoses and toxicities. Ann Emerg Med. 1988; 17: 135-144.
Gaudreault P, Guay J. Theophylline poisoning: Pharmacological considerations and clinical management. Med Toxicol Adverse Drug Exp. 1986; 1:169-191.
Blake KV, Massey KL, Hendeles L, Nickerson D, Neims A. Relative efficacy of phenytoin and phenobarbital for the prevention of theophylline-induced seizures in mice. Ann Emerg Med. 1988; 17:1024-1028.