Pharmacokinetics : Phenytoin is poorly absorbed from the intestine 1



It is metabolised in the liver2.

The half life of phenytoin at low doses (giving subtheraputic concentrations) is 6-24 hours{Clinical Pharmacology,9th edition, P.N.Bennet & M.J.Brown}.

It increases to 20-60 hours at higher doses.

Phenytoin is administered orally or by intravenous injection. 3

Phenytoin sodium may be given by slow intravenous injection, with ECG monitoring, followed by the maintenance dosage.


Intramuscular use of phenytoin is not recommended (absorption is slow and erratic){BNF 53 section 4.8.2}. Fosphenytoin is a water soluble pro-drug of phenytoin which can be administered by intramuscular injection.


Magnesium sulphate is superior to phenytoin for the prevention of eclampsia in hypertensive pregnant women. 4


Phenytoin can cause fetal hydantoin syndrome.

The following are the features of fetal hydantoin syndrome

mild to moderate growth deficiency

 

mild mental deficiency

 

craniofacial abnormalities
ocular hypertelorism
broad nasal bridge
low set abnormal ears
cleft lip and palate
hypoplasia of distal phalanges
micro - brachycephaly 


Hirsutism is a side effect of phenytoin.Phenytoin could also cause folate deficiency and this is reason why epileptic patients needs higher dose of folic acid(5mg).

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