& Dosage | Action
| Interacations |
Effects & Precautions |
Brands available in Market
Phenytoin exhibits zero order kinetics and has limited aqueous solublility. If further dose increments are to be given it should be in low dose and done gradually to avoid toxicity of drug. It is metabolised in liver and only about 2% of the drug is excreted unchanged in urine. There is good correlation between serum level of phenytoin and control of epilepsy. Serum assay of phenytoin is useful in treatment of epilepsy.
VENTRICULAR ARRHYTHMIAS (ESPECIALLY DIGITALIS INDUCED): With ECG monitoring, 3.5-5mg/kg via Caval catheter, slow inj at a rate of 50mg/min.
Acting on the motor cortex, phenytoin prevents spread of seizure activity. Stabilises threshold for hyperexcitability and reduces posttetanic potentiation at synapsi. Reduces brain stem activity responsible for the tonic phase of tonic-clonic convulsion. Also, a useful antiarrhythmic,
There are complex interactions between antiepileptics and toxicity may be enhanced without a corresponding increase in antiepileptic activity. Such interactions are very variable and unpredictable and plasma monitoring is often advisable with combination therapy. Since phenytoin is extensively bound to plasma proteins it can be displaced by drugs competing for protein-binding sites, thus liberating more free (pharmacologically active) phenytoin into the plasma. However, elevation of free phenytoin is reported to be of little clinical significance provided hepatic function is not impaired.A potentially more serious type of interaction may occur because phenytoin metabolism is saturable: toxic concentrations of phenytoin can develop in patients given drugs which inhibit phenytoin metabolism even to quite a minor degree. Phenytoin is a potent enzyme inducer, and induces the metabolism of a number of drugs, including some antibacterials, anticoagulants, corticosteroids, quinidine, and sex hormones (notably, oral contraceptives). The hypotensive properties of dopamine and the cardiac depressant properties of drugs such as lignocaine may be dangerously enhanced by intravenous administration of phenytoin.
Adverse Effect & Precautions
Gingival hypetrophy, Hirsutism, Acne, coarsening of facial features is seen with prolonged therapy. Anorexia, nausea, epigastric distress, constipation, vomiting, hyperglycaemia, megaloblastic anaemia, neutropenia, lymphadenopathy, hypersensitivity reaction and osteomalacia may also occur. It may cause ataxia, vertigo, drowsiness, confusion and impairment of higher intelectual fuction. Psychiatric changes.
Precaution: Chloramphenicol, Coumarin anticoagualants, isoniazid, metronidazole, cimetidine and disulfiram inhibit the metabolism of phenytoin which may rsult in phenytoin toxicity. Durgs which increase metabolism of phenytoin are barbiturates Carbamzepine and ehtanol thereby leading to lower plasma levels of phenytoin. In patients of hepatic disease phenytoin dose adjustment may be required. The drug should be withdrawn gradually to avoid rebound epihepatic imapirment. I.V. administration may cause hypotension, skin necrosis at i.v. site.