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Drug Information
Pharmacokinetics
| Indication
& Dosage | Action
| Interacations |
Adverse
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.
Indication
& Dosage
IV
VENTRICULAR ARRHYTHMIAS (ESPECIALLY DIGITALIS INDUCED): With ECG
monitoring, 3.5-5mg/kg via Caval catheter, slow inj at a rate of
50mg/min.
Action
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,
Interaction
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.
