Phenytoin Tablets

Mechanism

Phenytoin is an antiepileptic drug which can be useful in the treatment of epilepsy.
Through hyperpolarisation it stabilizes the membranes of the central and peripheral nerves and thereby prevents spread of seizure activity rather than abolish the primary focus of seizure discharge. An increase in inhibitory impulses contributes to the anticonvulsive effect.
It appears to stabilise rather than raise the seizure threshold and prevents spread of seizure activity rather than abolish the primary focus of seizure discharge.
The mechanism by which phenytoin exerts its anticonvulsant action has not been fully elucidated however, possible contributory effects include:

1. Non-synaptic effects to reduce sodium conductance, enhance active sodium extrusion, block repetitive firing and reduce post-tetanic potentiation.
   
2. Post-synaptic action to enhance gaba-mediated inhibition and reduce excitatory synaptic transmission.
3. Pre-synaptic actions to reduce calcium entry and block release of neurotransmitter.

Pharmacokinetics

• Absorption: Slowly but well absorbed after oral administration
• Bioavailability: Approximately 70–90%
• Distribution: Highly protein bound (~90%)
• Metabolism: Hepatic metabolism via CYP enzymes (capacity-limited)
• Half-life: Variable (approx. 22 hours; dose-dependent kinetics)
• Excretion: Mainly excreted in urine as inactive metabolites

Side Effects

• Gingival hyperplasia
• Dizziness
• Ataxia
• Nausea
• Rash

Contraindication

• Hypersensitivity to Phenytoin
• Sinus bradycardia
• Heart block

Interaction

• Oral contraceptives (reduced efficacy)
• Warfarin (altered levels)
• Alcohol (affects metabolism)
• Other anticonvulsants

Warning and Precautions

• Monitor blood levels regularly
• Avoid abrupt withdrawal
• Use cautiously in liver disease
• Maintain oral hygiene (gingival hyperplasia)