Phenytoin Pharmacology


About Phenytoin
Hydantoin derivative, Anticonvulsant, Class Ib Antiarrhythmic Agent.
Mechanism of Action of Phenytoin
Phenytoin acts on the motor cortex where it stabilizes neuronal membrane and inhibits the spread of seizure discharge. It prolongs the inactivated state of voltage sensitive neuronal sodium ion channel either by increasing the efflux or decreasing the influx of sodium ion across the cell membrane and inhibits the high frequency discharge.
Antiarrhythmic action of Phenytoin is due to its membrane stabilizing effect and blockage of sodium channels. It causes depression of automaticity in ventricular and Purkinje fibres.
At high concentrations Phenytoin
1. Enhances calcium binding to phospholipids in neuronal membrane and results in a more stable neuronal membrane.
2. It enhances the concentration of inhibitory neurotransmitter GABA
3. It inhibits the excitatory glutamate receptors.
All these indicate that Phenytoin limits the development of maximal seizure activity and reduce the spread of the seizure process from the active focus.
Antiarrhythmic action: Phenytoin exerts its action by normalizing sodium entry to Purkinje`s fibers in patient with cardiac glycoside induced arrhythmias.
Pharmacokinets of Phenytoin
Absorption: It is slowly absorbed after oral administration.
Distribution: It is very widely distributed in the body in protein bound form.
Metabolism: It is metabolised in the liver to inactive metabolites.
Excretion: It is excreted mainly in urine
Onset of Action for Phenytoin
10 days
Duration of Action for Phenytoin
1 day
Half Life of Phenytoin
12 - 24 hours
Side Effects of Phenytoin
1. Gum hypertrophy
2. Hirsuitism
3. Acne
4. Slurred speech
5. Ataxia
6. Decreased coordination
7. Nystagmus
8. Diplopia
9. Nausea
10. Vomiting
11. Megaloblastic anaemia
12. Neutropenia
13. Rash
14. Dermatitis
15. Coarsening of facial expressions
16. Abdominal discomfort
17. Constipation
18. Anorexia

Contra-indications of Phenytoin
1. Hypersensitivity to Phenytoin and other Hydantoins
2. Atrioventricular block
3. Sinus Bradycardia
4. Sino Atrial block
Special Precautions while taking Phenytoin
1. Hepatic impairment
2. Myocardial infarction
3. Hypotension
4. Renal impairment
5. Diabetic patient
6. Respiratory depression
7. Debilitated patient
8. Slowly withdraw the drug with caution

Pregnancy Related Information
Old Age Related Information
Use with caution
Breast Feeding Related Information
Children Related Information
Use with caution
NEONATES : Use with caution
Indications for Phenytoin
1. Anticonvulsant (treatment of all epilepsy except absence seizure)
2. Trigeminal neuralgia,
3. Migraine,
4. Cardiac arrhythmia
Interactions for Phenytoin
Increased pharmacological effects of Phenytoin may occur when any of the following drugs are administered concomitantly with Phenytoin: Allopurinol, Amiodarone, Benzodiazepines, Chloramphenicol, Cimetidine, Disulfiram, Acute ethanol ingestion, Fluconazole, Isoniazid, Metronidazole, Omeprazole, Phenylbutazone, Succinimides, Sulfonamides, Trimethoprim, Valproic acid, Salicylates, TCAs, Phenothiazines, Chlorpheniramine, Ibuprofen.
Decreased pharmacological effects of Phenytoin may occur when any of the following drugs are administered comcomitantly with phenytoin: Diazoxide, Barbiturates, Carbamazipine, Rifampicin, Chronic ethanol ingestion, Theophylline, Antacids, Charcoal, Sucralfate, Antineoplastics, Folic acid, Influenza virus vaccine, Loxapine, Nitrofurantoin, Pyridoxine.
Phenytoin may decrease pharmacological effects of the following drugs: Acetaminophen, Amiodarone, Cardiac glycosides, Corticosteroids, Dicoumarol, Doxycycline, Haloperidol, Oestrogens, Methadone, Mexiletine, Oral contraceptives, Quinidine, Frusemide, Cyclosporine, Mebendazole, Non depolarising muscle relaxants, Sulfonylureas, Valproic acid.
Corticosteroids: Systemic manifestation of phenytoin induced hypersensitivity reactions masked.
Dopamine: Severe hypotension.
Meperidine: Decreased analgesic effect and increased toxicity with phenytoin.
Primidone: Pharmacological effects of primidone enhanced.
Warfarin: Pharmacological effects enhanced leading to bleeding disorders.
Lab tests: May interfere with Metyrapone and 1mg Dexamethasone tests.

Typical Dosage for Phenytoin
Adults: Anticonvulsant
300mg /day in 3 divided doses with food or milk. If needed dose can be increased to 600 mg / day
Maintenance dose: 300 - 400 mg / day.
Neuritic pain (Trigeminal neuralgia, Migraine): 200 - 600 mg / day in divided doses
Arrhythmia: 50 - 100 mg IV every 10 - 15 minutes
Maximum dose: 15 mg / kg
Anticonvulsant: 5 mg / kg body weight in 2 or 3 divided doses
Maintenance dose: 4 - 8 mg / kg body weight
Maximum dose: 300 mg / day
Children above 6 years: 300 mg / day in 3 divided doses

Schedule of Phenytoin
Storage Requirements for Phenytoin
Store at 15 - 30 degree C. Protect from heat, light and moisture
Effects of Missed Dosage of Phenytoin
Take the missed dose as soon as noticed and if it is the time for next dose then skip the missed dose. Continue the regular schedule. Do not double the dose.
Effects of Overdose of Phenytoin
Give supportive measures and symptomatic treatment. Drug can be removed from the body by gastric lavage or emesis. Maintain fluid and electrolyte balance. Haemodialysis or peritoneal dialysis can be done if necessary.

Home Delivery for Phenytoin in Your City

Medicine India is just a publishing medium for medicine related information and does not provide services or sales of medicines including phenytoin.

However, we do publish a comprehensive directory of Pharmacies, Chemists and Druggists in cities all over India. You can use this directory to find the medicine stores in your city (or area) that provide home delivery services for phenytoin and other medicines and health products. Home delivery services for phenytoin may be free or they may cost you depending on the pharmacy and the minimum order requirements. It would be best to get this clarified while placing the order.

Please be aware that you should take phenytoin only if a doctor has recommended or prescribed it. Some or all pharmacies who provide a home delivery service for medicines might insist on a prescription for phenytoin before they complete the sale. You can get this information while placing the order for phenytoin with the pharmacy.

Phenytoin is a generic medicine name and there are several brands available for it. Some of the brands for phenytoin might be better known than phenytoin itself. If the pharmacy that's willing to deliver medicines to your home doesn't have phenytoin in stock, you can ask for one of the branded alternatives for phenytoin.