Phenobarbitone + Phenytoin Pharmacology

Phenobarbitone + Phenytoin

About Phenobarbitone + Phenytoin
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Mechanism of Action of Phenobarbitone + Phenytoin
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Pharmacokinets of Phenobarbitone + Phenytoin
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Onset of Action for Phenobarbitone + Phenytoin
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Duration of Action for Phenobarbitone + Phenytoin
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Half Life of Phenobarbitone + Phenytoin
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Side Effects of Phenobarbitone + Phenytoin
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Contra-indications of Phenobarbitone + Phenytoin
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Special Precautions while taking Phenobarbitone + Phenytoin
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Pregnancy Related Information
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Old Age Related Information
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Breast Feeding Related Information
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Children Related Information
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Indications for Phenobarbitone + Phenytoin
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Interactions for Phenobarbitone + Phenytoin
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Typical Dosage for Phenobarbitone + Phenytoin
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Schedule of Phenobarbitone + Phenytoin
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Storage Requirements for Phenobarbitone + Phenytoin
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Effects of Missed Dosage of Phenobarbitone + Phenytoin
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Effects of Overdose of Phenobarbitone + Phenytoin
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Phenobarbitone

About Phenobarbitone
Barbiturate derivative, Sedative-Hypnotic,Anticonvulsant.
Mechanism of Action of Phenobarbitone
Sedative and antiepileptic actions: Phenobarbitone mainly acts on Picrotoxin site of GABA -BZD receptor Cl- (chloride ion) channel complex. It prolongs the duration of GABA induced chloride channel opening and produces hyperpolarisation and decreases firing rate of neurons. This will alters the normal body functions. Phenobarbitone also has GABA mimetic action. At high concentrations Phenobarbitone directly increases chloride ion conductance and decreases transmission of calcium ion (Ca2+ ) dependent neurotransmitters. Phenobarbitone also inhibits excitatory glutamate neurotransmitters. At high concentration it also inhibits sodium ion channel and potassium ion channel. Phenobarbitone is used as an antiepileptic to control partial and generalised tonic-clonic seizures.
Barbiturates activate inhibitory GABA-A receptors and inhibit excitatory AMPA receptors and produces CNS depression.


Pharmacokinets of Phenobarbitone
Absorption: It is well absorbed after oral administration. Distribution: It is widely distributed in the body. Metabolism: It is metabolised in the liver. Excretion: It is excreted mainly through urine.
Onset of Action for Phenobarbitone
1 hour
Duration of Action for Phenobarbitone
10 - 12 hour
Half Life of Phenobarbitone
5 - 7 days
Side Effects of Phenobarbitone
1.Hang over
2.Drowsiness
3.Lethargy
4.Somnolence
5.Paradoxical excitement
6.Nausea
7.Vomiting
8.Depression
9.Confusion
10.Headache
11.Nystagmus
12.Ataxia
13.Hypothermia
14.Urticaria
15.Rash
16 Hypotension
17.Exacerbation of Porphyria
18.Dependnce
Contra-indications of Phenobarbitone
1.Hypersensitivity to Phenobarbitone and other barbiturates
2.Porphyria
3.Hepatic impairment
3.Renal impairment
4.Emphysema
5.Obstructive sleeps apnoea
Special Precautions while taking Phenobarbitone
1.Cardiovascular diseases
2.Patient with acute or chronic pain
3.Depression
4. Alcoholics
5. Drug abuse
6. Shock
7. Debilitated patients
8. Driving vehicles, operating machines and people involving any other dangerous activities
9. Slowly withdraw the drug with caution
Pregnancy Related Information
Use with caution
Old Age Related Information
Use with caution
Breast Feeding Related Information
Contraindicated
Children Related Information
Use with caution
Indications for Phenobarbitone
1.Anticonvulsant
2.Sedative.
Interactions for Phenobarbitone
Alcohol & other CNS Depressants (other sedatives, hypnotics, anesthetics, antihistamines, tranquilizers, phenothiazines): Additive CNS depression.
Valproic acid: Increases efficacy of barbiturates. Half life of valproic acid may be decreased.
Chloramphenicol: Efficacy of barbiturates increased. Barbiturates may decrease efficacy of chloramphenicol.
MAOIs: Effects of barbiturates prolonged.
Efficacy of the following drugs decreased by barbiturates: Oral anticoagulants, digitoxin, TCAs, corticosteroids, doxycycline, oral contraceptives and estrogens, acetaminophen, beta-blockers, quinidine, rifampicin, theophylline and metronidazole.
Phenytoin: Effect on phenytoin metabolism is unpredictable.
Frusemide: May produce or aggravate orthostatic hypotension.
Griseofulvin: Interferes with absorption of oral griseofulvin leading to decreased blood levels.
Typical Dosage for Phenobarbitone
Adult
Anticonvulsant (Not used for absence and febrile seizure): 60 - 180 mg / day at bed time or 3 divided doses
Sedation: 30 - 120 mg / day in 2 or 3 divided doses
Insomnia: 100 - 200 mg/ day
Maximum dose: 400 mg / day
Children
Anticonvulsant: 1 - 8 mg / kg body weight / day in single dose or 2 divided dose.
Sedation: 8 - 32 mg/ day
Schedule of Phenobarbitone
X DRUG
Storage Requirements for Phenobarbitone
Store at 15 - 25 degree C in a tightly closed container. Keep out of the reach of children.
Effects of Missed Dosage of Phenobarbitone
Take the missed dose as soon as noticed and if it is the time to take the next dose then skip the missed dose.
Effects of Overdose of Phenobarbitone
Give supportive measures and symptomatic treatment. Induce emesis or gastric lavage has to be done. Charcoal can be given to reduce the absorption of the drug. Urine alkalinization also helps to reduce the absorption of the drug. Haemodialysis can be done in severe cases.

Phenytoin

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
Contraindicated
Old Age Related Information
Use with caution
Breast Feeding Related Information
Contraindicated
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
Children:
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
G
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.

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