Source: Medicines & Healthcare Products Regulatory Agency (GB) Revision Year: 2018 Publisher: Macarthys Laboratories Ltd, T/A Martindale Pharma, Bampton Road, Harold Hill, Romford RM3 8UG
ATC Code: N03AA02 (antiepileptics, barbiturates and derivatives)
Phenobarbital is a long-acting barbiturate, which because of its depressant effect on the motor cortex, is used in the treatment of epilepsy. Phenobarbital has a widespread depressant action on cerebral function.
It has selective anticonvulsant activity and, used in hypnotic doses, it alters the stages of sleep in a dose dependent manner.
It has sedative effects and has some protective action against all varieties of human partial and generalised epilepsy, with the exception of absence seizures. Phenobarbital is also effective in preventing seizures in the corresponding experimental animal models of epilepsy.
In different studies phenobarbital appears to have had inconsistent effects in suppressing experimental epileptic foci, and epileptic after-discharges, but it inhibits synaptic transmission, at least in the spinal cord. The drug’s probable biochemical mechanism of action is through prolonging the opening time of Cl- ion channels in postsynaptic neuronal membranes. This effect causes membrane hyperpolarisation and thus impairs nerve impulse propagation. Phenobarbital also decreases intraneuronal Na+ concentrations, and inhibits Ca2+ influx into depolarised synaptosomes. It raises brain serotonin levels, and inhibits noradrenaline (norepinephrine) reuptake into synaptosomes. These additional biochemical actions may contribute towards the anticonvulsant effects of the drug.
The plasma half-life is about 75 to 120 hours in adults but is greatly prolonged in neonates, and shorter (about 21 to 75 hours) in children. The half-life is increased in the elderly and in neonates and is prolonged by renal and hepatic disorders. There is a considerable interindividual variation in Phenobarbital kinetics. Phenobarbital is only partly metabolised in the liver.
It is about 40% plasma bound.
Excretion is mainly in the urine (and is increased in alkaline urine) with about 30% of the drug unchanged. The remainder is inactivated in the liver.
Phenobarbital crosses the placenta and is secreted in the milk of nursing mothers.
No additional pre-clinical data of relevance to the prescriber is available.
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