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Phenobarbital in the serum
Medical expert of the article
Last reviewed: 05.07.2025
The serum concentration of phenobarbital when used in therapeutic doses is 10-40 mg/l (65-172 μmol/l). The toxic concentration is more than 45 mg/l (more than 194 μmol/l).
The half-life of phenobarbital in adults is 96 hours, in children - 62 hours, in newborns - 103 hours. The time it takes for the drug to reach equilibrium in the blood is 3-4 weeks.
Phenobarbital is primarily used as an anticonvulsant. It is taken orally, the drug is almost completely (up to 80%) absorbed in the small intestine. The maximum concentration of the drug is achieved 2-8 hours after a single oral dose, 1.5-2 hours after intramuscular administration. In blood plasma, phenobarbital is bound to proteins by 40-60%. Metabolism occurs in the liver by oxidation by the microsomal cytochrome P450 system. Approximately 50% of the drug is excreted unchanged by the kidneys. Phenobarbital is monitored in patients with epilepsy receiving this drug.
Rules for taking blood for research. The material for research is blood serum. For research, a sample of venous blood is taken before receiving the next dose of the drug. The first measurement of the drug concentration is carried out 2 hours after intravenous (initial) administration, and then 3-4 weeks after the start of treatment. The next control studies of the drug in the blood are carried out in the event of:
- changes in the dose of phenobarbital;
- introduction of another antiepileptic drug into the course of treatment;
- the appearance of signs of intoxication;
- recurrence of epileptic seizures;
- in pregnant women every 2-4 weeks.
Signs of drug overdose: drowsiness, impaired coordination, ataxia, nystagmus.
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