Fenobarbital

Additional information

Pharmacokinetics and dosing

Several studies have shown that patient’s sex does not significantly improve the estimate of phenobarbital clearance, neither in neonatal, infants or elderly [4, 5]. A review article on clinical pharmacokinetics in pediatric patients concludes that no sex differences on phenobarbital C/D ratios (ratio between plasma concentrations and dose/kg) were found [6].

Effects

No studies with a clinically relevant sex analysis regarding the effects of phenobarbital have been found.

Adverse effects

A study evaluated prolactin secretion in patients with partial or generalized epilepsy (78 men, 56 women; aged 13-60 years) treated with phenobarbital alone or in combination with either phenytoin or benzodiazepines. An increase in baseline prolactin values was observed in men in all treatment groups, but not in women, compared with healthy controls [7]. Abnormal prolactin levels may be associated with sexual dysfunction [8].

Reproductive health issues

Enzyme-inducing antiepileptic drugs, such as carbamazepine, phenytoin and phenobarbital, may have potentially negative effects on reproductive endocrine function in men and women. These antiepileptic drugs increase concentrations of sex hormone-binding globulin (SHBG) and thereby reducing the concentrations of unbound biologically active androgens [1-3]. This may result in sexual dysfunction. 

Concurrent administration of phenobarbital and oral contraceptives may decrease the effect of estradiol. Regarding drug-drug interactions aspects, please consult Janusmed Interactions (in Swedish, Janusmed interaktioner).

In pregnant women, the concentration of phenobarbital is decreased up to 50-55%. The lowered serum concentration is even more pronounced in primidone derived phenobarbital, up to 70% [9]. This may necessitate dose adjustments and therapeutic drug monitoring. Regarding teratogenic aspects, please consult Janusmed Drugs and Birth Defects (in Swedish, Janusmed fosterpåverkan).