The choice of treatment for epilepsy should take into account the delay in action, the side-effects and the cost of the drug administrated. Phenobarbital is often prescribed in the first instance. During long-term treatment, the serum-phenobarbital concentration can be measured to verify that the concentration is above that known to be effective and below potentially toxic levels. The dose administered can be adjusted according to the results. The measurement should be taken when the concentrations are equilibrated (about three weeks after the start of treatment). If crises are under control, then a biennial check-up is recommended because serum concentrations and biological effects vary during prolonged treatment.