Avoid use in patients with hepatic or renal impairment, as phenobarbital has a low clearance rate and may accumulate. Avoid use in the elderly who exhibit nocturnal confusion or restlessness when taking sedative hypnotics. Patients should be advised not to discontinue phenobarbital abruptly or to excede the prescribed dose as they may experience physiologic dependence and a risk of withdrawal symptoms following prolonged use. Phenobarbital may enhance the metabolism of beta-blockers. Beta-blocker dosage may have to be adjusted when phenobarbital is started or stopped.
Phenobarbital reduces the plama concentration of verapamil and nifedipine.
Higher doses than usual of these agents may be needed to achieve the desired effect. Phenobarbital in combination with other CNS depressants may have additive effects. Phenobarbital may decrease chloramphenicol concentrations and chloramphenicol may increase phenobarbital concentrations. Chloramphenicol dosage may have to be increased to minimize the risk of treatment failure. Alternative methods of contraception or oral contraceptives with higher estrogen content should be considered.