A review of epidemiological data on epilepsy, phenobarbital, and risk of liver cancer.

Phenobarbital is not genotoxic, but has been related to promotion of liver cancer (as well as inhibition) in rodents. In October 2012, we carried out a systematic literature search in the Medline database and searched reference lists of retrieved publications. We identified 15 relevant papers. Epidemiological data on epileptics/anticonvulsant use and liver cancer were retrieved from eight reports from seven cohort (record linkage) studies of epileptics, and data on phenobarbital use from a pharmacy-based record linkage investigation of patients treated with phenobarbital (three reports), plus a case-control study nested in one of the cohort studies and including information on phenobarbital use. Of the studies of cancer in epileptics, two showed no excess risk of liver cancer. A long-term (1933-1984) Danish cohort study of epileptics found relative risks (RRs) of 4.7 [95% confidence interval (CI) 3.2-6.8] of liver cancer and of 2.2 (95% CI 1.2-3.5) of biliary tract cancers. Such apparent excess risks could, however, be largely or completely attributed to thorotrast, a contrast medium used in the past in epileptic patients for cerebral angiography. A Finnish cohort study of epileptics obtained an RR of 1.7 (95% CI 1.2-2.4). Such an apparent excess risk, however, was not related to phenobarbital or to any specific anticonvulsant drug. The long-term follow-up of two UK cohorts found some excess risk of liver cancer among severe, but not among mild, epileptics. Some excess risk of liver cancer was also found in cohort studies of patients hospitalized for epilepsy in Sweden and Taiwan, in the absence, however, of association with any specific drugs. A UK General Practice database, comparing epileptics treated with valproate with unexposed ones, found a very low incidence of liver cancer. Of the studies of cancer in patients treated with phenobarbital, a large US pharmacy-based cohort investigation showed no excess risk of liver cancer. In a case-control study, nested in the Danish cohort of epileptics, no association was observed between phenobarbital and liver cancer among patients who had not received thorotrast (RR=1.0 for liver and 0.8 for biliary tract cancers). Thus, some, although not all, studies reported excess risk of all cancers and liver cancer in severe, but not in milder epileptics. There is no evidence of a specific role of phenobarbital in human liver cancer risk, but data on the topic are limited.