ObjectiveTo study the human risk and benefit of oral pyridoxine (vitamin B6) treatment during pregnancy.Design and settingThe analysis of cases with 25 congenital abnormality (CA) groups and their all-matched controls without CAs in the population-based dataset of the large Hungarian Case-Control Surveillance of Congenital Abnormalities, 1980–1996.Study participants22 843 cases of pregnant women who had newborns or fetuses with CAs and 38 151 pregnant women who had newborn infants without any CAs (control group).Main outcome measuresPrevalence of pyridoxine use in early pregnancy among mothers of cases with different CAs and control mothers with infants without any CA.Results2013 (8.8%) case mothers and 4086 (10.7%) control mothers were treated with pyridoxine (adjusted prevalence odds ratio [POR] 0.8; 95% confidence interval [CI] 0.7, 0.9). The analysis of cases with different defects and their all-matched controls did not indicate any obvious teratogenic potential of pyridoxine use during the second and third months of gestation, i.e. in the critical period for the development of most major CAs. However, some protective effect was found for cardiovascular malformations (adjusted POR 0.8; 95% CI 0.7, 0.9).ConclusionTreatment with pyridoxine during pregnancy does not indicate a teratogenic risk to the fetus, but may provide some protective effect for cardiovascular malformations.