Because periconceptional folic acid (FA) supplementation is widely recommended for substantial advantages in preventing neural tube defects, concerns about potential side effects have recently been raised on both mothers and their infants, researchers first assessed the impact of the use of FA supplements on gestational diabetes mellitus (GDM) with regard to both doses and durations in order to further understand the safe dose and duration of the use of FA supplements with respect to GDM risk. For the final analysis, 4,353 women were eligible. Data reported that the incidence of GDM was 8.6%. According to findings, FA supplement use ≥ 800 μg/day has been found to be associated with higher GDM risk from pregnancy through mid-pregnancy. Given periconceptionally, an FA dose of 400 μg/day was shown to be effective in preventing neural tube defects. Unless otherwise indicated, there is no scientific justification for the long-term use of a higher dose of FA. The potential risk of excess FA intake is a concern that will need further assessment to examine the mechanism underlying it.