Limitations
Our findings should be interpreted in the light of the following limitations: 1) This is an observational case-control study with its inherent drawbacks; however, we selected the control group from a population-based cohort to minimize the risk of bias. 2) We excluded more than two-thirds of our patients due to a history of taking vitamin B supplements in the last six months, which might bias our findings; nonetheless, more than half of the screened individuals from TeCS were excluded for the same reason as well. This eye-catching use of supplements may be attributed to the concurrency of the coronavirus disease 2019 (COVID-19) pandemic and our study because the usage of vitamin supplements dramatically increased during the pandemic.34, 35 3) The sample size of this study is small; nevertheless, it should be noted that the non-COVID-19 healthcare utilization dropped by one-third during the pandemic, which substantially limited the number of eligible participants for this study.36 4) It is more sensitive to screen for vitamin B12 or folate deficiency by measuring serum levels of methylmalonic acid and homocysteine,37 while we did not measure methylmalonic acid in this study.