Vitamin and homocysteine (Hcy) alternations have been associated with psychiatric disorders. The aim of this meta-analysis was to assess the association of serum vitamin and Hcy levels with obsessive-compulsive disorder (OCD). Following PRISMA protocol, we used the databases including Scopus, PubMed, Google Scholar, and Web of Science with no time restriction. Data were pooled using a random-effects model and/or fixed-effects model to estimate the standard mean difference (SMD) for evaluation of the strength of association analyses. Our data showed a significant reduction in vitamin B12 (SMD = −0.58, 95% CI = −1.08 to −0.08, p = 0.02, I2 = 65%; pheterogeneity = 0.06), vitamin E (SMD = −0.89, 95% CI = −1.23 to −0.56, p < 0.00001, I2 = 23%; pheterogeneity = 0.26), and vitamin C (SMD = −1.40, 95% CI = −2.44 to −0.36, p = 0.008, I2 = 92%; pheterogeneity < 0.0001) in OCD patients. In addition, the findings showed significantly higher levels of Hcy (SMD = 1.11, 95% CI = [0.48, 1.75], p = 0.0006, I2 = 73%; ph = 0.02) in patients compared to controls. Also, our data showed that vitamin B9 and D levels are not associated with OCD (vitamin B9: SMD = −0.23, 95% CI = −1.01 to 0.55, p = 0.56, I2 = 88%; pheterogeneity < 0.0001; vitamin D: SMD = −0.63, 95% CI = −1.41 to 0.15, p = 0.11, I2 = 88%; pheterogeneity = 0.0002). Our findings support significant impacts of Hcy and vitamin B12, E, and C levels in OCD pathogenesis. This will be important for prevention and treatment of OCD. However, further studies are recommended to elucidate more accurate conclusions.
