OBJECTIVE: This study was aimed to investigate the effectiveness of vitamin C supplementation in preventing and treating respiratory tract infections.

METHODS: We used the Cochrane, PubMed, and MEDLINE Ovid databases to conduct our search. The inclusion criteria were placebo-controlled trials. Random effects meta-analyses were performed to measure the pooled effects of vitamin C supplementation on the incidence, severity, and duration of respiratory illness.

RESULTS: We found ten studies that met our inclusion criteria out of a total of 2758. The pooled risk ratio (RR) of developing respiratory illness when taking vitamin C regularly across the study period was 0⸳94 (with a 95% confidence interval of 0⸳87 to 1⸳01) which found that supplementing with vitamin C lowers the occurrence of illness. This effect, however, was statistically insignificant (P= 0⸳09). This study showed that vitamin C supplementation had no consistent effect on the severity of respiratory illness (SMD 0⸳14, 95% CI -0⸳02 to 0⸳30: I2 = 22%, P=0⸳09). However, our study revealed that vitamin C group had a considerably shorter duration of respiratory infection (SMD -0⸳36, 95% CI -0⸳62 to -0⸳09, P = 0⸳01).