Review Manager (RevMan 5, The Nordic Cochrane Centre) was used to combine findings of studies in a meta-analysis. Studies were pooled if homogeneity was considered by assessing study population, intervention, and outcome. RevMan was used to determine homogeneity by the inverse variance method for a random or fixed effects model. If homogeneity was not obtained studies were excluded from meta-analysis. Heterogeneity was expressed by I2. An I2 of >75% was considered as substantial heterogeneity. Meta-analysis was performed on observational studies and RCTs, in which data from observational studies and RCTs were not combined in the same meta-analysis. For the outcome of interest, risk ratios and 95% confidence intervals were used.