CYP19A1 and venous thromboembolism: Five out of six randomised controlled trials directly comparing the risk of venous thromboembolism in aromatase inhibitor users versus tamoxifen users estimated large protective relative risks ranging from 0.25 to 0.61,28, 29, 32, 38, 39, 44 with one further randomised controlled trial finding no association (I2=0%, Cochrane Q test P=0.70, fig 4).