Modern COCs contain 10 to 35 μg EE, but typically 20, 30, or 35 μg. The initial formulations of COCs that were introduced in the 1960s contained 100 to 150 μg EE. However, it was soon found that EE is associated with increased risk of VTE and that the risk is dose-dependent. Following these events, the dose of EE was greatly reduced, and is now always less than 50 μg. These lower doses have a significantly reduced risk of VTE with no loss of contraceptive effectiveness. Gerstman et al. (1991) found that COCs containing more than 50 μg EE had 1.7-fold and COCs containing 50 μg EE 1.5-fold the risk of VTE of COCs containing less than 50 μg. A 2014 Cochrane review found that COCs containing 50 μg EE with levonorgestrel had 2.1- to 2.3-fold the risk of COCs containing 30 μg or 20 μg EE with levonorgestrel, respectively. COCs containing 20 μg EE are likewise associated with a significantly lower risk of cardiovascular events than COCs containing 30 or 40 μg EE. However, discontinuation of COCs is more common with doses of EE from 10 to 20 μg due to problematic changes in bleeding patterns.