Patient health and safety is at stake. A review of the clinical literature concluded that bioidentical hormones are associated with lower risks, including the risk of breast cancer and cardiovascular disease, and are more effective than synthetic or animal-derived hormones. On the other hand, studies have consistently demonstrated that synthetic hormones are associated with an increased risk of breast cancer. Synthetic and animal-derived hormones have also been shown to produce negative cardiovascular effects and to negate the cardio-protective effects of estrogen.