What are some common side effects associated with pro-hormones?

The selected answer highlights significant side effects commonly associated with pro-hormones, particularly hair loss and breast tissue enlargement. Pro-hormones are substances that serve as precursors to hormones in the body, and their use can lead to hormonal imbalances. Hair loss occurs due to the conversion of certain pro-hormones into testosterone or its derivatives, which can affect hair follicles. Additionally, the enlargement of breast tissue, known as gynecomastia, can result from increased estrogen levels or an imbalance caused by elevated testosterone levels in the body.

The other options focus on benefits of pro-hormones, such as increased muscle mass, improved endurance, enhanced recovery, reduced fatigue, and improved cardiovascular health and flexibility. While these benefits may be desired outcomes from the use of pro-hormones, they do not address the common adverse effects that users might experience, thus emphasizing why the first choice correctly identifies side effects rather than benefits.