There are valid health concerns with it. Oftentimes it's not inspected or approved by the FDA, and you are usually injecting it straight into your bloodstream. There are also conditions, that may have not yet been diagnosed that could make taking additional hormones dangerous.

Just be very well read concerning the literature, make sure you understand your body well (underlying conditions, potentially hereditary conditions too), ask for medical advice and try your best to buy from a reputable source or source FDA approved drugs.