system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: How do steroids work, and what about them makes them so dangerous and illicit?
gpt
First of all, by definition a steroid is just a specific chemical structure. Most of them just exist naturally: hormones like testosterone or oestrogen, vitamin D, cholesterol, ... Performance enhancing steroids are called anabolic steroids, and are mostly some variations on the male testosterone. When it comes to muscle development, they mostly stimulate protein synthesis and muscle recovery. 

What makes them dangerous is that testosterone is a sex hormone, impacting a lot more male "features" than just muscle development. It impacts your sex drive, facial hair, baldness, mood (roid rages), fertility, growth (cancers), ... Artificially injecting it also disrupts your natural production, making problems even worse. 

Besides wannabe bodybuilders, they are also used under prescription when teenagers don't enter puberty for example, so not all use is terrible.