ELI5 Steroids - What makes them dangerous and illicit?

How do steroids work? What about them makes them so dangerous? Why are they illicit and banned from most professional sports?   ... We explain like i'm five!   Thank you to the r/explainlikeimfive community, and specifically the following reddit users whose questions and responses formed the basis of this conversation: deknegt1990, drkobbe, lifteatsleeprepeat2, sa250039 & swollymamoth   To the ELI5 community that has supported us so far, thanks for all your feedback and comments. Send us an e-mail: [email protected] ... we love hearing from you!