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

 

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