STEROIDS and LAW

All right...here's the deal-I'm using testosterone prescribed by a doctor as a treatment for my medical condition.
And I have to continue using it for a long period of time probablly for the rest of my life.I take shots every 1-2 weeks.
But I was thinking I'm using it anyway,why not bulding some muscles...the problem is that steroids are very expensive here in US and of course illegal.However I have a friend who can send me some from abroad.Can that put me (or him) in some kind of legel trouble if we get caught.I mean I have a prescription so I think everything sould be all right,but I just wanna make sure?
What about if he sends me steroids different from what my prescription says?For example he sends me deca durabolin or sustanon or dianabol and I have a prescritpion for testosterone?Dianabol for example is far from treatment of my medical condition and I don't think I can get a prescription for that,so will I get in trouble with the law if smth goes wrong?What the law says by the way?I tried to find smth online but I couldn't.Is it one law for US or it's differnet for every different state?I'm in SC.Do U know where can I find some more info online?
What about if I possess steroids without or with expired prescription?What are the consequences?
I was also wandering if the steroids are illegal here then why you can buy from so many online stores what they claim to be "legal'" steroids and what makes them "legal"?Is it safe(the drug) and am I safe(with the law)?
Is clenbuterol considered a steroid and where can I find a list with all prohibited steroids and illegal drugs?
And one more last question-will prescription from abroad be valid here in US?
Thank you!