That didn't seem to be a factor in alcohol prohibition.

Anyone with half a brain can make alcohol himself. However, I for one would have no idea where to start with making steroids.

You said yourself that if steroids were legal, cheap and transdermal, you might try them. So, in you, it would cause an increase in usage if they were legalized.
If I had a reliable source of steroids right now, I'd be using them again. So, in me, it would cause an increase in usage if they were legalized. Same goes for several people I know.
I also know people who, because it is illegal, think it's too dangerous to use. They'd use it if it were legal.

Now, it could be that we are the only people in the world this goes for. It's possible. Doesn't seem likely though. So, an increase in usage if it gets legalized seems extremely likely.