Addicts are going to get fucked up on whatever they can find. If they can't find opiates or whatever, they're going to get fucked up on cough syrup, or booze or huffing paint or whatever.

In general, though, addicts don't necessarily go straight to the hard stuff or inevitably to the hard stuff, if other options are available. Prohibition actually encourages people to go to the hardest purest forms, though.

When you're moving around contraband, you want the most expensive high you can get in the smallest volume. It's easier to move around heroin or crack than something like codeine or coca wine, so that's what's going to be available on the street.