I disagree. Yes, the people who love a drug addict are affected by the addiction. That does not negate the fact that a consenting adult should have the right to do whatever he/she wishes with his/her own body, be it taking drugs, drinking alcohol, smoking tobacco, whatever. If drugs were legal, global crime cartels would implode overnight, and the money saved on drug enforcement could be funneled into low-cost rehabilitation and education facilities.
Prostitution should also be legal and regulated. Unless the prostitute is being forced, a violation of many other laws, then she is plying her trade of her own free will. Her clients are there of their own free will. If you're thinking they harm others because of STD's, a legal, regulated prostitute is cleaner and more savvy than the mistress the married guy bonks in the office. If you're thinking about the wives being deceived, that's on the married man who fools around, not on the prostitute.
Prohibition never works. History has shown us that prohibiting what people can ingest or do with their own bodies does nothing but make felons out of non-criminal citizens, and prop up a profitable crime syndicate to provide a black-market service.
Both drug laws and prostitution laws are on the books for one reason and one reason only: The continuing need for lots and lots of law enforcement jobs, and the continuing need for government enforcement bureaucracies, like the DEA. If we made drugs and prostitution legal, the supporting crimewave would fold and they'd lose a lot of jobs and power.