What percentage of people have a drug problem?

Its really hard to say for certain what percentage of homeless people imbibe illegal/legal drugs. Until there is a 100% accurate census of the homeless, no truly accurate percentages can be deduced.

With that said, it depends on where you're at. If its a big city, then the percent of homeless using illicit drugs is increased, and if its a smaller city, then the chances of alcohol abuse is increased.

This would make for an interesting study, actually, but the legwork involved would be much too tedious and difficult for a single person to collect any significant amount of data.