Drugs are not completely personal. Depending on drug and supply chain, we are also talking about drug gang activity, drug-fueled economies in the developing world, social problems related to drug abuse, and so on. If you grow a harmless drug on your own or get it from a harmless source, you're probably okay, but let's not fool ourselves into thinking that drugs are completely down to the individual.
Prohibition causes most of the troubles though (especially cartel violence). And I think the social aspects of any increased addiction rates would be more than offset by not imprisoning hundreds of thousands annually on drug charges.
It's vastly cheaper to subsidize rehab for the relatively small number of actual addicts than it is to pay to arrest and process a million people a year. Not to mention the effect on GDP of the lost production from all the incarcerated people.
