At any rate, the number of people harmed makes little to no difference so long as they are: 1.) Harming only themselves, 2.) Paying the costs for any future medical care they may need to the proportion that the average user of the drug would end up needing it, and 3.) Made aware of the nature of risks for which they are already paying.
I would agree with this, but point numbers 1, 2, and 3 are all questionable at best in many cases. You don't harm only yourself when you destroy yourself with drugs. You harm your family, anyone who relies on you, anyone who cares about you, and possibly children who are modeling after you. You don't pay the costs of all future medical care yourself, even if you live in the USA where there isn't universal health care. You still suck up medical resources and drive prices for them up through higher demand. And people are often NOT made aware of the nature of risks involved, both because they are not fully researched and because these drugs come from sources that don't provide much of a warranty.
A little pot isn't likely to hurt you all that much. But cocaine and other hard drugs can do some serious damage, both to you, and to those around you.