I, personally, think that making the use of drugs a crime is a bad idea. I think not only is it bad for the drug users themselves it is bad for society as a whole and it is also bad for the ever-sacred taxpayer.
Reasons it is bad for the drug users themselves is pretty obvious. If one buys drugs off the streets it is impossible to actually know the contents of the drug, meaning that the drug could be cut with anything including harmful substances. It creates a stigma in which it is harder to get help for a drug addiction. Putting drug addicts in prison is bad for them -- it is not the help that they need to recover from this. Not only this, but a prison sentence ruins further life chances, meaning that if an addict recovered and tried to live a normal life with a normal job, they would find employment difficult to find. Although former addicts may find it hard to do this anyway, a conviction makes it more difficult, and regarding the non-addicted drug users it would definitely have an impact as they could function normally in society prior to conviction.
Obviously the main negative impact on society is the crime factor. People often resort to crime to get another fix, and drugs are sold by the generally corrupt and the profits may go towards funding other illegal ventures.
I think it negatively affects the taxpayer. The first reason is that prison sentences are in the long run more expensive than rehabilitation for drug addicts. Rehabilitated drug addicts can then go on to lead a normal life meaning having a job and paying tax, thus less tax for the rest of us.
Locking people up in prison also effectively makes them unemployable, because very few people are prepared to employ an ex-con and they have also been out of the jobs market so to speak for the duration of their prison sentences. This means that a person locked up for a drug offense might end up stuck on benefits, costing the taxpayer money.
The other point is that if drugs were legal they could be taxed in a similar way to cigarettes or alcohol, again, making more money for the government meaning either taxes could be cut elsewhere or spending could be increased.
Do not get me wrong: I don't like drugs in general and I would never choose to do them. But other people will, no matter how illegal they are(indeed, perhaps because they are illegal). I think the best way is to make them legal and safer(ie. cut with less crap, in more controlled doses, etc) and set up programs for rehabilitation and better programs of education about drugs. At the end of the day, the best one can really do is to educate people and let them make their own choices, to draw an analogy re: sex education, the best we can do is to tell children about condoms and birth control and how effective they are, the negatives of STDs and the risk of pregnancy, oral and anal sex and how they can still spread disease et al. and then hope that they take this advice into account when they have sex.
NB: I think it is fairly clear, but some of these points apply only to people who actually suffer with addiction, and others apply to drug users in general. Of course, these two things are different, in fact, "drug use" is fairly heterogeneous a category but it tends to be all lumped in together.