I agree that there are negative effects an individual experiences when using drugs, but there are negative effects of doing a lot of things. I understand and agree with the idea that some substances are too dangerous for the general population to be allowed to possess, but in terms of regulation, I would agree only if those dangers relate to effects felt by people beyond the user. For example, possessing strongly radioactive material will cause harm to people based upon proximity, regardless of who actually possesses the substance. When it comes to growing a plant and consuming it in such a way so as to enjoy the effects produced in the body and brain, to me that's a bit absurd.
The origins of criminalizing cannabis were cultural and racist -- portraying "the wrong sort of people" as being frequent users, there became a movement to ban the behavior as a way of punishing those people, and it created a snowball effect that we now call the drug war. The main forces who wish to keep it illegal are those who gain financially from it being so -- dealers who make more profit because of how supply and demand are affected by the laws, and law enforcement agencies who are paid to keep chasing them down. Neither wants the system to change, because then they'll be out of a job. The main proponents of decriminalization are the people who wish to continue enjoying their usage in the privacy of their own homes without fear of arrest or paying fines for doing so.
When a movement began to criminalize alcohol, a thriving black market arose, because demand for the product didn't disappear when it suddenly became illegal. Suppliers saw this demand and potential for profit, and competition led to violence. Law enforcement saw another way to wield power, as well as accepting bribes to "look the other way." The effect was that for people who still desired to consume the product, costs went up dramatically, and because the product was an illegal substance, the threat of incarceration was ever present.
And what do we have today? Basically the same system, only the substance has changed.
I can't remember which country off the top of my head (I'll have to dig around and get back to you), but I remember seeing a documentary about the drug war here that examined how things are working in a European country that recently decriminalized many of its drugs, and had government-funded rehabilitation provided free for addicts seeking to get clean. The part that raised my eyebrows was when the interviewer asked "Don't the taxpayers object to the idea of paying for addicts' rehab?" and the person responded "Well, in actuality, we have determined that it costs less taxpayer money to do it this way than it costs to maintain the addicts as criminals in jail."
Hmmmm......
