Even advocates of drug legalization concede that the move risks an increase in drug use. Milton Friedman, a passionate advocate of drug legalization, noted that a fall in price would be expected to bring about a greater level of demand. Those who advocate decriminalisation of drugs, rather than legalization, often make this point in defense of their position.
Indeed, if marijuana were legal, it might be incredibly cheap. One book estimates that a joint may be as cheap as things we commonly regard as free  (such as sachets of artificial sweeteners).
In practice, the effects of drug legalization on total consumption are unknown. The Netherlands, with its liberal drug policy, provides some interesting insights, but it is impossible to say whether these would carry over in other countries. For example, adult use of marijuana in the Netherlands is about the same as in other European countries .
It is important to separate total consumption from the more important factor: the impact of legalization on addicts. A rise in non-dependent users of drugs should not be a cause for concern. If people can use drugs recreationally without feeling reliant on them, then all the better. We should embrace the different ways in which people choose to enjoy their lives. It is those who become addicted that we should be more concerned for.
By reducing the price of drugs, we can help to free the vulnerable from the costs of their addiction. It is the high prices of drugs (as a result of their legal status) which causes users to resort to crime to pay for their vice. It is these high prices, and the inelastic nature of the demand addicts have for the product, that causes users to reduce their consumption of other goods rather than of drugs. This in turn means that drug users face worsening diets and unsafe living conditions. These can be more threatening than the drugs themselves.
There are many other benefits that would follow from liberalising drug policy (such as taking the economic activity of drug distribution out of the black market), but our chief concern in considering the issue of drugs should be those most at risk. Government intervention that drives the most vulnerable to commit crime is heinous indeed. By compounding the problems of addicts with higher costs we make their position worse. Addiction need not be so devastating.