The only way we can restrict who buys cannabis is to legalise its sale. That way we have sanction over someone who sells to the underage or otherwise unsuitable. Right now, anyone whose got the money can buy cannabis and that is far from ideal.

It is true that you cannot completely eliminate the black market but when you look at alcohol prohibition in the US you see that it has reduced in size dramatically. In addition, the regulation around alcohol has worked to reduce the harms associated with it. Finally, as with alcohol prohibition, the dealers either change or lose their business. No one wants to buy from sketchy people on the street.

Most of the problems we see from drug use, either those caused to the individual or society, come from prohibition itself. Prohibition makes it harder for people in trouble to seek help. It makes it harder to communicate the dangers of drugs to drug takers and it creates a fantastically rich class of gangsters who will kill to protect their turf.

The problem with finding them and punishing them is that it extremely expensive, as is widely known because finding and punishing them is exactly what we are doing now. After 70 years of supply side intervention, drugs are still widely available and drug dealers are still here. The US has more prisoners per capita than anywhere in the world. You simply cannot lock up all the drug dealers and throw away the key. Where there is a great demand you will always find people willing to risk supplying it.

The American history during the period of "The Prohibition" (of alchohol) in the 1920's clearly shows how prohibition can increase the demand of a popular item. If people really want it there will be people risking to supply. This will result in black market violence between gangs for control of the supply and further enhance difficulties for people in need of help to seek it out.

Drug use is a victimless crime. We have no right to legislate what someone can or cannot consume. Moreover, we have no ability to. Drug users will continue to take drugs regardless of the law. All the law does is create a system where people hide their use and feel guilty about taking drugs. This, contrary to popular belief, does not result in them stopping drugs but rather results in a binge culture where their drug consumption continues in an even more harmful way.

Alcohol and tobacco are both regulated. The laws attempt to minimize the harms associated with them. The same can and should be done with cannabis. Dangerous behavior such as driving high should be illegal but that doesn't mean all behavior involving cannabis should be illegal. Such blanket laws criminalize even the safe and responsible use of cannabis.

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Alcohol causes more crime, abuse and costs to the NHS as well as society as a whole and it is legal, so why should cannabis be illegal?

Sure, you could argue alcohol should be illegal, therefore that must rationally be your position if you argue against legalisation of cannabis. Are you?

Parity would be a great start.

Unless you are suggesting actually alcohol is "better", "nicer", "less harmful", and so should be legal?

The real point is that we as a society choose to allow access to all sorts of potentially harmful activities and substances because we have free will - including over our own bodies.