By legalising it would be more accessible to young people. It has been recorded that young people are more susceptible to mental health problems related to smoking weed. Mental health issues in general should be a concern, they're expensive to deal with and can lead to lifelong dependency on medical services.

Legalising it will increase the number of smokers, since we culturally smoke it with tobacco this will increase the amount of tobacco smokers.

There are many reasons why we choose to smoke it with tobacco that won't be solved by new smoking techniques. People can't afford to smoke pure weed all the time, it's easier to smoke socially in a joint and some people prefer to have a smoke to break up activities such as gaming or research.

The US may be very anti-smoking but they also don't tend to smoke weed with tobacco so it's kind of a redundant point.

We've tried all sorts to reduce tobacco smoking and it isn't really working, offering a new incentive to smoke tobacco will increase the amount of smokers.

Notice I said "I don't think" not "it definitely will". Neither of us are in a position to say whether it will but simply saying it will lead to a reduced demand on the NHS is pointless.

Personally, I think it will reduce crime rates by reducing public intoxication, and to an extent it will probably reduce A&E stress at the weekend but that doesn't guarantee a reduced stress.