Probably a bad thing...

If California legalizes marijuana, consumption will likely increase. But is that a bad thing? - LA Times: The data from Colorado and Washington, where voters legalized recreational marijuana in 2012, are still preliminary. We do know, however, that the number of Coloradans who reported using marijuana in the past month increased from about 10.5% in 2011-12 to nearly 15% in 2013-14. In Washington, reported use increased from just above 10% to almost 13%.

Given that both states' preexisting medical systems already provided quasi-legal availability, it is hard to imagine that commercial legalization did not account for at least some of these increases. (That said, other factors could influence marijuana use and it will be some time before researchers have enough data to conduct rigorous analyses. Some of the increase could also come from respondents being more honest now that marijuana is legal in their states).

But is an increase in marijuana consumption a bad thing from a public health standpoint? Not necessarily.

I didn't realise the increases were that large, but this article is from a pro-legalisation advocate.

On the matter of public health, the problem is partly the length of time it takes to work this stuff out.  The rate of increase in smoking in the relatively young is the major issue, but its full effect may take years to clearly establish.

Given that the worst possible health effect (apart from possible car accident death) is a really debilitating mental illness (schizophrenia), surely you don't need too much of an increase in the rate of that to say that its increased use is a real public health negative.