Legalizing marijuana in your given state, province or country has the additional benefit of being able to place a tax on the commercial sale of a drug normally sold on the black market: since they've passed legalization bills, California, Washington and Colorado have all generated more than $1 billion. This is a sum of money too small to ignore (although not necessarily in contrast to the overall budget of these states).
Where should states/nations invest this revenue? Some ideas proposed include directing funding towards awareness campaigns, public schools, or human services. Since the United States is currently in an opioid epidemic, it also seems wise to consider forwarding resources to address this public health crisis concerning more dangerous drugs. Given the money comes from vending citizens an unhealthy substance, what's the most ethical solution?
The tax money could be useful if invested in public/merit goods, such as education and healthcare. However, it is very difficult to track where this tax money is used, and is extremely unique to the country/circumstance. Therefore, I don't think the "use" of the tax money, or the potential benefit to society, is a sufficient argument in itself to justify the legalization of marijuana.