The current “marijuana stocks” are for small companies that are likely to be pushed aside when larger well-established companies enter the business. In effect you have two problems buy stocks in that industry. If the movement towards legalization reverses, you own shares in a company whose main activity is illegal. If the movement towards legalization continues, the companies in similar businesses (tobacco, food, alcohol, etc.) are likely to move in and crush the small players you are investing in.
If you are not a professional investor, you are likely to get burned playing the stock market.