Greetings and salutations Bonken,
If you want to get into cannabis on regular markets your only option would be to buy on the London Stock Exchange. GWP.L GW Pharmaceuticals is the only company I know of on a regular exchange that has any cannabis products in their business. They make a cannabinoid spray to treat the spasticity related to symptoms of Multiple Sclerosis.
Everything else is on OTC markets (from what I understand) is considered the "gutter" of the markets - especially PNK sheets. But life isn't without risk, so I say why not!? Prior to CO & WA legalizing, MDBX was trading @ $3.14 a share on Nov 5th, but by the 15th it had jumped to $205.00 per share!!! - There are quite a few more cannabis related stocks so, I think there is a high probability that another one will POP! - Especially since CO and WA are STILL laying the footwork to legalization, so cannabis commerce isn't going to proceed on any kind of grand scale until 2014. My biggest gripe is the fact that the FDA says cannabis has zero medicinal value, yet the federal government registered patent 6630507, which specifically details the medical values of the main compounds in marijuana - cannabinoids.
Personally I think if you want to invest in cannabis stocks but can't get access to OTC markets, you're hosed. I don't think cannabis based stocks will make it to the regular markets until more states legalizes or the fed caves on their archaic stance of flawed science and fear, which will be a long long while. Right now there is just too much risk that the fed will come in and take everything. I think that is the biggest problem with marijuana stocks.