I think a few people are confused. Genetic modification is not the same thing as selective breeding, and I'm pretty sure there aren't any cannabis growers doing GM
Selective breeding is where you take a male and female plant that have the traits you want, put them together and let nature take its course. Yes, there is human intervention but you have no real control over the output, that is determined 100% by the genetics of the plants you happened to choose. This is what cannabis growers do, and what farmers have done for 1000s of years.
Genetic modification is where you "cut and paste" the DNA in a chemical/laboratory method, to create what are essentially frankenstein plants that could never possibly have evolved with selective breeding. This would be like putting seaweed DNA in cannabis because you want it to grow in salt water.
The main problem with GM is that its a purely commercial venture. The corporations are pushing it under the guise of 'easing global poverty' but its a complete and utter lie.
Heres the real reason behind GM, read it and think about the implications:
Genetically modified plants, unlike 'normal' plants, can be patented.
Having a patent on a plant might not seem like a big deal until you realise that GM is slowly taking over staple foods like wheat, rice, soy, corn etc. So wheat isn't just "wheat" anymore, its becoming "Wheat (TM)" and you have to pay someone if you want to grow it.
You might think "well the farmers can grow what they choose" but thats where things get uglier. Other genetic modifications are designed specifically to force the farmers to use GM seeds.
Companies like Monsato sell not just seeds, but pesticides. Their seeds are modified so they can survive their own brand pesticides (of course). But the pesticides are so toxic that nothing else can grow on the field afterwards. So the farmer is 'locked in' and the only way they can survive is to keep buying more Monsato seeds and pesticides every year.
They have also developed what are known as 'terminator' seeds. Its a genetic modification designed to prevent farmers from seed saving (ie. taking profits away). The seeds will always die after a set amount of time, so the farmer is forced to buy new seeds every season. This creates a situation where a large corporation has total control over the food supply because they are at the top of the chain with nobody to answer to except shareholders.
And this is only the tip of the iceberg.
Another big problem with GM/food industries, they are becoming a monoculture ie. everyone is growing the exact same breed of corn, potatos etc. all over the world, and all the sub-species are being lost. This leaves the food supply extremely vulnerable to disease, and is exactly what caused the great famine in Ireland. They were only growing 1 kind of potato, when a certain plant disease sprung up which destroyed that species of potato. After the famine was over, they decided they needed to grow many different kinds of potatos to spread the risk, and keep genetic diversity.
I don't know if there are health risks with GM food. But I don't need any more reasons than those I listed above.