The irony is that while marijuana has definitely been "genetically modified" to contain higher levels of THC, that change didn't involve the tools of modern biotechnology.
Instead, the changes were achieved using rather clumsy methods from the past.
New plant varieties have often been based on chance mutations in their DNA. For instance the sweet corn varieties we enjoy today include a mutation that allows them to retain their sugar content after picking. In the 1950s and 1960s many plant breeders employed "mutation breeding" to increase the chances of finding DNA changes that would result in new traits.
They exposed the plants or seeds to doses of chemicals or radiation that would cause random mutations, and then looked for the rare cases where there was some desirable effect. They would also use the toxic chemical colchicine to induce the plants to double their number of chromosomes, something that sometimes leads to more vigorous growth. These same methods were quite successfully employed by the extra-legal marijuana industry over the last few decades.
Both mutation breeding and chromosome doubling can lead to genetic changes which are undesirable but which remain undetected. If you go through the list of "what ifs" that are typically raised by the opponents of genetically engineered crops (new allergens, changes in regulatory pathways or other patterns of gene expression...), these old methods are far more likely to create such problems.
Unlike a modern biotech crop where the exact nature of the genetic change is known, we really have no idea what all has changed in crops improved using these "old school" methods. Kevin Folta posted an excellent comparison of transgenics and mutation breeding.
However, the crops developed using these clumsy tools have never been regulated or safety tested like biotech crops. They qualify for use in organic farming. Such crops would not have to be labeled under the various bills and initiatives that have been proposed.
Now the truth is that foods developed using these old methods have a decent track record of safety. But applying the same "you never know about long-term effects" logic used against biotech crops, we really can't be sure they are ok, even after decades. I guess the advantage is that you are less likely to worry about such questions after partaking in some mutant, highly-ploidy weed.
Cannabis raid image from the West Midlands Police
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