I did a little research on this, and it seems like there are health risks involved with eating Genetically modified foods. Since these foods are not "natural," in a sense, then they may have allergenic properties that their naturally-occurring counterparts do not. In other words, you might not be allergic to regular soybeans, but you are allergic to genetically modified soybeans. It also seems that diseases have been caused by people eating genetically modified foods, and some people have experienced gene transfers of these foods into their own systems. All in all, even though I only spent a few minutes looking up this topic, I feel like there aren't really very many positives to this issue yet, but there are plenty of negatives.
Since you are only interested in the cons of GM foods, here is a link that should be interesting:
There is a link on this page to a PowerPoint called “The Documented Health Risks of Genetically Engineered Foods.” There is also a link to a script for this PowerPoint.
The website has a summary of the risks included in the PowerPoint that is better than I can give you, so I am quoting from the website:
"The GMO health risks section highlights many of the adverse findings revealed through laboratory experiments and reported by farmers, doctors, and investigators. It also introduces theoretical risks based on the current state of the science.
All risks are broken down into five categories:
1. The process of creating a GM crop creates unpredicted changes in DNA and plant composition
2. The protein produced by the inserted gene may be harmful
3. The protein produced by the inserted gene may be different than intended
4. There are more herbicide residues in herbicide tolerant crops
5. Genes may transfer to gut bacteria or into our DNA"
I hope you find this helpful.
One issue is GM techniques create irreversible changes to natural species and disturb the enviroment. Eg. Corporations have developed GM rice (and other crops and vegetables) which are resistant to many diseases. These plants have been planted out. Of course it is natural for plants to reproduce and spread, so this super-rice is now out there, competeing in nature for its personal advantage. If it can colonise as much area as possible, it will.
There are far too many examples of catastrophes due to man introducing new species into environments.
Nobody knows the effect of introducing plant super-strains. It is a highly irresponsible activity and is driven by money. American companies like Monsanto make billions from their "products" (modified plants) and they are too eager to make a profit. There have been accusations of big corporations blocking research.
It is a trillion dollar industry, in the future it will be one of the world's biggest industries, and the companies trying to control it are not good companies. This is a serious problem.