This is a good, thoughtful question. However, it is impossible for science to absolutely prove a negative. I can’t prove that you won’t be attacked by an emperor penguin next year; I can report only that, from our knowledge of penguins, almost certainly this will not occur, but no matter how much research is done, I can’t prove it. The same applies to GM foods. The science says that there should be no harm, and reliable studies to date have not shown any harm, but we cannot prove that there will never be any harm. All we can say is that all the evidence points toward GM crops being no different from non-GM crops of the same species with regard to safety for the traits currently approved and on the market.
GM crops are not expected to present any new food risk, as the changes are simply in molecules (protein and DNA) that have been in our diet for ever, and the small quantity of added material is degraded in our gut in exactly the same way as has always occurred. There has been very extensive safety testing carried out in many countries around the world, and GM crops have been found to be no different from non-GM crops of the same species in terms of food safety or environmental effects.
The European Union has said, “The main conclusion to be drawn from the efforts of more than 130 research projects, covering a period of more than 25 years of research and involving more than 500 independent research groups, is that biotechnology, and in particular GMOs, are not per se more risky than conventional plant breeding technologies” ("A decade of EU-funded GMO research," Directorate-General for Research and Innovation, European Union, 2010).
A multitude of studies by academic scientists have been conducted to assess the food safety of GM crops. Summing them up, the American Association for the Advancement of Science recently said, “The science is quite clear: crop improvement by the modern molecular techniques of biotechnology is safe."
After 16 years of consumption by billions of livestock, pets and humans, there have been no cases of allergy, cancer or death, or indication that GMOs are of any health concern. Claims of effects have been found to be anecdotal and without merit and are rejected by the overwhelming majority of scientists worldwide.
Should any new information or studies be discovered indicating adverse information about a GM product on the market, the law requires that such information be brought to the FDA, which regulates the safety of all foods and food products—including from GM plants.