What if you knew that detractors fear that GM foods might pose health risks for certain people?
Some people, including children, are highly allergic to peanuts and other foods. Some critics of GM foods feel the possibility exists that those genetically modifying food crops may unintentionally introduce a new allergen. Given that genes can be introduced from unrelated species -- for example, a fish gene can be put into a plant -- some critics argue that the possibilities of allergies might be greater than with traditionally bred crops.
Another potential hazard to human health is the possibility that bacteria in our guts could pick up antibiotic-resistance genes found in many GM foodstuffs. (Food geneticists often add such genes to GM plants as 'markers' to tell them which plants have taken up exotic genes.) If this transfer happens, in principle it could exacerbate the already worrisome spread of disease-causing bacteria that have proven able to withstand our antibiotics.
"Today the vast majority of foods in supermarkets contain genetically modified substances whose effects on our health are unknown. As a medical doctor, I can assure you that no one in the medical profession would attempt to perform experiments on human subjects without their consent. Such conduct is illegal and unethical. Yet manufacturers of genetically altered foods are exposing us to one of the largest uncontrolled experiments in modern history."
--Dr. Martha R. Herbert, pediatric neurologist 
"With genetic engineering, familiar foods could become metabolically dangerous or even toxic. Even if the transgene itself is not dangerous or toxic, it could upset complex biochemical networks and create new bioactive compounds or change the concentrations of those normally present. In addition, the properties in proteins may change in a new chemical environment because they may fold in new ways. Further, the potential toxic or carcinogenic effects could have
substantial latency periods."
--from The Need for Greater Regulation and Control of Genetic Engineering: A Statement by Scientists Concerned About Trends in the New Biotechnology 
"Lots and lots of people -- virtually the entire population -- could be exposed to genetically engineered foods, and yet we have only a handful of studies in the peer-reviewed literature addressing their safety. The question is, do we assume the technology is safe based on an argument that it's just a minor extension of traditional breeding, or do we prove it? The scientist in me wants to prove it's safe."
--Dr. Margaret Mellon, director of the agricultural and biotechnology program, Union of Concerned Scientists 
1: Herbert, Martha. "Genetically Altered Foods: We Are Being Exposed To One Of The Largest Uncontrolled Experiments In History," Chicago Tribune, 9/3/00.
2: Published by Third World Network, Penang, Malaysia, 1995, p.18. Quoted in "Why the Genetic Engineering of Our Food Offends Principles of Most Religions," www.bio-integrity.org/RelReject.html.
3: Quoted in "Does the World Need GM Foods?," Scientific American, April 2001.