From Wikipedia

GM Food, Panacea or Poison

The genetically modified foods controversy is a dispute over the relative advantages and disadvantages of genetically modified (GM) food crops and other uses of genetically-modified organisms in food production. The dispute involves biotechnology companies, governmental regulators, non-governmental organizations and scientists. The dispute is most intense in Japan and Europe, where public concern about GM food is higher than in other parts of the world such as the United States, where GM crops are more widely grown and the introduction of these products has been less controversial.

Safety is a major issue in this controversy. Adverse health effects need to be screened for, because health effects are dependent upon the modifications made. The need for screening and testing increases as more changes are made, and "second-generation" GMs will require more testing. To date no adverse health effects caused by products approved for sale have been documented, although two products failed initial safety testing and were discontinued, due to allergic reactions. Most feeding trials have observed no toxic effects and saw that GM foods were equivalent in nutrition to unmodified foods, although a few reports attribute physiological changes to GM food. However, some scientists  and advocacy groups such as Greenpeace  and World Wildlife Fund consider that the available data do not prove that GM food does not pose risks to health, and call for additional and more rigorous testing before marketing genetically engineered food.

Another area of controversy is what effect pest and herbicide-resistant crops have on ecosystems, by for example reducing the numbers of pest insects in farmland and impacting biodiversity, or by decreasing the use of insecticides. Attempts have been made to measure these effects by farm-scale trials of GM crops, although the interpretation of the results of these trials has been controversial. The risk and effects of horizontal gene transfer have also been cited as concerns, with the possibility that genes might spread from modified crops to wild relatives.