According to WebMD, possible disadvantages of genetically modified crops include environmental hazards such as the creation of herbicide-resistant weeds, altering the nutritional content of food, resistance of crops to antibiotics, the presence of toxins and allergens and the risk of contamination between modified and unmodified crops.
According to a 2013 article in Scientific American, the threat of herbicide-resistant "super weeds" is real, as cotton farmers in the southeastern United States have reported such infestations, although it points out that this is due to the use of a single weedkiller rather than the presence of GM crops. The WebMD article points out an instance of modified corn deemed fit for animal but not human consumption being accused of causing allergies when it inadvertently was used in fast foods. This was an isolated case, however, and the EPA was not able to prove that the genetically modified corn was responsible for the allergies.
Governments widely differ in their response to genetically modified crops. The United States does not require labeling of modified foods, whereas the European Union is much more strict in its efforts to label and limit such food products. According to an article in Forbes Magazine, proponents point out that genetically modified crops are vital to quell world hunger, that they contribute to a reduction of greenhouse gas emissions caused by agricultural production and that genetically modified crops are no more dangerous than those grown by conventional breeding methods. In addition, the article points out the difficulty of adequate human testing, as opposed to laboratory and animal testing, as it would involve monitoring subjects over much of the length of a human lifetime.