What are the concerns about it?

• Ordinary familiar foods may become allergenic. As an example, allergic reactions to nuts can be serious and even fatal. A soybean engineered to contain genes from a brazil nut was found to produce allergic reactions in persons with nut allergies.
• Familiar foods may become toxic. The Bt toxin as it appears in bacteria is considered relatively safe for humans because in the bacteria it exists as a protoxin which becomes dangerous for insects only after it is activated in the insect's digestive system. However, some genetically engineered Bt crops produce toxin in the activated form. When the toxin is incorporated into common foods, we are exposed each time we eat those foods.
• GE crops can indirectly promote antibiotic resistance, making it difficult to treat human disease. In the technique of transplanting genes there is need for a marker to identify which cells have taken up the foreign gene. One way is to attach a gene for antibiotic resistance. Following the attempt to insert the new genes, antibiotics can be applied to determine which cells survive, and are therefore carriers of the implanted DNA. Once these resistant genes are into the food chain, more disease causing bacteria may become antibiotic resistant, increasing the problems of public health.