Some of the concerns are related to food allergens.  People are scared that genetically altered foods will cause allergic reactions, as peanuts do to some people.  Steve Taylor, head of the Department of Food Science and Technology at the University of Nebraska in Lincoln, says the chances of creating a new allergenic food is actually quite small because of what is known about allergens currently. 

Another concern with genetically modified crops is related to the environment.  Each variety that is released has to go through federal review to ensure it is safe for the environment.  Even today we have the ability to produce a variety of corn that is resistant to Roundup, but because of the fear of the modification being spread to Johnson Grass, a major weed in corn, it is not being produced.  

So far over 40 crop varieties have been cleared through the federal review process with enhanced agronomic and or nutritional characteristics or one of more features of pest protection and tolerance to herbicides.  These crops are safe as well as cheaper and easier to produce.  Most of the time the nutritional changes to the crops causes them to be better, as well as safer, for the consumer than non genetically altered foods.

Scientists that help produce genetically altered crops are concerned about the environment as well as the people who consume the crops they produce.  The government also has several regulations on the distribution of genetically altered crops to help ensure the safety of people here and around the world.