A concern about what affect genetically modified material could have on human health. For example, transgenic crops have been suggested to cause allergies in some people, although it is uncertain whether transgenic crops are the source of this reaction. Furthermore the antibiotic resistance genes placed in these crops has been suggested to cause resistance to antibiotics leading to super bugs that cannot be killed with antibiotic treatments. The idea of a population being uncomfortable with ingesting DNA that originated from another source, such as a virus or bacteria, must also be considered when thinking about producing transgenic crops. However, to date, there is no evidence of the DNA from transgenic crops being any different from the DNA ingested from conventional crops.
A concern about whether transgenic crops cause damage to the natural environment. One example includes pollen from transgenic corn, which has been suggested to kill the Monarch butterfly larvae. It has been shown that hybrid corn expresses a bacterial toxin in its pollen, which is then dispersed over 60 meters by wind. In this range, the corn pollen is deposited on other plants near cornfields where it can be ingested by non-target organisms including the monarch butterfly. These butterflies have been found to eat less, have a slower growth rate and higher death rate. A second example is the hybridization of crops with nearby weeds. This could cause these weeds to attain resistance to herbicides or other things that we have been trying to avoid for many years. Genes that provide resistance to viral disease or other traits allowing them to survive in their environment could end up benefiting weed populations around a crop field. This trait could make that population more difficult to control. To date, there has been little evidence to support this theory.