Are GMO foods safe?
       Safety concerns regarding GMO foods fall mainly into two categories, safety for human health and safety for the environment. In both cases, concerns about safety stem from the fact that biotechnology can potentially incorporate a broader array of phenotypic traits into plants than was previously possible, and with this comes an increased possibility of unintended genetic effects (NAS 2002, 2004).

        Regarding human health concerns, the National Academy of Sciences concludes that to date there have been no adverse health effects attributed to genetic engineering (NAS 2004). Their analysis of this question finds that unexpected and unintentional compositional changes arise in all forms of genetic modification in plants and animals with both conventional and biotechnological breeding methods. However, they judged the likelihood of unintended genetic effects to be generally higher among techniques involving biotech transfers of rDNA and mutation breeding techniques (NAS 2004). Whether such changes result in unintended health effects, however, is dependent on the specific substances and consequences involved (NAS 2004).

References:

National Academy of Sciences, 2002. "Environmental Effects of Transgenic Plands: the Scope and Adequacy of Regulation." (http://www.nap.edu).

National Academy of Sciences, 2004. "Safety of Genetically Engineered Foods: Approaches to Assessing Unintended Health Effects." (http://books.nap.edu/catalog/10977.html)
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