While the risks of genetic engineering are often exaggerated or misrepresented, GE crops do have the potential to cause a variety of health problems and environmental impacts. For instance, they may spread undesirable traits to weeds and non-GE crops, produce new allergens and toxins, or harm animals that consume them.

At least one major environmental impact of genetic engineering has already reached critical proportions: overuse of herbicide-tolerant GE crops has spurred an increase in herbicide use and an epidemic of herbicide-resistant "superweeds," which will lead to even more herbicide use.

How likely are other harmful GE impacts to occur? This is a difficult question to answer. Each crop-gene combination poses its own set of risks. While risk assessments are conducted as part of GE product approval, the data are generally supplied by the company seeking approval, and GE companies use their patent rights to exercise tight control over research on their products.

In short, there is a lot we don't know about the long-term and epidemiological risks of GEwhich is no reason for panic, but a good reason for caution, particularly in view of alternatives that are more effective and economical.

Excerpt from:

Genetic Engineering in Agriculture | Union of Concerned ...