Some of the disadvantages of genetic engineering are health risks, environmental impacts, impacts on agriculture and industry, and ethical issues that may arise based on the engineering. The disadvantages differ based on the kind of genetic engineering that is taking place. Agricultural genetic engineering is more likely to impact health, and animal engineering is more likely to impact the environment.
The main disadvantage of genetic engineering is the risk it poses to health. Some detractors claim that genetically engineered crops are inherently unsafe, but this can't really be verified because much of the data on health impacts is controlled by the genetic engineering industry. What scientists do know is that health hazards are present. In 1996 in Brazil, and in 1990 in America, there were documented cases of engineered food causing allergic reactions, the passing of diseases and even death.
Another disadvantage to genetic engineering is the problems it causes the environment. Engineered crops may become toxic to wildlife that need the food to survive, and engineering has also created types of weeds that are resistant to pesticides. These new "super weeds" can choke crops and destroy agriculture and habitats.
Certain ethical issues are brought up by genetic engineering. Scholars, politicians and scientists have debated the use of engineering to improve humans, animals and plants. Organizations like Greenpeace vehemently oppose the use of genetic engineering, according to Greenpeace's website, and others think that genetic engineering is the key to making safer and more abundant food, as well as healthier people and animals.