From The Food and Agriculture Organization of the United Nations:
The main arguments that have been put forward against the use of GMOs in agriculture include:
Potential negative effects on the environment
Genes can end up in unexpected places: Through "gene escape" they can pass on to other members of the same species and perhaps other species. Genes introduced in GMOs are no exception, and interactions might occur at gene, cell, plant and ecosystem level. Problems could result if, for example, herbicide-resistance genes got into weeds. So far, research on this is inconclusive, with scientists divided - often bitterly. But there is scientific consensus that once widely released, recalling transgenes or foreign DNA sequences, whose safety is still subject to scientific debate, will not be feasible.
Genes can mutate with harmful effect: It is not yet known whether artificial insertion of genes could destabilize an organism, encouraging mutations, or whether the inserted gene itself will keep stable in the plant over generations. There is no conclusive data on this issue.
"Sleeper" genes could be accidentally switched on and active genes could become "silent": Organisms contain genes that are activated under certain conditions -- for example, under attack from pathogens or severe weather. When a new gene is inserted, a "promoter" gene is also inserted to switch it on. This could activate a "sleeper" gene in inappropriate circumstances. This is especially relevant in long-lived organisms - such as trees. Sometimes the expression of genes is even "silenced" as a result of unknown interactions with the inserted gene.
Interaction with wild and native populations: GMOs could compete or breed with wild species. Farmed fish, in particular, may do this. GM crops could pose a threat to crop biodiversity, especially if grown in areas that are centres of origin of that crop. In addition, GM crops could compete with and substitute traditional farmers' varieties and wild relatives that have been bred, or evolved, to cope with local stresses. For example, local varieties in Latin America permitted the recovery from the catastrophic potato blight in Ireland in the 1840s. Today such plants often help improve climate tolerance and disease resistance. If genetically modified crop varieties substitute them, they could be lost, but the same applies to improved varieties developed by conventional breeding methods.
Impact on birds, insects and soil biota: Potential risks to non-target species, such as birds, pollinators and micro-organisms, is another important issue. Nobody quite knows the impact of horizontal flow of GM pollen to bees' gut or of novel gene sequences in plants to fungi and soil and rumen bacteria. Besides, it is feared that widespread use of GM crops could lead to the development of resistance in insect populations exposed to the GM crops. Planting "refuge" areas with insect-susceptible varieties is advised to reduce the risk of insect populations evolving resistance due to the widespread growing of GMO Bt-crops.
Potential negative effects on human health
Transfer of allergenic genes: These could be accidentally transferred to other species, causing dangerous reactions in people with allergies. For example, an allergenic Brazil-nut gene was transferred into a transgenic soybean variety. Its presence was discovered during the testing phase, however, and the soybean was not released.
Mixing of GM products in the food chain: Unauthorized GM products have appeared in the food chain. For example, the GM maize variety Starlink, intended only for animal feed, was accidentally used in products for human consumption. Although there was no evidence that Starlink maize was dangerous to humans, strict processing controls may be required to avoid similar cases in the future.
Transfer of antibiotic resistance: Genes that confer antibiotic resistance are inserted into GMOs as "markers" to indicate that the process of gene transfer has succeeded. Concerns have been expressed about the possibility that these "marker genes" could confer resistance to antibiotics. This approach is now being replaced with the use of marker genes that avoid medical or environmental hazards.
Potential socio-economic effects
Loss of farmers' access to plant material: Biotechnology research is carried out predominantly by the private sector and there are concerns about market dominance in the agricultural sector by a few powerful companies. This could have a negative impact on small-scale farmers all over the world. Farmers fear that they might even have to pay for crop varieties bred from genetic material that originally came from their own fields when they buy seeds from companies holding patents on specific genetic modification "events". Some argue that the World Trade Organization's agreement on Trade-Related Intellectual Property Rights (TRIPS) encourages this, but there are options to protect farmers' traditional practices within that agreement. Also, the new International Treaty on Plant Genetic Resources for Food and Agriculture recognizes the contributions of farmers to the conservation and use of plant genetic resources over time and for future generations. It provides for an international framework to regulate access to plant genetic resources and establishes a mechanism to share the benefits derived from their use.
Intellectual property rights could slow research: The proprietary nature of biotechnology products and processes may prevent their access for public-sector research. This might have a stronger negative impact in developing countries where no private research initiatives are in place. In addition, most developing countries still do not provide patent protection to biotechnological products and technologies. Because patents have a national scope, the entry of products developed through proprietary biotechnologies could be prevented in those external markets where patent protection exists.
Impact of "terminator" technologies: Although these are still under development and have not yet been commercialized, they would, if applied, prevent a crop from being grown the following year from its own seed. This means that farmers could not save seeds for planting the next season. Some believe that this technology, also known as the Technology Protection System, could have the advantage of preventing out-crossing of GM seeds.
Go to The Food and Agriculture Organization of the United Nations' website.