Some GMOs — for example, a variety called Bt corn — actually do contain a pest-killing toxin. That may sound pretty dangerous, until you stop to consider that unlike, say, chemical insecticides and herbicides, the toxin in the Bt corn is engineered to work on specific ravenous insects, and doesn't affect other species, including humans.
A recent Scientific American article that summarized the research on Bt corn noted, "The evidence overwhelmingly demonstrates that Bt toxins are some of the safest and most selective insecticides ever used. Claims that Bt crops poison people are simply not true." Indeed, the article argues that when properly managed, fields of Bt corn actually help protect the environment, because they reduce the use of broad-spectrum insecticides that kill off insects, including beneficial ones, indiscriminately [source: Jabr].
Even so, there are downsides. One problem is that pests can become resistant to genetically engineered toxins, just as they can develop immunity to pesticides. For example, farmers in the Midwest have discovered that one variety of Bt corn no longer repels the root-chomping beetle larvae that it's meant to stop. If GMOs fail to be pest resistant, that might lead farmers to start dousing their fields with problematic chemicals again [source: Jabr].