There are no human toxicology problems with Bt toxin proteins. Even though the toxins are integrated into the plasma membrane in insects, they are not toxic to humans and are simply digested as part of the diet. The most important concern is that insects will develop resistance to these Bt toxins. It has been argued by some that this is unlikely to occur, because Bt spores have been used over the past 20 years and there have been essentially no reports of insect resistance developing against these controls. However, there are a number of important differences between using Bt spores as insecticides and expressing a Bt toxin in transgenic plants.
Bt spores are quite short lived and disappear quickly after application.
This is compared to the continual exposure to Bt toxin in transgenic plants. Prolonged exposure to a selective agent, such as a Bt toxin, is likely to accelerate the development of resistance. There is also a major qualitative difference between spores and transgenic plants. Transgenic plants express only a single Bt toxin protein. In contrast, Bt spores express more than one toxin and this mixture has a synergistic effect, where the combined action of multiple toxins is more than the effect of the toxins added together.