Why are scientists concerned about the use of the bt toxin which is used to kill insects in genetic engineering?

Many scientists aren't concerned about the use of Bt toxins. Consumers, some scientists, some medical care professionals, and others are usually the ones who are concerned about Bt crops used to feed animals and humans. To understand why, you need to understand that Bt crops are genetically modified to produce the toxin that kills insects in every cell of the plant itself. Essentially, the plant itself becomes an insecticide. Though it is claimed that science shows those plants have no effect on animals or humans, many believe that real life anecdotal evidence shows eating Bt foods may have unintended harmful effects on the intestinal flora and digestive system. Those who believe that also believe that research needs to be conducted to find out if Bt is causing harm to health, and many try to avoid eating GMO foods.