As they are genetically engineered and a desired gene is expressed, the crop yield may also contain the modified gene which may result in many health problems to the livings.
Bt cotton was the very first to get popularized around the world as genetically modified crop. As it was the period for the farmers where they were worried with low yield, low price, water scarcity, weed management, pest control and many more, the Bt crop reached every one as a very easier way with good advertisement by the seed company. Even though they had invested huge amount in the seeds their yield was low but had a little control against pests.