The issues surrounding G.M.O.s — genetically modified organisms — have never been simple. They became more complicated last week when the International Agency for Research on Cancer declared that glyphosate, the active ingredient in the widely used herbicide Roundup, probably causes cancer in humans. Two insecticides, malathion and diazinon, were also classified as “probable” carcinogens by the agency, a respected arm of the World Health Organization.

Roundup, made by Monsanto for both home and commercial use, is crucial in the production of genetically engineered corn and soybean crops, so it was notable that the verdict on its dangers came nearly simultaneously with an announcement by the Food and Drug Administration that new breeds of genetically engineered potato and apple are safe to eat. Which they probably are, as are the genetically engineered papayas we’ve been eating for some time. In fact, to date there’s little credible evidence that any food grown with genetic engineering techniques is dangerous to human health — unless, like much corn and soybeans, it’s turned into junk food. But, really, let’s be fair.

Fair, too, is a guess that few people are surprised that an herbicide in widespread use is probably toxic at high doses or with prolonged exposure, circumstances that may be common among farmers and farmworkers. Nor is it surprising that it took so long — Roundup has been used since the 1970s — to discover its likely carcinogenic properties. There is a sad history of us acting as guinea pigs for the novel chemicals that industry develops. For this we have all too often paid with our damaged health.

Rarely is that damage instantaneous, but it’s safe to say that novel biotechnologies broadly deployed may well have unexpected consequences. Yet unlike Europeans, Canadians, Australians and others, we don’t subscribe to the precautionary principle, which maintains that it’s better to prevent damage than repair it.