Maggie Koerth-Baker warns:
Nothing has been proven yet — most of the data comes from disembodied cell cultures and animal testing, which doesn't necessarily give you an accurate picture of what's happening in humans — but several studies over the last few years have drawn connections between lubricant use and increased rates of STD transmission. (It also looks like some lubricants might kill off natural vaginal flora — the good bacteria that live "up there" and make the difference between a healthy vagina and, say, a raging yeast infection.) Some of these studies have provided evidence suggesting that the ingredients in lubricants damage the cells lining the vagina and rectum — which would explain why those lubricants might facilitate STD transmission.
Lauren Wolf has more:
Right now, the Food & Drug Administration doesn’t typically require testing of personal lubricants in humans. The agency classifies them as medical devices, so the sex aids have to be tested on animals such as rabbits and guinea pigs. Rectal use of lubricants is viewed by the agency as an "off-label" application—use at your own risk.
Questions about lubricant safety arose nearly a decade ago when microbicide developers were testing whether the detergent nonoxynol-9 could block HIV transmission. Manufacturers had been incorporating the compound into spermicidal lubricants for years because of its ability to punch holes in the cell membranes of sperm. In 2002, however, a Phase II/III clinical trial of a nonoxynol-9 vaginal gel failed to protect women from HIV infection. Not only that, but the detergent actually increased the risk of HIV infection in the sex workers tested—women living in countries such as South Africa and Thailand who used the product three or four times per day. Lab work eventually revealed the reason for the paradoxical increase: Nonoxynol-9 is so good at punching holes in cell membranes that it not only bores into sperm but also into the cells lining the vagina and rectum.