People who used lubrication during anal sex were three times more likely to have rectal sexually transmitted infections (STIs) than those who didn’t use lube, according to a study presented at the International Microbicides Conference in Pittsburgh.

Why the increased risk? A separate study, also presented at the conference, may offer answers. It evaluated the safety of lubricants used for anal sex and found that many of them may make the body more vulnerable to STIs, including HIV. The tests found that Astroglide and KY Jelly were the most damaging lubes, while Wet Platinum and PRÉ were the safest. Results of both studies are summarized in ScienceDaily.

In one analysis regarding anal sex and lubricants, researchers with the University of California at Los Angeles looked at data from 302 men and women in Baltimore and Los Angeles. About half (147) said they used lube during anal sex. Of the 302 people, 25 (8.3 percent) tested positive for rectal gonorrhea or chlamydia.

Researchers found that 11.7 percent who tested positive had used lube, compared with 5.1 percent who did not use lube. In other words, people who used lube were three times more likely to have a rectal STI, a difference that was statistically significant. Data showed that 61 percent of African Americans used lube for anal sex, as did 40.4 percent of Latinos and 23.2 percent of whites. In addition, 56 percent of HIV-positive participants used lube, compared with 43.7 percent of HIV-negative participants. And 76 percent of all lube-using participants said they used water-based lubricant.

The second study evaluated the safety of five water-based lubes—Astroglide, Elbow Grease, ID Glide, KY Jelly and PRÉ—and one silicone-based lube Wet Platinum. Laboratory tests looked at the toxic affect the lubes might have on the epithelium, the layer of cells that act as a barrier inside the rectum.

Researchers evaluated lubes based on viscosity (slipperiness), pH and osmolarity, which is a measure of its dissolved salts and sugars. Products that are hyperosmolar have a higher concentration of salts and sugars than skin cells. As a result, they leech water out of the skin cells, which dries them out and even kills them. Isomolar lubes contain the same amount of salts and sugars as skin cells. (PRÉ was the only isomolar lube in the study.) Researchers also looked at each lube’s affect on different types of cells and bacteria.

Test results found that Wet Platinum and PRÉ were the safest, Astroglide was most toxic to cells and tissues, and KY Jelly has the worst effect on good bacteria (it wiped out an entire colony).

Researcher noted that studies in humans are needed to determine the affects of lube on HIV and STI transmission. The study was conducted at the University of Pittsburgh and Magee-Women’s Research Institute in collaboration with the International Rectal Microbicides Advocates and the Microbicide Trials Network.

The ScienceDaily article also includes results of a survey regarding pre-exposure prophylaxis (PrEP) awareness among men who have sex with men.