Your friend (let’s call her Becky) is the improbably hot manager of a fast food joint. Despite being cool and attractive and having her shit together, she’s attracted to a schlubby, terminally indecisive dude named Dante. Dante and his best friend Randal have strong opinions on the grossness of never going ass to mouth—that is, a dick never going straight from the butt to the mouth of the same person. But Becky wonders if, in the heat of the moment, it’s actually just fine. Is she right?
If you’re going down on a guy who’s been inside your butt, and said butt isn’t very clean, then it's obvious there could be traces of fecal matter on that penis—meaning you could be transferring flecks of your own poop into your mouth. But is that as gross as it sounds? Maybe not. Yes, poop is loaded with bacteria—which includes shigella, a group of bacteria that can cause diarrhea and dysentery. There’s a risk of transferring the organisms from your rectum to your mouth, and thus your gastrointestinal system. But as long as you're ingesting your own feces—and not your partner's—the risk of getting sick is actually, surprisingly, quite small.
Timothy Sly, an epidemiologist and expert in public health risks based at Canada’s Ryerson University, explains that when it comes to “the transfer in the same body of NORMAL enteric (intestinal) pathogens—those that cause gastroenteritis—we wouldn’t expect to see an increase in risk. If you have enterohemorrhagic E. coli (that's E. coli that can cause diarrhea or coliti) or Campylobacter (a type of bacteria that causes food poisoning) in your gut, adding some more at the top end doesn’t noticeably change things in the alimentary canal.” In which case, the only obstacle may be your own gag reflex. Right? Well, kind of.
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The Worst That Will Happen
It’s not probable, but bacteria and other organisms in the rectum could be transferred to the mouth. “There may be some risk of infections like E. coli or intestinal parasites," says June Gupta, associate director of medical standards at Planned Parenthood Federation of America. (Although, as Sly just explained, the risk is usually minimal.) "While anal sex followed by oral sex performed by the receptive partner is unlikely to be harmful, it’s best to avoid putting anything—your fingers, penis, sex toys, and so on—that’s been in an anus directly into the mouth," she says.
The risks are higher for certain individuals and situations, such as multiple partners when ass-to-mouth play isn’t protected via condoms or another barrier method. And, Sly says, while it may seem obvious, people with immune-suppressed conditions, or who are post-transplant, have HIV, or are receiving cancer treatments could respond with a range of infections due to the new species they had not encountered before outside of the heavily-populated but well-balanced bacterial flora in the gut. In other words, the organisms that normally live in the gut might not take kindly to being relocated to the mouth, via the bum. This could also apply to people taking antibiotics, which have wiped out the friendly bacteria that normally live in the gut, and allowing harmful bacteria introduced via ass-to-mouth to take charge without competition.
Some parasitic worms can also cause autoinfection. And when it comes to infections more commonly associated with sex, people can also re-infect themselves with herpes, though it's highly uncommon. This would involve transmitting the virus from an open sore on one part of the body to another. Erin Basler, a sexuality educator for the Center for Sexual Pleasure and Health, notes that “rectal tears from anal penetration increase the risk of blood-borne STIs being transferred through microabrasions caused by brushing one's teeth.” Using lube would help with this.
Finally, there's some limited research suggesting that gonorrhea can be transmitted between the rectum and pharynx, as researchers at Melbourne University in Australia proposed in 2017. Pharyngeal gonorrhea tends to be asymptomatic, according to the CDC, although it might cause a sore throat. But that doesn't mean it's harmless: Gonorrhea in the throat can still be passed on to sexual partners. And in general, as the CDC also points out, “It’s possible to have an STD in more than one area at the same time.” In short: Infection or re-infection is possible if one or more of the people getting it on are already carrying something, and more likely if anyone’s in a weakened state.
What to Tell Your Friend
First of all, it’s weird for you to be talking to a fictional character. Your friend Becky is clearly the love interest played by Rosario Dawson in the underappreciated Clerks II. Possibly even more pressing than its central theme—how to nurture middle-aged friendship—is the question of proper hygiene when moving from anal to oral sex.
In any case, you can tell Becky she can mostly chill. If she and her partner are free of infections, ass to mouth isn’t likely to cause any health issues. But for peace of mind, it’s easy enough to avoid the situation. The guy could clean off his dick or stick a condom on before it goes into her mouth. Specifically, Basler suggests, “an additional safety measure for folks who want the feeling of ass to mouth without the bacterial risk is to use an internal condom in the receptive partner. Internal condoms—with the inside ring removed—can be used anally, warm up to the body's temperature, and are fairly unobtrusive to both parties.”
Or Becky and her beau could simply go mouth to ass. It may not seem that sexy to schedule the order of sex acts. Sly acknowledges that in the heat of the moment, careful intentions can go out the window; pathogenic spread from ass to mouth is “one of those risks that could be minimized if planned and limited appropriately, but whether it all transpires as intended is up to the participants in their moment of excitement.”
But as Ruth Neustifter, an assistant professor and sex educator at the University of Guelph in Ontario once put it, “We want to keep the booty bacteria just in the booty, right where it belongs.”
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