A reader asked the following:

“Are there any health effects of swallowing semen? Is it better to spit it out instead of swallowing?"

Thanks for these very interesting questions. Let me start by saying that if you perform oral sex on a man who has an STI (e.g., chlamydia, gonorrhea, syphilis), you run the risk of contracting that infection. It doesn't matter whether his semen is actually swallowed—the risk comes from simply having his ejaculate in your mouth. So, if you know your partner has an infection or you aren’t sure of his status, it would be advisable to use a condom to prevent contact with his semen, thereby lowering your infection risk (read more about the potential STI risks of oral sex in this article).

Assuming he is uninfected, swallowing semen is unlikely to pose any health risks. For one thing, it's not going to make you gain weight. Some people seem to be under the impression that semen is loaded with calories, but it's not. Most estimates I’ve seen put the number of calories in a single “serving” of semen between 1 and 5.

Women don’t need to worry about pregnancy from swallowing semen either because sperm deposited in the digestive tract can't get to the reproductive tract. That is, unless you were to get stabbed in the stomach shortly after performing oral sex--and the odds of that happening are pretty low. I read a case report a few years back that described an African girl who was performing oral sex on her boyfriend when her ex walked in on them [1]. Apparently a knife fight ensued and she was stabbed in the stomach. Soon after, she discovered that she was pregnant. After visiting a doctor, the mode of conception was attributed to the stabbing.

Now, you're probably thinking that vaginal intercourse is surely a much more likely explanation; however, it turns out that it wouldn't have been possible in this case because this girl did not have a vagina. She had a genetic condition (Mullerian agenesis) that prevented vaginal development, leaving just a small dimple in the spot where the vagina typically appears. In light of this, the post-oral sex stomach wound seemed to provide the most plausible explanation for how sperm reached her reproductive tract.

If this seems hard to believe, you're not alone--I have my doubts about it too. All I can tell you is that it was published as a case report in a medical journal.

Beyond the potential for STI transmission and the extremely remote possibility of a post-oral sex stomach stabbing, the only other risk I’m aware of when it comes to swallowing ejaculate would be if the receptive partner has a semen allergy. Indeed, research has found that some women are allergic to their male partner's semen. To learn more about semen allergies and how doctors can treat them, check out this article.

Related to this, I heard a conference talk a few years ago on “semen ingested induced diarrhea,” in which the authors discussed how swallowing semen produces serious gastrointestinal distress in some individuals (presumably as a result of a fructose intolerance, given that fructose is one of the primary components of semen); however, this is thought to be very rare.

We've focused on risks so far, but are there any potential health benefits of swallowing semen? Believe it or not, some research has found a correlation between swallowing semen and a reduced risk of preeclampsia [2]. Preeclampsia is a condition in which a women develops very high blood pressure, usually in the later stages of pregnancy. If left untreated, it is potentially fatal to both mother and fetus.

Why would women who swallowed more semen through oral sex have a lower risk of developing preeclampsia? The favored hypothesis is that mothers are exposed to foreign antigens during pregnancy because half of the genetic material of her child is provided by the father; however, to the extent that a woman is exposed to more of these antigens prior to pregnancy, she may develop a tolerance to them, thereby reducing the risk of an immune reaction occurring during pregnancy.

That said, research on the potential benefits of swallowing semen isn’t conclusive enough to necessarily recommend it to women who are thinking of becoming pregnant. What is clear, though, is that as long as a man is uninfected and his partner is not allergic to semen, swallowing semen is unlikely to have negative health effects.

For past Sex Question Friday posts, see here.

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[1] Douwe, A.V. (1988). Oral conception. Impregnation via the proximal gastrointestinal tract in a patient with an aplastic distal vagina. Case report. British Journal of Obstetrics and Gynaecology, 95, 933-934.

[2] Koelman, C. A., Coumans, A. B., Nijman, H. W., et al. (2000). Correlation between oral sex and low incidence of preeclampsia: A role for soluble HLA in seminal fluid? Journal of Reproductive Immunology, 46, 155–66.

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