Chlamydia. Gonorrhea. HIV. Teen pregnancy.

These are common buzzwords in discussions of sex, during which education programs and the media tend to focus on its darker side. Other sources take the opposite tack, touting sex as a miracle panacea, with Top 10 lists and headlines like “Not Just Good, but Good for You” and “Is Sex Necessary?”

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The truth about sex, though, falls somewhere in between. Although many of the benefits reputed to come from sex are drawn from inconclusive studies, there are, in fact, many positive health benefits that have been substantiated by scientific research.

One study, conducted by psychiatrist Gordon Gallup of the State University of New York, Albany, found that females having unprotected sex were significantly less depressed than both sexually inactive females and females who had sex with protection — indicating a connection between exposure to semen and elevated mood in women. Gallup said some of the chemicals found in semen can be found in the female bloodstream after sex — having been absorbed through the vaginal epithelial tissue — within about an hour of intercourse.

His study, published in 2001, also found that depression scores went up as time after the sexual encounter increased, suggesting the possibility of a “semen-related withdrawal effect.” Finally, they found that women who had unprotected sex in a previous relationship became sexually involved with another male more quickly than did women who had protected sex, further supporting the idea.

“It’s very interesting from an evolutionary perspective,” Gallup said. “[The chemical make-up of sperm] may have been designed by evolutionary history to commandeer the female reproductive tract and manipulate it in the male’s best interest.”

But Gallup said the study’s results did not recommend having unprotected sex, because the potential consequences, such as STDs and pregnancy, far outweighed the possible benefits.

He also advised against drawing unfounded conclusions — lack of depression does not necessarily indicate general happiness, he said.

“You can’t say that semen makes women happy, because it’s not true,” he said. “Semen only modulates mood.”

Another study, conducted by Yale fertility doctor Harvey Kliman, explored the effects of orgasm on endometriosis — a condition in which the uterine lining grows in other areas of the female body, he said. This is often caused by menstrual debris failing to exit the body normally and, instead moving backwards, exiting through the Fallopian tubes and floating around the body cavity.

Kliman attributes this phenomenon to the female body being ill-equipped to deal with 400 periods in a lifetime — the current average. 500 years ago, he said, women only experienced about 12 periods in their whole life because of shorter life spans and earlier, more frequent childbirth.

Kliman’s study found that women who had a high frequency of orgasm — either through intercourse or masturbation — had the lowest instances of endometriosis. While orgasms ordinarily produce upwards contractions to suck semen inwards, during menstruation, orgasms instead intensify the outward motion of regular menstrual contractions, thereby pushing debris out instead of pulling it in, he said.

Kliman said he was surprised by the results, which oppose his original hypothesis that orgasm would pull debris backwards.

He also cautioned against extracting from his research the overly simplified conclusion that “sex is good,” as he said many media sources did.

“This research was to understand how to prevent a particular disease,” he said. “It just so happens that orgasm has a medical benefit … sex is good, but it’s not that having sex cures all sorts of diseases.”

Some sources, though, purport lists of health benefits that come from sex.

For example, Logan Levkoff, a sexologist who spoke at Yale on the female orgasm yesterday, said sex increases the production of DHEA, a natural steroid, and releases hormones that reduce stress, improve muscle pain and cramping.

Hugh Taylor, a doctor at the Yale Fertility Center, said the hormonal benefits of sex mentioned by Levkoff had been supported by various studies, but were not definitively proven.

Another study, conducted by Carl Charnetski at Wilkes University in Pennsylvania, found that students who had sex once or twice a week — as opposed to never, once a week, or three or more times a week — had significantly higher levels of immunoglobulin A, IgA, a chemical that indicates immune system health.

But Charnetski said it was not clear that there was a causal relationship between the level of sexual activity and levels of IgA.

In the same vein — though online sources make claims that sex improves cardiovascular health or reduces the risk for prostate cancer — the original studies often say that they are inconclusive.

Kliman said the lack of research on sexual issues was due to people’s aversion to it, which he called “a sick problem.”

“People don’t want to do it,” he said. “The problem is that it’s so controversial and titillating.”