Katherine Sharpe will never know what caused her first serious relationship to end. And that still bugs her. What she does know is that over time, her physical desire for the man she dated in college began to wither. And as she points out, "sex inside a stable relationship is sometimes the glue that holds people together." She also admits that the flame could have gone out on its own. But there's another reason the love affair may have hit the skids, says the 34-year-old, and it doesn't sit quite so well. Its fate could have been sealed by a prescription medication she never really needed in the first place.

"The drug definitely diminished my interest in sex," says Sharpe, the author of Coming of Age on Zoloft. She was given antidepressants after a visit to the campus health center to talk about her anxiety, an ordinary window of distress she now views as minor. "I'll never know what role the drug played," she says, clearly wrestling with the thought. "I don't like having to wonder that. We went out for two years—a long time when you're 18. He was my first lover; we were crazy about each other. I wish I could be certain it failed entirely on its own merits."

I've had the same problem. When I was on Zoloft for two years in my early 30s, sex became like a footrace in snowmobile boots. The drug eased my anxieties during the day but jumpstarted them in bed. Those are well-known side effects, but it turns out that delayed orgasm and losing that itch are only two of the many ways antidepressants can handicap romance. These drugs do their work in the brain, which is why they can influence not just erections but also affection, connection, love, and attachment. That can leave you wondering if your lover's indifference derives from her heart, her head, her med, or a jumble of all three.

We live in the age of the foursome: you, her, and your respective pharmacists. With mood meds, a veritable rite of passage for many born after, say, 1980, large portions of the dating pool have known only pharmacologically influenced sex. Many are inserting meds into functioning but difficult relationships with no idea of the cost. Still others may be feeling effects that lingered after the drugs left their system.

This isn't a diatribe against mood-lifting drugs. People take them for good reasons. If your girlfriend is trying to pull herself together, it's a sign of maturity to seek help. If you want to tackle your problems without getting lost in work, partying, sex, or designer vodka, it's a sign of courage. But the generational legacy of our love affair with antidepressants is only now becoming apparent, and the "side effects" of these drugs may include everything from widespread singledom to the soul-destroying excesses of hookup culture to porn addiction to the problems of too many friends offering too many benefits.

With so many new unknowns, it pays to learn what antidepressants can mean for your ability to love another person, and how you show those feelings in the bedroom. Is it a happy pill, or something else?

Starting in the 1990s, a new generation of antidepressants were more aggressively marketed to a broader population. The main and most successful target group of these campaigns: women. According to data from the National Health and Nutrition Examination Survey, women are 2 1/2 times as likely to take antidepressants as men are. And with 264 million antidepressant prescriptions written in 2011, the selective serotonin reuptake inhibitors (SSRIs)—including Celexa, Zoloft, Prozac, and Lexapro—are some of the most prescribed pharmaceuticals in the nation. That means if your girlfriend is taking something from a brown bottle, chances are it's for her head.

College kids are presumably a peak demographic. Audrey Bahrick, Ph.D., a researcher on the sexual side effects of SSRIs and a staff psychologist at the University of Iowa's counseling service, says 20 to 30 percent of students coming to the service are already taking psychotropic meds. "Another 20 percent choose to start medication during the course of therapy, so of the college students seeking help, about 50 percent are on a psychotropic medication."

How do these meds work? SSRIs increase the availability of serotonin, but whether raising serotonin is the reason for the drugs' effectiveness is unclear. (Contrary to the marketing claim, depressed people do not have a "chemical imbalance" of low serotonin.) The drugs' effectiveness could be due to the placebo effect, or to mild sedating effects, or to mild mental energizing properties. But the bigger question is this: What are the drugs better at—improving depression scores or disrupting sexual function?

Research finds that only about half of patients respond to antidepressant treatment, and even among those who benefit, there may be a significant placebo effect to take into account—an astonishing 82 percent, according to a data analysis from the University of Connecticut. The researchers speculate that if these two findings are cumulative, the drug-specific benefit for the user may be "clinically negligible."

The percentage of SSRI users who take a hit to the libido, however, may be significant. "I think we can be confident that the majority will be affected by sexual side effects, perhaps somewhere between 50 and 70 percent of people who take them," says Bahrick. In a recent Iranian study, researchers concluded that sexual side effects affected 75 percent of people using SSRIs. Another study found the effect as high as 98 percent. It's a nearly perfect score for exactly the wrong thing.

If you are slack-jawed at this slack-penis (or vague-vagina) effect, it's probably because until recently, the labels on these drugs have reported a much lower risk. According to research by Bahrick, many of the SSRI package inserts even acknowledge that the sexual-dysfunction side effects reported in clinical trials, affecting 2 to 16 percent of users, may be an underestimation. How does 16 percent jump to 70 percent? In the initial clinical trials, the patients were not directly questioned about their sexual functioning. They were supposed to volunteer the embarrassing, confusing fact that they seemed to have been having lame sex an awful lot lately.

Kara (not her real name), a 22-year-old student in Washington state, was prescribed Cymbalta at age 18 for depression. She later took Zyprexa, Lexapro, Remeron, Pristiq, and then Prozac. She first noticed an effect while on Lexapro, when she masturbated "and had the weakest orgasm ever," and "total numbness in my vagina." Her emotional connection to lovers had been broken as well. "I had two long-term boyfriends that I really loved," she says. "As soon as I started taking Lexapro, there was this marked decrease in my ability to feel love for them and connection to them."

Bahrick thinks the effects on women don't receive enough attention. "With men it's easier to assess. A man who can no longer achieve an erection can't hide that fact from his partner, but a woman's loss of sexual functioning is less overt—she can choose to give pleasure even if she's not capable of receiving pleasure.

That might be acceptable in the short run, but it can wear thin over time."

As Kara says, "I don't know if I'm capable of falling in love anymore. The function isn't there. As for sex, I can't feel anything and just lie there like a sex doll. That's no fun for me."

That numbing effect happened to a man I'll call Rob, a real estate property manager who went on Lexapro for a lifelong case of dour outlook. Now, at 41, he remembers that he'd have been overjoyed to accept his lousy mood if he could have regained his libido when he quit the drug. When offered the med, "I was, like, 'Why not, what have I got to lose?'" he says. Lots, as it turned out. "For about a year it worked really well," he recalls. Sure, his libido ebbed, but at first he didn't mind. "Honestly, it was helpful, because I was so oversexed before then," he says.

But after a year the drug stopped working, so Rob decided to quit taking it. When he did, he discovered that his sexual function remained on pause—for five years. Once, in his first return to the bedroom with a woman he liked, he found himself struggling to reach half-mast. When it became clear that he couldn't perform, "she very cheerily got up and said she was going to go sleep in the guest bedroom," he says. "She'd left to go finish on her own, and I remember lying there thinking, 'Wow, how bad has my life become?'"

"There's a proportion of people for whom the change seems to be permanent," says David Healy, M.D., the author of Pharmageddon and founder of the drug side-effects database Rxisk.org. Reports of post-SSRI sexual disorder (PSSD, for short) are becoming more common, and the condition is being reported in medical journals. Prozac now carries the following ominous warning: "Symptoms of sexual dysfunction occasionally persist after discontinuation of fluoxetine treatment."

Thankfully, PSSD isn't the norm. Katherine Sharpe's desire peaked whenever she missed her pill, and Rob is now in a happy relationship. In case you might be wondering, I've shed my snowmobile boots in the bedroom. But terrible questions remain: How can one class of drugs turn off so many important pleasure zones? How can they diminish one of the most essential aspects of being human?

As it happens, the chemicals that throttle lust are the same ones that make life enjoyable. Antidepressants may interfere with nitric oxide, a blood molecule that relaxes smooth muscle and triggers the engorging of blood vessels in the penis, nipples, and clitoris during sex. The drugs also affect the way you think, feel, and relate to others. By raising serotonin, SSRIs overload receptors designed to keep dopamine activity high. That means less dopamine between neurons that serve as reward pathways of the brain. It is this neurotransmitter that makes possible thoughts like I want her, I need her, and even I love her. Depleting dopamine might distort, delay, or doom relationships.

The effect has a clinical name: emotional blunting. We have known since the late 1980s that antidepressants (and antipsychotics like Seroquel, Zyprexa, Abilify, and Risperdal) can induce apathy and indifference. Emotional blunting can be helpful in reducing feelings of sadness in depression, or fear and anxiety. But blunting also diminishes the kind of positive emotions you need to make a genuine connection with a romantic partner.

In one of the earliest accounts of emotional blunting, in the British Journal of Psychiatry in 2009, researchers at the University of Oxford, England, reported on SSRI effects in 38 users. The scientists found that "some participants felt reduced love or affection toward others and, in particular, reduced attraction toward their partner." They also discovered that SSRI use may cause "emotional detachment from other people, and reduced concern for other people's needs." Users of SSRIs can also be sloppier gauges of their own errors in judgment.

For instance, after only a week on the drug Celexa, even people who aren't depressed become less competent at detecting negative emotions in facial images, another Oxford study found. The evolutionary biologist Helen Fisher, Ph.D., who has written about the impact of SSRIs on relationships, notes: "When it comes to love and attachment, you want all your natural abilities in place. You don't want them blunted or altered."

Fisher, who has used fMRI scans to identify brain systems associated with desire, romantic love, and emotional attachment, is troubled about blunting among people who are not suffering from severe, chronic depression.

"It's one thing to lose the sex drive. But when you have sex it drives up testosterone, and more testosterone makes you want to have more sex, and having sex stimulates the genitals, and stimulating the genitals drives up the dopamine system, and that is needed to push yourself over the threshold and into falling in love," says Fisher. Orgasms also drive up oxytocin and vasopressin, she explains, "and that helps give you feelings of emotional attachment."

According to Lauren Starr, a spokeswoman for Pfizer, which manufactures Zoloft and Effexor, "Depressive and anxiety disorders, with or without treatment, can be associated with the emergence of adverse events such as sexual dysfunction." (The makers of Cymbalta and Lexapro declined to comment for this article. The maker of Paxil passed along a link to the drug's prescribing information.)

"Diminished sex drive really isn't a symptom of mild depression," says Stuart Shipko, M.D., a psychiatrist based in Pasadena, California. "However, diminished sex drive afflicts two-thirds of people who are prescribed an antidepressant. The sexual dysfunction caused by the drugs is much worse than sexual dysfunction from depression."

After eight years on antidepressants, Sharpe stopped at age 26. She felt that she was in the wrong phase of life to be foggy about how she felt. "College is a time of figuring out who you are, what kind of a path you want to put yourself on, and how you feel about things," she says. "One of the bad things about the overdiagnosis of young people is that it says the way you feel is a mistake, a product of an illness."

It's also a period when young people traditionally look for mates. "We're living in this time when the 20s have become a time of intense independence. Instead of partnering with someone in college, more and more people are living with friends and delaying settling down," Sharpe says. "I wonder if being mildly depressed, of having an acute sense of your need for comfort, keeps you in a relationship. If antidepressants give you a way to be on your own, they support this independent, self-centered social lifestyle," she says.

But there's a cost, says Fisher. "This game of love really matters during the reproductive years. We've evolved an enormous number of brain mechanisms to choose the partners we want. If you don't feel anything for anyone and go from one hookup to another, you will keep disappointing potential partners."

Plus, you may just miss your chance at the right one. "If you don't have any sex drive or can't feel any emotion," Fisher says, "how are you going to respond to the cute girl in algebra class or at the gym? These drugs are not a free ride."

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