What is desire?
What triggers it?
And how can we make it last?
Some people think oysters, chocolate and ginseng -- among other mythical things --are aphrodisiacs.
They are rich in nutrients and give energy, but no one has proven they give you passion.
Even the "little blue pill" Viagra just works on, shall we say, the plumbing -- it keeps the blood flowing in the right direction.
Now there's a drug in the pipeline that its makers say really will restore lost libido.
It's being tested and developed, in part, in a laboratory at Concordia University in Montreal by neuroscientist Jim Pfaus.
Full-time college professor, part-time punk rocker, Pfaus is using rats to test whether the new substance, bremelanotide (pronounced "BREE-ma-LAN-o-tide") -- or PT 141 -- triggers desire.
Rats stand in for humans, because, like people, they're social and they have a similar hypothalamus, the part of the brain that controls desire.
In his lab, Pfaus says he's finding that bremelanotide seems to put rats in the mood.
Given the peptide, female rats, he says, initiate sex four times more often than those who do not receive it.
And bremelanotide's makers are betting it will work the same way on both men and women.
"It brings back your libido," Pfaus said. "It doesn't make it something that it wasn't. It brings it back to where it probably was when you were having good sex."
Bremelanotide didn't start out as a drug for sexual dysfunction.
In fact, it was being developed as a tanning enhancer, until researchers noticed interesting side effects in the men involved in clinical trials.
"All treatments for sexual dysfunction, especially erectile dysfunction, have colorful stories about how they started," said Carl Spana, CEO of Palatin Technologies, which holds the patent on bremelanotide. "In this particular one, young college men were given this drug in a Phase 1 study and got spontaneous erections.
Spana hopes his company has stumbled on the holy grail of sexual dysfunction.
And how does bremelanotide work?
Spana says it activates parts of the brain that are involved in regulating normal sexual function.
He says other products on the market, such as Viagra, Cialis and Levitra, "work by affecting peripheral blood flow, [rather than] the central nervous system."
Palatin Technologies, a biotech company in New Jersey, hopes to have bremelanotide on the market for men within a couple of years -- and for women shortly after that.
If approved, it would be the first drug sold in the United States to specifically target lack of sexual desire in women.
It's for those experiencing serious sexual problems -- women who've gone through menopause or had a hysterectomy, for instance, and lost desire as a result.
Michael Perelman, a New York City sex and marriage therapist, screened women for Palatin's human trials, and has kept an eye on the results.
"Some of the women put it in colorful language that I'm not comfortable using on television," Perelman told ABC News' "Nightline."
"But what was said was that, 'The quality of my orgasm was more the way it used to be, and I like that.' 'I had some thoughts about my husband.' 'I initiated sex with him for the first time even though I've thought about doing it for a long time.' Those were some of the things that they said," Perelman said.
If and when bremelanotide makes it to market, it will be administered as a single-use nasal spray.
That way, it goes straight to the brain and avoids being broken down in the digestive system.
Dr. Annette Shadiack, Director of Research at Palatin, says the results last six hours or eight hours, with the onset as early as 30 minutes.
But bremelanotide likely will encounter tough resistance on its way to receiving approval from the Food and Drug Administration.
One concern is abuse. Some young partyers have been known to mix Viagra with Ecstasy for recreational use.
"I think that there'll be enormous sales over the Internet. I think that there'll be sales in bars, on street corners," said sexual psychologist Leonore Tiefer, associate clinical professor of psychiatry at the New York University's School of Medicine. "Certainly when the thing is first approved, I think it will be widely misused."
In 2004, Tiefer helped block FDA approval for Proctor & Gamble's hormone patch Intrinsa, which was being touted as the female Viagra.
Besides the potential for abuse, she also worries about the overall impact of these sorts of drugs on the culture.
In particular, she's worried about the way it will be marketed.
"It raises the bar on performance expectations. So that women, couples, will somehow feel that if arousal, orgasm, desire, is not a regular routine, every day, every week, womb-to-tomb kind of thing that there is something wrong with you," Tiefer said.
"If women do go in with that message, their physician should clearly tell them that this is not what this is for," said Palatin CEO Spana. "It's for women who feel a real lack of desire, real lack of ability to get aroused."
Back in his lab in Montreal, Pfaus says he thinks women should have the option, but he hopes bremelanotide will give women confidence, not doubt.
"Women don't need another thing staring at them from a billboard telling them that they are not good enough," Pfaus said.
"But I think that if the drug is available for people who actually have true desire disorders that are defined by a psychiatrist and a clinical psychologist, then we're talking about a different group of people," he said.
But Pfaus says critics should not be overly concerned about bremelanotide's potency.
"This is not going to make people have orgies in the street," he said.