Past research has attributed at least part of the “love effect” of Ecstasy to an increase in brain levels of the natural antidepressant serotonin. Australian neuropharmacologist Iain McGregor, of the University of Sydney, has another explanation. He says Ecstasy users are under the influence of a massive surge of oxytocin—the brain’s “love” hormone, normally released during nursing or orgasm—which cements pair bonds.

Earlier research had shown that Ecstasy causes an increase in blood levels of oxytocin, but because the hormone doesn’t readily cross the blood-brain barrier, no one was certain whether it was responsible for the feelings of empathy, euphoria, and openness reported by users. After giving rats the human equivalent of two to three Ecstasy tablets, McGregor found that the drug activated oxytocin-containing neurons in the hypothalamus, the part of the brain that normally releases the hormone. Moreover, when McGregor then gave the rats an oxytocin blocker, the Ecstasy-induced social behavior—lying next to each other and cuddling—all but disappeared.

So just how long might an Ecstasy-instigated relationship last? “There was an old bumper sticker in California that said ‘Don’t get married for 6 months after Ecstasy’—that could be the approximate length of time, but who knows?” McGregor says. A more specific answer could be coming soon: McGregor is currently devising an experiment to test whether rats prefer to be with rats they’ve taken Ecstasy with.