Ecstasy – a stimulant that provokes feelings of euphoria and emotional warmth – causes a two-year drop in serotonin levels, a new study reveals. Serotonin helps regulate mood, appetite, sleep, learning, and memory.

New research suggests the effects of ecstasy “may be sustained over time, with no evidence of reversal,” said study co-author Dr. Ronald Cowan of Vanderbilt University in Tennessee.

The study involved 24 women, 14 of whom were ecstasy users for three to four years. The number of pills taken ranged from five to 375. When the study began, the ecstasy users had abstained from drug use for at least 90 days. Only women were used in the research to avoid gender-specific differences in serotonin levels.

Brain scans identified a rise in the number of serotonin receptors among the ecstasy using subjects, which indicates a drop in serotonin levels. Notably, the more ecstasy a woman had taken in her lifetime, researchers found, the bigger the drop in serotonin levels.

“We also have no proof that a drop in serotonin levels would be forever,” Dr. Cowan added. “So, this work raises serious concerns and questions, but more research is needed.”

About 16 million people in U.S. over the age of 12 have reportedly used ecstasy, the authors note.