A new study has linked ecstasy use to dementia-like memory problems.
Although the drug's effects on memory have been studied before, results have been muddied by the possibility that users already had memory problems.
To avoid this, researchers from the University of Cologne focused on young people who had taken the drug in the past and expected to use it more in future.
At the end of a year, 23 people had become regular ecstasy users, having taken between ten and 62 ecstasy pills since the start of the study.
Those who had become regular users showed a clear deterioration in episodic memory in comparison with others.
This memory details personal experiences, combining information about what happened with when and where - such as remembering not only the last film you saw but also who you went with and where you sat.
Lapses in memory are seen as an indicator of the first stages of dementia.
Importantly, users of ecstasy, which is also known as MDMA, a Class A drug, may not realize their brain is being affected until the damage was done.
"As the nature of the impairments may not be immediately obvious to the user, it is possible people wouldn't get the signs that they are being damaged by the drug until it is too late," the Daily Mail quoted a spokesman for the researchers as saying.
Lead author Dr Daniel Wagner, of the University of Cologne, said that the study had helped them start isolating the precise cognitive effects of the drug.
"By measuring the cognitive function of people with no history of ecstasy use and one year later identifying those who had used ecstasy at least ten times and re-measuring their performance, we have been able to start isolating the precise cognitive effects of this drug," Dr Wagner said.
"Our findings may raise concerns with regard to MDMA use, even in recreational amounts over a relatively short time period," he said.
Valerie Curran, professor of psychopharmacology at University College London, said that any effects of the drug on memory are likely to be small and transient.
"The general agreement that is emerging about ecstasy is that while you are using the drug, you might expect a very subtle memory impairment but it's probably not significant in the real world," she said.
"When you stop using it, as most people do, things go back to the way they were," she added.