© UnknownRave on: Taking ecstasy, even in relatively small amounts, can damage memory, scientists have warned
Worryingly, the memory lapses are similar to those that occur in the early onset of dementia.
Even ten pills a year - less than one a month - caused problems, says the journal Addiction.
Ecstasy, also known by its chemical name MDMA, is a Class A drug.
But there are disagreements over how dangerous it is.
Government chief drugs advisor Professor David Nutt was fired three years ago after claiming taking ecstasy is no more dangerous than riding a horse.
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 tried the drug in the past and expected to use it more in future.
They were tested on their memory, learning, brain processing speed and attention at the start of the study and a year on.
At the end of a year, 23 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 the 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 who you went with and where you sat.
Lapses in it are seen as an indicator of the first stages of dementia.
Even low numbers of pills still caused memory problems.
Importantly, users of ecstasy, which is also known as MDMA, may not realise their brain is being affected until the damage was done.
A spokesman for the researchers said: '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.'
© UnknownEven ten pills a year, less than one a month, caused problems, the journal Addiction reports
'Our findings may raise concerns with regard to MDMA use, even in recreational amounts over a relatively short time period.'
Valerie Curran, professor of psychopharmacology at University College London, said that any effects on memory are likely to be small and transient.
She said: '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.
'When you stop using it, as most people do, things go back to the way they were.'
Young men and women in the UK are thought to be Europe's biggest users of ecstasy and the drug has been linked to more than 200 deaths since 1996.
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