Even small amounts of the illegal drug ecstasy can be harmful to the brains of first time users, researchers say.
Half a million Britons are believed to take ecstasy each week
The University of Amsterdam team took brain scans and carried out memory tests on 188 people with no history of ecstasy use but at risk in the future.
They repeated the tests 18 months later, and found for the 59 people who had used ecstasy there was evidence of decreased blood flow and memory loss.
Long-term ecstasy use is already known to be harmful.
The class A drug is used by about 500,000 people in the UK, mostly on the club scene.
Lead researcher Maartje de Win said: "We do not know if these effects are transient or permanent.
"Therefore, we cannot conclude that ecstasy, even in small doses, is safe for the brain, and people should be informed of this risk."
Research has shown that long-term or heavy ecstasy use can damage neurons and cause depression, anxiety, confusion, difficulty sleeping and decrease memory.
However, no previous studies have looked at the side-effects of low doses of the drug on first time users.
The study, presented to the annual meeting of the Radiological Society of North America, said there was no evidence of damage to the neurons or alteration to mood and it was unclear whether the effect of early use of the drug was permanent.
Blood provides the brain with energy, and decreased flow can lead to memory loss and attention problems.
Of the people who were tested who had taken ecstasy, the average use was six tablets.
Dr Fabrizio Schifano, professor of pharmacology at the University of Hertfordshire, said it was clear that early use of the drug did have some effect, but what there was not a consensus on was how long that would last.
He said: "We know long-term use has a lasting impact, so it makes sense that damage starts as soon as someone starts to use the drug.
"But we cannot say exactly how much damage is sustained at the start and need more research to be categorical about this."