The brains of ecstasy users show low levels of a certain protein, a finding that may explain why many feel they need to turn to higher doses of the drug.

In Tuesday's issue of the journal Brain, Stephen Kish of the Centre for Addiction and Mental Health in Toronto and his colleagues confirmed that low levels of the serotonin transporter, or SERT, are found in part of the brain of chronic ecstasy users who typically take about two tablets twice a month.

The transporter is a protein responsible for regulating levels of serotonin, a neurotransmitter important for mood and impulse control.

"It is possible that the low levels of the transporter means that there's actual brain damage, loss of serotonin neurons in the brain," Kish said. "But we can't prove that."

Ecstasy interacts with the transporter to cause the release of serotonin, which may explain some of the "friendly effect" of increased sociability that users report, the researchers said.

Since the transporter is needed for ecstasy to act, the low transporter levels might also explain why most ecstasy users in the study reported the first dose is the best for getting high, but the effect begins to decline and higher doses are needed.

"The need for higher doses, possibly caused by low SERT, could well increase the risk of harm caused by this stimulant drug," Kish said.

About 84 per cent of ecstasy users in the study showed some degree of drug tolerance. Participants were typically 26 years old when their brains were scanned and had been using the drug for four years when recruited for the study.

Using the same probe developed in Toronto, researchers at Johns Hopkins University in Baltimore also reported that SERT levels are low in the cerebral cortex of chronic ecstasy users.

For 12 years, scientists worldwide have struggled with conflicting brain-scanning results on how ecstasy might harm brain cells that use serotonin. The new independent replication makes the finding more believable, Kish said.

The study looked at 49 drug users and 50 control subjects. Hair analysis confirmed that ecstasy users actually used the drug, and an imaging probe measured SERT throughout the brain.

The hair analysis also showed many ecstasy users were also using methamphetamine, perhaps unknowingly. The meth itself might damage serotonin cells, the researchers said. MRI scans showed people who used both drugs showed a slightly thinner cerebral cortex.

But low SERT was found in both those who used ecstasy and meth and ecstasy alone.

Those with low SERT also showed a slight reduction in memory performance.