"Neurotoxicity" is a term thrown around in most studies and reports on Ecstasy.

Any substance which makes permanent or temporary changes to the central nervous system, and the brain in particular, is deemed "neurotoxic".

Note the word "temporary". Neurotoxic does not automatically imply permanent changes. Nor does it necessarily mean "damage" or adverse effects for the human or animal experiencing it.

Neurotoxicity is suspected to be at the root of Ecstasy's memory bending effects. Specifically changes in the brain's serotonin (or '5-HT') system.

serotonin
Serotonin is an important neurotransmitter chemical which has many roles in the body, including heat regulation, modulating mood, pleasure and pain, balance, and memory.

Animal tests have shown that high or repeated doses of MDMA can produce long-lasting changes in the serotonin system, most worryingly, neurotoxic effects to the actual nerve cells (McCann).

However these neurotoxic effects were only detected at high doses. Specifically 5mg per kilo.

To translate that into human terms, for example a 70kg person, you multiple 70kg by 5mg (70 x 5mg) which makes an equivalent human dose of 350mg - or (roughly) three and half E pills (the average E pill contains 100mg of MDMA).

Neurotoxicity was not observed at lower dose of 2.5mg/per kilo, a 175mg dose for a 70kg person. In human clinical experiments with MDMA, they use 1.7mg/per kilo (119mg).

The exact neurotoxicity level of MDMA for humans is unknown. It could be more. It could be less. But it probably lies in between the two.