"Neurotoxicity" is a term used frequently 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 the changes in the brain's serotonin (or '5-HT') system.
Serotonin is an important neurotransmitter chemical which plays 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 of at least 5mg per kilo. Take a person who weighs 70 kilograms, for example. Multiply 70kg by 5mg (70 x 5mg) and we'll get an equivalent human dose of 350mg - or roughly three and half Ecstasy pills (the average E pill contains 100mg of MDMA).
Neurotoxicity was not observed at lower doses 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, however, remains unknown.