MDMA is mildly neurotoxic, which means it can cause 'brain damage' if overused.
Basically: once your brain runs out of serotonin from rolling, free radicals can reach the serotonin receptor and potentially damage them. During a roll, you can help prevent this by taking some vitamin C - it's a powerful anti-oxidant that will reach your brain and help neutralize the free radicals before they can cause damage. The other main thing you can do to prevent this in the long term is to space out your rolls by at least a couple weeks each. If you roll twice in a short time, your brain is already low on serotonin and depletes it more quickly, so there's a longer time where the free radicals can do their nasty stuff. Some people also use 5-HTP to boost their serotonin levels before, during or after a roll. The jury's still out on how effective that is. Some people swear by it, others say it does nothing or even weakens their roll.
It's not particularly great for you at any dose, but the common wisdom is that for a healthy person, rolling once a month or so isn't too hard on the brain, but rolling more often than that with any regularity is probably not a good idea. Of course, the only way to be absolutely, 100% safe from any potential risk is never to take it, but IMHO, MDMA used in moderation is safe enough that an otherwise healthy person shouldn't lose any sleep over it.