The worst side effect I saw from mephedrone was blue body part extremities; however, this side effect never happened to me. Also, I never got bad comedowns from either mephedrone or MDMA, except the time I did 3.5g 4-MMC in one night and got emotional the next morning. Mephedrone, despite its 5-HT releasing properties, feels more like a good, addictive stimulant to me, whereas MDMA has more of a serotonergic feel to me. As far as OD deaths go, they are both quite safe. And I agree with JWills20 that I wouldn't categorize either as terribly "neurotoxic," which has been explained to me to be defined as brain cell death. By this definition, MDMA is not a neurotoxin, but alcohol is. I think tolerance is an issue with both if you use them enough.