Could you tell me if drugs like cocaine/mephedrone cause brain damage?

They can and do: There are rare cases of people who suffer serious consequences, particularly with mephedrone and other "bath salt" products, on their very first use with long-lasting and perhaps permanent damages. Others suffer those consequences (psychosis, depression, organic brain syndrome) after long-acting and/or heavy use. Bath salts in particular are a total crap shoot -- you don't know what you get.