Highly advise against cocaine for many health reasons besides the results of a FUE hair transplant. Cocaine also reduces blood flow thus decreasing graft survival. - Dr. Keaney
The blood supply of the scalp is very rich and even a vaso-constrictor such as Cocaine or Smoking may have minimal effect. In any event, to pay it safe, stay away from heavy cocaine use after having a hair transplant for any potential compromise on blood supply. I know of no scientific study that either conforms or denies any impact of cocaine on hair transplants but because there is no evidence one way of the other, that should not be a reason to take the theoretical gamble