My guess is it means he was sweaty from taking drugs, presumably cocaine (coke).
Student or Learner
I am read a Micheal Connelly's book called The Lincoln Lawyer and saw the phrase in the following context:
She arranged her liaisons through a website which carried photos, phone numbers and e-mail addresses of escorts.
"Did he say where he was from?"
"No. He was Mexican or Cuban or something. He was sweaty from using."
"When he gave you the coke, did you see if he had any more?"
Does using refer to their intercourse? And does sweaty from using refer to the man's state after the thing?
Absent context, "using" refers to taking illegal drugs.
Did it look like she was still using.
I left him - said until he was no longer using, he and I were through.
At least, that's what I understand from TV.
I'm not a teacher, but I write for a living. Please don't ask me about 2nd conditionals, but I'm a safe bet for what reads well in (American) English.