Teenagers say words like 열라, 겁나, 졸라 a lot to just add emphasis. I'd say 열라 and 겁나 is just slang and they are not swear words while 졸라 is a very vulgar word.
I use 졸 with my very close friends who don't judge me by how I speak. There're some other variations of this one that men use far more ofen than women, I rarely use any of them.
If you have some extremely close Korean male friends, you could use it with them in my opinion.