On a hair follicle drug test, does it only test for heavy users?

I slipped up and took a pain pill last week and I have to take a hair follicle test maybe this week or in the next 45 days for underground coal mining, I am not a chronic every day user in fact thats the first time i have taken a pain pill in the last 2 years. But, its not likely for me to be taking a hair test this week probably just a urine but when they hire onto company after 45 days they definately do a hair test. I was told that those hair tests only test for heavy users, and say I did that last week I should be fine as long as I only did it one time in the last 3 months. Thanks for any answers i am just curious.