Want to join marines but I failed a drug test for previous job?

Hi, I am planning on signing up for the marines and had a question. I previously failed a drug test for a job I had about 3 months ago. I have been clean for 6 months now but they did a hair follicle drug test so it went back far. What should I say if they ask me about it I want to just tell them about it, but everybody keeps telling me not to? If I dont tell them about it will they look into my previous job history?