When I joined the Air Force, they helped us fill out the clearance forms. One question was related to marijuana use in the past. The NCO helping us told us “if you have used it before, be honest. They will know.” But then followed it up with “remember: you used it less than 5 times and you didn’t like it”.
I remember similar advice.
In Navy boot camp the person reviewing my security clearance application (which was filled out weeks before) was very helpful in the way he asked the critical question. “It says here you tried marijuana once. Is that true?”
"Well, some guy I didn't know very well said it was marijuana - but how would I know? All it seemed to do was make my eyes water, and give me a headache..."