at least they know marijuana itself isnt harmful yet its hard to argue their defense. Saying that you could always tell instantly wether it was laced makes you sound like a I-think-I-know-it-all-but-I-dont teenager.

My 2 cents is that you should just come forward and lay it out in the open. If you smoke frequently and live with your parents you are almost guarenteed to slip up at some point, leaving pipe in your pants pocket or weed on the table etc... When you come out about it they wont think you have a problem. As long as your parents arnt anti-weed zealots if you speak logically and rationally it will make them look stupid to flare up. In short it will really be worth it in the longer term.