What are possible negative effects of one time use of marijuana?

No. Yes. Consider a question. "Have you ever, at any time, used any illegal drugs or substances?" You will never be able to honestly answer this, "No."

Does it even matter if you can answer that honestly? What does it do to your character? I've heard people say, "I can't trust a person who's never smoked pot". Essentially, they're saying that a person who's tried it is someone who has half a brain and can think for themselves. You don't just follow rules blindly because someone says so. I would however say there are different tiers to drugs. Someone who has been smoking pot every other month for the past 3 years is harmless. Absolutely harmless. Someone <cough, cough, bankers> who've been doing coke for the last three years in order to stay up all night and close deals are.... well... think about it.