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It's never a good sign if someone is going out of their way to hide something. Even if they can't quite justify their urge to be secretive, hiding even low-level substance use is cause for concern. "It's a sign of addiction in that, on some level, the individual is recognizing what they're doing is inappropriate or problematic," says Compton. When someone is feeling self-conscious about their use of a substance, even in small amounts, it may indicate serious potential for a future problem.