Well, I'm not sure but I think a drug is a substance that does something to your physiology. As previously stated, not all drugs are addictive (it's even called drug-store...) and not all things addictive are drugs.
Sex is a drug because it's addictive!
Look how wrong that statement is.
Most addictions are bad, making you less a part of the real world. World of Warcraft can be extremely bad. But it doesn't alter your physiology.
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