On the human body and/or mind?
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Don't underestimate a milligram. An mg of water still contains about a million million million molecules, more than we can really fathom. We generally have far less than a million million million sites for a molecule to act on, and so the substance and its target interact quite frequently. Not to mention that a single binding event can trigger a much larger cascade of responses - like methamphetamine causing an increase in dopamine through four separate pathways.
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Sometimes 1 mg is too much. The body may be sensitive to substances in much lower concentrations than 1 mg. It really depends on how our body has evolved and what substance it may be. Half a milligram of tetrodotoxin, for example, if injected directly into a 70 Kg (150 lb.) person, would be lethal. These substances affect our nervous system, often by blocking the transmission of signals between nerve cells, so it only takes a small amount to have a huge effect.