Exposure to drugs early in life can have long-lasting implications for brain structure and function. Effects on the developing nervous system, before homeostatic regulatory mechanisms are properly calibrated, differ from those on mature systems. Recent studies show that permanent alterations in brain pharmacology and cell signaling are induced by early drug exposure, producing hypo- or hyperresponsiveness to environmental or pharmacological challenges later in life. As a result, children exposed to drugs pre- or postnatally might respond abnormally to therapeutics used to treat the very disorders that they later exhibit as a result of their previous drug exposure.