The mechanism of action of the antiepileptic and antinociceptive drugs of the gabapentinoid family has remained poorly understood. Gabapentin (GBP) binds to an exofacial epitope of the α₂δ-1 and α₂δ-2 auxiliary subunits of voltage-gated calcium channels, but acute inhibition of calcium currents by GBP is either very minor or absent. We formulated the hypothesis that GBP impairs the ability of α₂δ subunits to enhance voltage-gated Ca²⁺channel plasma membrane density by means of an effect on trafficking. Our results conclusively demonstrate that GBP inhibits calcium currents, mimicking a lack of α₂δ only when applied chronically, but not acutely, both in heterologous expression systems and in dorsal root-ganglion neurons. GBP acts primarily at an intracellular location, requiring uptake, because the effect of chronically applied GBP is blocked by an inhibitor of the system-L neutral amino acid transporters and enhanced by coexpression of a transporter. However, it is mediated by α₂δ subunits, being prevented by mutations in either α₂δ-1 or α₂δ-2 that abolish GBP binding, and is not observed for α₂δ-3, which does not bind GBP. Furthermore, the trafficking of α₂δ-2 and Ca_V2 channels is disrupted both by GBP and by the mutation in α₂δ-2, which prevents GBP binding, and we find that GBP reduces cell-surface expression of α₂δ-2 and Ca_V2.1 subunits. Our evidence indicates that GBP may act chronically by displacing an endogenous ligand that is normally a positive modulator of α₂δ subunit function, thereby impairing the trafficking function of the α₂δ subunits to which it binds.