Analysis of 11,911 patients across two datasets reveals that combining gabapentin with dihydropyridine calcium channel blockers (common blood pressure medications like amlodipine) creates reversible cognitive decline through dual neuronal calcium signaling blockade. The self-controlled study of 3,058 patients showed 67% higher rates of cognitive symptoms and 34% more falls during combination use versus after discontinuation, with effects tripling encephalopathy risk in dementia patients. This finding addresses a critical gap as gabapentin prescriptions have surged 123% since 2010, now reaching 15.5 million patients annually. The reversibility suggests these aren't permanent brain changes but functional impairments from disrupted calcium signaling essential for memory and coordination. The research design cleverly used patients as their own controls and included a negative control with non-dihydropyridine calcium blockers, strengthening causal inference. However, this preprint awaits peer review, and results could change. For the millions taking this combination, the findings suggest routine medication reviews could prevent reversible cognitive decline, particularly in older adults with kidney disease where gabapentin accumulates.