A bipolar disorder patient developed lithium neurotoxicity after starting a GLP-1 receptor agonist, marking the first documented case of this potentially dangerous drug interaction. The patient experienced neurological symptoms characteristic of lithium poisoning, requiring immediate medical intervention. This interaction likely occurs through GLP-1's effects on kidney function and fluid balance, which can rapidly concentrate lithium to toxic levels in the bloodstream. The timing is particularly concerning given the explosive growth of GLP-1 medications like semaglutide and tirzepatide for weight management. Millions of Americans now use these drugs, and many psychiatric patients receive them specifically to counteract metabolic side effects from mood stabilizers. The finding exposes a critical blind spot in prescribing practices, as lithium has an extremely narrow therapeutic window where small concentration changes can mean the difference between therapeutic benefit and life-threatening toxicity. This case demands immediate protocol changes requiring enhanced lithium monitoring whenever GLP-1 agonists are introduced, and highlights how popular new medications can create unexpected risks when combined with established psychiatric treatments.