QX 222 is a permanently charged lidocaine derivative primarily used in research settings as a selective intracellular blocker of voltage-gated sodium channels (NaV), particularly those involved in neuronal action potential generation and propagation. Due to its quaternary ammonium structure, it does not cross cell membranes and is applied via patch pipettes in electrophysiological recordings. This allows researchers to isolate and study other ionic currents (e.g., potassium or calcium) by suppressing sodium currents. Its applications are crucial in investigating neuronal excitability, synaptic transmission, and disorders such as epilepsy, neuropathic pain, or cardiac arrhythmias. QX 222 aids in elucidating ion channel mechanisms and developing targeted therapies through precise control of cellular excitability in patch-clamp experiments.