Tetrodotoxin (TTX) is a potent marine neurotoxin that blocks voltage-gated sodium channels (VGSCs). VGSCs play a critical role in neuronal function under both physiological and pathological conditions. TTX has been extensively used to functionally characterize VGSCs, which can be classified as TTX-sensitive or TTX-resistant channels according to their sensitivity to this toxin. Wex Pharmaceuticals is investigating tetrodotoxin for the treatment of chronic and breakthrough pain. The toxin is currently undergoing Phase III clinical trials in Canada as a systemic analgesic for inadequately controlled pain due to advanced cancer, especially where the pain has neuropathic features. In addition, TTX is in Phase II clinical trials to study its ability in moderate to severe neuropathic pain caused by chemotherapy.