This will be a multicentre, open-label, continuation trial of the efficacy and safety of tetrodotoxin in patients with inadequately controlled moderate to severe pain associated with cancer. All patients who participated in the TEC-006 study (tetrodotoxin and placebo-treated), who would like to continue with tetrodotoxin treatment and meet the inclusion/exclusion criteria, are eligible to receive the First Treatment Cycle for this continuation study.