This will be a multicentre, randomized, double-blind, placebo-controlled, parallel-design trial of the efficacy and safety of tetrodotoxin in patients over 18 years of age with stable but inadequately controlled moderate to severe pain associated with cancer. Approximately 15 centers across Canada, New Zealand and Australia are expected to participate. Subcutaneous tetrodotoxin (30 µg b.i.d.) or placebo will be administered to 127 patients per group for four consecutive days.