Inclusion Criteria:

All patients who participated in the TEC-006 study (tetrodotoxin and placebo-treated) will be eligible for inclusion in this study provided they wish to continue to receive treatment and meet the inclusion and exclusion criteria.

NOTE: The blinded treatment assignment of patients in the TEC-006 will not be revealed to either the investigator or the patient. Patients will not be aware which treatment they received in the TEC-006 study. All patients will receive tetrodotoxin treatment in the TEC-006OL study.

A patient will be eligible or continue to be eligible for inclusion in this study (First Treatment Cycle and subsequent Treatment Cycles) only if all of the following criteria apply:

1. Male or female 18 years of age and over
2. In-patients or out-patients with a diagnosis of cancer
3. Patients must be experiencing somatic, visceral and/or neuropathic pain related to cancer.
4. Inadequately controlled pain: Pain intensity described as 'moderate', 'severe' or 'excruciating', as assessed by the VRS during the screening/baseline period of the First Treatment Cycle, and a baseline pain intensity score of ≥4 as assessed by NRS (worst, average, or component-specific pain).
5. Life expectancy of > 3 months.
6. Ability to communicate well with the Investigator and to comply with the requirements of the entire study.
7. Willingness to give written informed consent (prior to any study-related procedures being performed) and to be able to adhere to the study restrictions, appointments and examination schedule.

Exclusion Criteria:

A patient will not be eligible for inclusion in this study if any of the following criteria apply:

1. Planned initiation of chemotherapy, radiotherapy or bisphosphonates within 30 days prior to enrolment.
2. Use of anaesthetics.
3. Use of lidocaine, and other types of antiarrhythmic drugs.
4. Use of scopolamine and acetylcholinesterase-inhibiting drugs such as physostigmine.
5. History of CO2 retention, or SaO2 <80% either on room air or O2 of not greater than 2-4 L/min by nasal cannula.
6. Second or third degree heart block or prolonged QTc interval (corrected for rate) on screening ECG (confirmed > 450 msec on repeated occasion) or any other active cardiac arrhythmia or abnormality that would constitute a clinical risk.
7. Coagulation or bleeding defects if in the opinion of the Investigator this represents a risk to the subject considering the subcutaneous (s.c.) route of administration.
8. Known hypersensitivity to puffer fish, tetrodotoxin and/or its derivatives.
9. Received an investigational agent other than tetrodotoxin within 30 days prior to screening or who is scheduled to receive an investigational drug other than tetrodotoxin during the course of the study.
10. Females who are lactating or at risk of pregnancy (i.e., sexually active with fertile males and not using an adequate form of birth control).
11. Females with a positive pregnancy test at screening or on admission to study site.
12. Any other condition that, in the opinion of the investigators, is likely to interfere with the successful collection of the measures required for the study or poses a risk to the patient.