Opioid Methadone, often used as Heroin replacement therapy, weakens tumor cell’s resistance against chemotherapy and radiation and sometimes outright kills them on its own. German chemist Dr. Claudia Friesen discovered this accidentally in 2007 while trying to explore opioid chemistry, using cancer cells as test candidates.
German article; English article. Methadone was used in a trial on 80 patients with terminal cancer and ALL OF THEM lived longer than expected, some still alive. Note that the trial only used terminal cancer patients. It would likely be beneficial for less severe cases as well. Though, don’t ask me about addiction potential and side effects of Methadone though. I would think that side effects are irrelevant – clean opioids are generally not a direct health issue; junkies die through accidental overdose and malnutrition because they spend all their money on drugs instead of healthy food. They don’t die from the substance itself.