Naltrexone is a pharmaceutical drug which has long been used at high doses in the treatment of alcohol and opiate medication addictions. Although it has been approved by the FDA for this purpose, research has shown that it might also be an effective off-label cancer treatment.
Low dose naltrexone, referred to as LDN, is given as a capsule in the 3 mg to 4.5 mg range for cancer, as opposed to the 50 mg dose given for addiction. 
Low dose naltrexone might exert its effects on tumor growth through a mix of three possible mechanisms:
- By inducing increases of metenkephalin (an endorphin produced in large amounts in the adrenal medulla) and beta-endorphin in the blood stream;
- By inducing an increase in the number and density of opiate receptors on the tumor cell membranes, thereby making them more responsive to the growth-inhibiting effects of the already-present levels of endorphins, which induce apoptosis (cell death) in the cancer cells; and
- By increasing the natural killer (NK) cell numbers and NK cell activity and lymphocyte activated CD8 numbers, which are quite responsive to increased levels of endorphins. 
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LDN works by increasing endorphins, which are the “feel good” peptides that are produced during activities such as exercise. Endorphins are responsible for the runner’s high, and in addition to improving well-being, they also improve immune system function
The mechanism of action of low dose naltrexone seems to be its effect on something known as opioid growth factor (OGF), which promotes the growth of cancer cells. OGF seems to be present in high amounts in all cancerous cells — much higher than in normal cells. One study looked at 31 human cancer cell types, representing 90 percent of all human cancer in the world, and all 31 cell types showed a high level of OGF. Low dose naltrexone’s ability to inhibit OGF should result in a slowing of the growth of tumor cells.
Research by others — on neuropeptide receptors expressed by various human tumors — has found opioid receptors in many types of cancer:
- Brain tumors (both astrocytoma and glioblastoma)
- Breast cancer
- Endometrial cancer
- Head and neck squamous cell carcinoma
- Myeloid leukemia
- Lung cancer (both small cell and non-small cell)
- Neuroblastoma and others
These findings suggest the possibility for a beneficial LDN effect in a wide variety of common cancers.
Another promising effect of LDN’s effect on OGF is that it may inhibit the formation of new blood vessels, known as angiogenesis. We know that cancer cells recruit new blood vessels, which is essential for cancer to survive and spread. One study on lab mice found that LDN reduced the number of new blood vessels formed as well as the total length of blood vessels.
“Low Dose Naltrexone (LDN) may well be the most important therapeutic breakthrough in over fifty years. It provides a new, safe and inexpensive method of medical treatment by mobilizing the natural defenses of one’s own immune system. LDN substantially reduces health care costs and improves treatment of a wide array of diseases. Unfortunately, because naltrexone has been without patent protection for many years, no pharmaceutical company will bear the expense of the large clinical trials necessary for FDA approval of LDN’s new special uses. It is now up to public institutions to seize the opportunity that LDN offers.” 
— Dr. David Gluck