Howard Cohen hopes it will boost his immune system and help him fight off his prostate cancer.
The California's Mothers' Milk Bank in the US says it has supplied about 28 adults with doctors' prescriptions in the past four years.
But cancer experts are sceptical and say there is no evidence that it works and might be risky for patients.
That is because drugs and viruses can be transmitted through breast milk, they said.
It doesn't taste all that pleasant. It's a bit oily and there's an after-taste.
Mr Cohen, who has a PhD in theoretical physics, read up on his cancer when he was diagnosed in 1999 and came across a piece of literature by scientists who had killed cancer cells in the laboratory using an ingredient of breast milk.
The team at Lund University in Sweden found a compound called alpha-lactalbumin-oleic acid killed brain tumour cells in the test tube.
Further studies showed the same compound appeared to treat warts caused by a virus linked to cancer of the cervix.
"My wife breast-fed all of our children and we are great proponents of its benefits."
"Breast-fed babies have lower risks of cancer, allergies and infections than babies who are not breast-fed."
He found a milk donor through an ex-colleague.
"His wife was nursing an eight month old, and she herself was a cancer survivor. She agreed to pump her breast milk for me."
A bottle a day
"For the next 11 months I was getting milk from her. I would go over about once a week and get the frozen milk. For a while I was drinking a 3.5 ounce bottle a day."
"When she decided to wean her child, my wife and I looked for another source."
They approached the California Milk Bank, which agreed to sell them the milk with a prescription.
It's a bizarre claim with no evidence for it having any clinical value or use.
David Kerr, professor of clinical pharmacology and cancer therapeutics at Oxford University
Mr Cohen said his urologist refused to prescribe the milk.
He went to see other doctors who also refused him, but eventually found an internist with an interest in alternative medicine who wrote the prescriptions.
He now drinks about two bottles per week and hopes to cut back more, but plans to drink it for life.
"I see it as an adjunct to the vitamins and minerals that I take. "
He says he is convinced that it is doing some good.
He said he refused to have surgery, radiation or hormonal therapy because he did not want to risk the potential side effects.
"They will always be available to me if the mother's milk doesn't work forever and it comes to that," he said.
When asked what it tastes like, he said: "It doesn't taste all that pleasant. It's a bit oily and there's an after-taste."
"I mix it with fruit and tofu and yoghurt into a smoothie to mask the taste as well as get other good things into my diet."
Cancer specialists in the US and the UK said there was no proof that breast milk could help cancer patients.
Dr John Stevens of the American Cancer Society said: "It has not been tested in any scientific fashion. "
"On a plausibility basis, it is hard to envisage how it might be working."
David Kerr, professor of clinical pharmacology and cancer therapeutics at Oxford University, said: "This is a tiny number of anecdotes with no proof whatsoever underpinning it."
"It's a bizarre claim with no evidence for it having any clinical value or use."
Pauline Sakamoto from the Mothers' Milk Bank in California said: "Part of it could be placebo effect. "
"No one really knows. It's an area that needs a lot of study. "
"Some patients truly believe this is the way to go for their cancers."
Gillian Weaver, of the UK Association for Milk Banking, welcomed more research.
She said even if there were evidence in the future that it did help, resources of donor milk in the UK were so scarce that the priority would be for premature and sick babies.