Another barrier is excitement. “These are old unsexy drugs. If you’re doing work with a new molecule there’s a huge amount of interest,” Mr. Pantziarka said. “But if you’re doing something with aspirin, it’s not that interesting. You don’t get that hype. We like to think medicine isn’t fashion driven, but there is a degree of that going on.”

Doctor and patient acceptance is another concern. While off-label prescribing is entirely legal — the Food and Drug Administration doesn’t have the legal authority to regulate the practice of medicine — many doctors still fear lawsuits.

As I was told by one woman who preferred not to be identified in order to protect her privacy found out, many doctors disagree with experimenting with off-label drugs.

Almost four years ago, that woman was diagnosed with an incurable, Stage 4 cancer of the bile ducts, known as cholangiocarcinoma. The cancer, which is extremely rare, had spread and 24 tumors had grown throughout her body.

The patient, who had three children younger than 5 at the time of her diagnosis, was told she had six to 12 months to live. She quit her job and dedicated her time to finding a way — any way — of surviving. She saw as many doctors as possible to gain insight into her rare cancer, but some were unwilling to prescribe drugs outside the standard cancer treatments.

“I saw a doctor who thought more out of the box, and she recommended me to look into metformin,” the patient said. Metformin does appear to inhibit the proliferation of tumor cells, but the mechanism underpinning that isn’t yet fully understood. Nevertheless, she added metformin to her chemotherapy for six months.

Her search for a cure didn’t end there, however. It led next to fenbendazole, an anti-parasitic drug for animals. Joe Tippens, an American who was diagnosed with Stage 4 small-cell lung cancer in 2017, claims that taking fenbendazole saved his life; he has been cancer-free now for a year, and his story has gone viral on the internet. While a few studies in mice and petri dishes have suggested that the drug may have anticancer properties, studies on humans is lacking.