An inexpensive drug that's already used to treat type 2 diabetes could find new life as a cancer fighter, with two new studies suggesting it can fight off colon cancer and even prevent lung cancer in smokers.

The drug is called metformin, and is sold as Glucophage. It's widely used by type 2 diabetics who do not use insulin propoerly. But new research conducted at the National Cancer Institute in the U.S. has found that metformin can also fight lung cancer tumour growth in mice that have been exposed to a common carcinogen in cigarettes.

Metformin has been previously shown to activate an enzyme that is known to inhibit a protein that regulates cell growth and survival in tobacco-induced lung tumours.

For this study, NCI researchers, led by Dr. Philip Dennis, gave the mice metformin either orally or by injection. Those treated orally had between 40 and 50 per cent fewer tumours, while those mice treated with injection had 72 per cent fewer tumours.

A second study, also to be published in the journal Cancer Prevention Research, suggested metformin may prevent colorectal cancer tumours in humans by targetting the earliest stages of the disease.

What excites researchers is that unlike chemotherapy and radiation, which burn and poison cancer cells, metformin's effects are more subtle.

It's thought the drug works by targeting a cancer tumour's stem cells which, if not killed off, can allow various cancer cell types to regenerate.

"It's not killing them; it is actually suppressing their growth and without growth, the cells eventually die," explains Vuk Stambolic, a cancer researcher at Princess Margaret Hospital in Toronto.

Metformin was originally designed for diabetics to help them better use insulin. It works by preventing the liver from breaking down starch into sugar, while also stimulating uptake of sugar by the body's muscles, causing blood sugar and insulin levels to fall.

But when a number of studies noticed that diabetic patients taking metformin also had a significantly lower cancer rate, scientists began investigating its cancer-fighting properties.

Dr. Michael Pollak, a professor of oncology at McGill University, said the findings that metformin might be a cancer fighter are "one of the most unexpected and innovative findings" he's seen in years.

"Here we have an old molecule, an old drug, a safe drug that may have an unexpected use in cancer prevention and cancer treatment. The findings represent an unexpected and exciting lead," he said.

These are not the first studies to note metformin's ability to fight cancer.

A study released last year found that combining metformin with the chemotherapy drug doxorubicin reduced breast cancer tumours faster than doxorubicin alone, when tested on mice. The drug combo also prolonged remission in the mice longer than chemotherapy alone.

Dr. Pamela Goodwin, a medical oncologist at Mount Sinai Hospital in Toronto has three studies underway investigating if metformin can slow or prevent breast cancer in 3,500 women.

She notes that it's been tough getting funding for her studies because metformin is "an orphan drug," a generic drug not backed by any pharmaceutical company. And with no patent, there is no financial incentive for drug companies to fund research.

"We first proposed this five years ago," she told CTV News. "It took us five years of multiple presentations and requests for funding to cobble together funding."

She notes that metformin is a generally safe medication with a long history.

"It's important to develop this as quickly as we can, because if it is beneficial, we need to get this out to patients," she says.

With a report by CTV's medical specialist Avis Favaro and producer Elizabeth St. Philip