Due to its distinctive effects on male physiology, Viagra is a drug that has changed the lives of countless men around the world over the last two decades.

But an unexpected side effect of the little blue pill hints at even greater transformative potential that scientists never knew about before – and one that could ultimately save thousands of lives.

Researchers studying the effects of Viagra (aka sildenafil) on mice have discovered a small, daily dose of the medication in the animals' drinking water significantly reduces their risk of developing colorectal cancer.

"Giving a baby dose of Viagra can reduce the amount of tumours in these animals by half," says biochemist Darren D. Browning from Augusta University.

(Phil Jones/Augusta University)

We should bear in mind that so far this surprising result has only been demonstrated in the animal model, not in people as yet.

But that's the exact next step the team wants to pursue, saying a clinical trial with patients considered at high risk of colorectal cancer – or with a family history of the disease – should be a research priority.

If such a trial gets the green light and the effects can be replicated in humans, it could be a huge step forward in saving lives lost to cancer.

In people, colorectal cancer is the third most common cancer worldwide, causing in excess of 1 million cases annually – some 50,000 of which end in death in the US each year.

There's a chance we might be able reduce these grim numbers with Viagra, the researchers think, thanks to the effect the drug appears to have on tissue that can become cancerous.

In the study, the researchers demonstrated that the daily Viagra dose halved the formation of polyps in mouse tissue: abnormal clumps of cells that form on intestinal lining, which have a tendency to become tumours.

Another drug the researchers experimented with, linaclotide – used to treat constipation and irritable bowel syndrome – was even more effective than Viagra.

But it comes with a nasty drawback: diarrhoea, even in small doses, which makes it ultimately unsuitable for prolonged use, the researchers say.

In contrast, Viagra at such low doses isn't known to produce side effects in humans, which could make it a safe, convenient way to stop the spread of polyps, thanks to it promoting production of a chemical called cyclic GMP inside the body.

The exact process by which cyclic GMP benefits the intestinal lining is still being investigated, but the research so far suggests the chemical suppresses excessive cell proliferation – the formation of new cells – in the gut.

Seeing as the gut has to deal with whatever we put in our mouths on a daily basis, it experiences high cell turnover – which is why polyp formation is so common. The more new cells forming, the more chance something could go wrong.

But cyclic GMP appears to boost normal cell formation while eliminating abnormal counterparts.

"When we give Viagra, we shrink the whole proliferating compartment," says Browning.

"Proliferating cells are more subject to mutations that cause cancer."

It's not the first time Viagra has demonstrated unexpected health benefits a world away from its origins in treating erectile dysfunction – which just goes to show how scientists are continually discovering new molecular mechanisms in old drugs.

In this case, existing polyps don't seem to be affected by the drug, so it looks like Viagra can't be used to fight tissue that has already turned cancerous. But as for cancer prevention, we could be onto a whole new thing here.

The findings are reported in Cancer Prevention Research.