A drug that reverses ageing, promotes DNA repair and could even help astronauts travel to Mars by reducing the impact of cosmic radiation, may be on the market in three years, scientists claim.

Researchers working with two biotech companies hope to begin testing the treatment on clinical trial patients in the next six months.

In early experiments the drug, nicotinamide mononucleotide (NMN), had a dramatic rejuvenating effect on ageing mice.

Lead scientist Professor David Sinclair, from the University of South Wales (USW) in Australia and Harvard Medical School in the US, said: "The cells of the old mice were indistinguishable from the young mice, after just one week of treatment.

"This is the closest we are to a safe and effective anti-ageing drug that's perhaps only three to five years away from being on the market if the trials go well."

NMN boosts levels of NAD+, the oxidised form of the chemical nicotinamide adenine dinucleotide, which is naturally present in every cell of the body and helps regulate protein interactions that control DNA repair.

Accumulated DNA damage is believed to be a major driver of natural ageing and a primary cause of cancer.

Levels of NAD+, a "co-enzyme" or "helper" chemical that assists essential proteins, decline with age.

Recent work highlighting the chemical's potential anti-ageing properties has led to an influx of NAD+ supplements available online. However there is no hard evidence that the low-dose supplements really can keep ageing at bay.

The new research, reported in the journal Science, showed that NAD+ boosts the activity of a well-known DNA repair enzyme called PARP1.

Reduced levels of NAD+ with age were thought to reduce the ability of PARP1 to repair damaged DNA.

The work has attracted the interest of the American space agency Nasa, which is looking for ways of shielding astronauts from the effects of radiation on the long voyage to Mars.

High levels of cosmic radiation mean that the chances of unprotected astronauts developing cancer could approach 100%.

A competition run by Nasa in search for possible solutions was won by Prof Sinclair's team last year.

Over the past four years, Prof Sinclair and USW colleague Dr Lindsay Wu have been working with two spin-off companies MetroBiotech NSW and MetroBiotech International to turn NMN into a drug treatment.

The first clinical trial is expected to get under way at Brigham and Women's Hospital in Boston, US, this year.

As well as helping astronauts, NMN could protect frequent flyers from the effects of radiation on passenger jets and combat the accelerated ageing seen in childhood cancer survivors, said Dr Wu.

He pointed out that 96% of childhood cancer survivors suffer a chronic illness by the age of 45 including heart disease, Type 2 diabetes, Alzheimer's and new cancers.

"It would be great to do something about that, and we believe we can with this molecule," said Dr Wu.

In 2003 Professor Sinclair found a link between the anti-ageing enzyme SIRT1 and resveratrol, a naturally occurring molecule in red wine.

He said: "While resveratrol activates SIRT1 alone, NAD+ boosters activate all seven sirtuins, and should have an even greater impact on health and longevity."