A drug discovered in the soil of a South Pacific island may help to fight the ageing process, research suggests.
When US scientists treated old mice with rapamycin it extended their expected lifespan by up to 38%.
The findings, published in the journal Nature, raise the prospect of being able to slow down the ageing process in older people.
However, a UK expert warned against using the drug to try to extend lifespan, as it can suppress immunity.
Rapamycin was first discovered on Easter Island in the 1970s.
It is already used to prevent organ rejection in transplant patients, and in stents implanted into patients to keep their coronary arteries open. It is also being tested as a possible treatment for cancer.
Researchers at three centres in Texas, Michigan and Maine gave the drug to mice at an age equivalent to 60 in humans.
The mice were bred to mimic the genetic diversity and susceptibility to disease of humans as closely as possible.
Rapamycin extended the animals' expected lifespan by between 28% and 38%.
The researchers estimated that in humans terms this would be greater than the predicted increase in extra years of life, if both cancer and heart disease were prevented and cured.
Researcher Dr Arlan Richardson, of the Barshop Institute, said: "I've been in aging research for 35 years and there have been many so-called 'anti-ageing' interventions over those years that were never successful.
"I never thought we would find an anti-ageing pill for people in my lifetime; however, rapamycin shows a great deal of promise to do just that."
Professor Randy Strong, of the University of Texas Health Science Center, said: "We believe this is the first convincing evidence that the ageing process can be slowed and lifespan can be extended by a drug therapy starting at an advanced age."
Rapamycin appears to have a similar effect to restricting food intake, which has also been shown to boost longevity.
It targets a protein in cells called mTOR, which controls many processes involved in metabolism and response to stress.
The researchers had to find a way to re-formulate the drug so that it was stable enough to make it to the mice's intestines before beginning to break down.
The original aim was to begin feeding the mice at four months of age, but the delay caused by developing the new formulation meant that feeding did not start until the animals were 20 months old.
The researchers thought the animals would be too old for the drug to have any effect - and were surprised when it did.
Professor Strong said: "This study has clearly identified a potential therapeutic target for the development of drugs aimed at preventing age-related diseases and extending healthy lifespan.
"If rapamycin, or drugs like rapamycin, works as envisioned, the potential reduction in health cost will be enormous."